Bruce E Tabashnik

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• Carriere, Y., Carriere, Y., Fabrick, J., Fabrick, J., Tabashnik, B. E., & Tabashnik, B. E. (2016). Advances in managing pest resistance to Bt crops: Pyramids and seed mixtures.. In R. Horowitz (ed.) Advances in Insect Control and Resistance Management.(pp 263-286). Switzerland: Springer.
• Soberon, M., Garcia-Gomez, I., Tabashnik, B. E., & Bravo, A. (2015). Countering pest resistance with genetically modified Bt toxins. In Bt resistance – mechanisms and strategies for preserving effectiveness(pp 150-161). CABI.
• Tabashnik, B. E., & Carriere, Y. (2015). Successes and failures of transgenic Bt crops: Global patterns of field-evolved resistance.. In Bt resistance – mechanisms and strategies for preserving effectiveness(pp 1-14). CABI.
• Tabashnik, B. E. (2013). Modeling resistance to juvenile hormone analogs: Linking evolution, ecology, and management.. In Juvenile Hormones and Juvenoids: Modeling Biological Effects and Environmental Fate(pp 99-126). Boca Raton, FL: CRC Press.

Journals/Publications

• Carrière, Y., & Tabashnik, B. E. (2023). Fitness Costs and Incomplete Resistance Associated with Delayed Evolution of Practical Resistance to Bt Crops. Insects, 14(3).
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  Insect pests are increasingly evolving practical resistance to insecticidal transgenic crops that produce (Bt) proteins. Here, we analyzed data from the literature to evaluate the association between practical resistance to Bt crops and two pest traits: fitness costs and incomplete resistance. Fitness costs are negative effects of resistance alleles on fitness in the absence of Bt toxins. Incomplete resistance entails a lower fitness of resistant individuals on a Bt crop relative to a comparable non-Bt crop. In 66 studies evaluating strains of nine pest species from six countries, costs in resistant strains were lower in cases with practical resistance (14%) than without practical resistance (30%). Costs in F progeny from crosses between resistant and susceptible strains did not differ between cases with and without practical resistance. In 24 studies examining seven pest species from four countries, survival on the Bt crop relative to its non-Bt crop counterpart was higher in cases with practical resistance (0.76) than without practical resistance (0.43). Together with previous findings showing that the nonrecessive inheritance of resistance is associated with practical resistance, these results identify a syndrome associated with practical resistance to Bt crops. Further research on this resistance syndrome could help sustain the efficacy of Bt crops.
• Carrière, Y., Degain, B., Unnithan, G. C., & Tabashnik, B. E. (2023). Inheritance and fitness cost of laboratory-selected resistance to Vip3Aa in Helicoverpa zea (Lepidoptera: Noctuidae). Journal of economic entomology, 116(5), 1804-1811.
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  The polyphagous pest Helicoverpa zea (Lepidoptera: Noctuidae) has evolved practical resistance to transgenic corn and cotton producing Cry1 and Cry2 crystal proteins from Bacillus thuringiensis (Bt) in several regions of the United States. However, the Bt vegetative insecticidal protein Vip3Aa produced by Bt corn and cotton remains effective against this pest. To advance knowledge of resistance to Vip3Aa, we selected a strain of H. zea for resistance to Vip3Aa in the laboratory. After 28 generations of continuous selection, the resistance ratio was 267 for the selected strain (GA-R3) relative to a strain not selected with Vip3Aa (GA). Resistance was autosomal and almost completely recessive at a concentration killing all individuals from GA. Declines in resistance in heterogeneous strains containing a mixture of susceptible and resistant individuals reared in the absence of Vip3Aa indicate a fitness cost was associated with resistance. Previously reported cases of laboratory-selected resistance to Vip3Aa in lepidopteran pests often show partially or completely recessive resistance at high concentrations and fitness costs. Abundant refuges of non-Bt host plants can maximize the benefits of such costs for sustaining the efficacy of Vip3Aa against target pests.
• Fabrick, J. A., Li, X., Carrière, Y., & Tabashnik, B. E. (2023). Molecular Genetic Basis of Lab- and Field-Selected Bt Resistance in Pink Bollworm. Insects, 14(2).
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  Transgenic crops producing insecticidal proteins from the bacterium (Bt) control some important insect pests. However, evolution of resistance by pests reduces the efficacy of Bt crops. Here we review resistance to Bt cotton in the pink bollworm, , one of the world's most damaging pests of cotton. Field outcomes with Bt cotton and pink bollworm during the past quarter century differ markedly among the world's top three cotton-producing countries: practical resistance in India, sustained susceptibility in China, and eradication of this invasive lepidopteran pest from the United States achieved with Bt cotton and other tactics. We compared the molecular genetic basis of pink bollworm resistance between lab-selected strains from the U.S. and China and field-selected populations from India for two Bt proteins (Cry1Ac and Cry2Ab) produced in widely adopted Bt cotton. Both lab- and field-selected resistance are associated with mutations affecting the cadherin protein PgCad1 for Cry1Ac and the ATP-binding cassette transporter protein PgABCA2 for Cry2Ab. The results imply lab selection is useful for identifying genes important in field-evolved resistance to Bt crops, but not necessarily the specific mutations in those genes. The results also suggest that differences in management practices, rather than genetic constraints, caused the strikingly different outcomes among countries.
• Guan, F., Dai, X., Hou, B., Wu, S., Yang, Y., Lu, Y., Wu, K., Tabashnik, B. E., & Wu, Y. (2023). Refuges of conventional host plants counter dominant resistance of cotton bollworm to transgenic Bt cotton. iScience, 26(5), 106768.
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  Transgenic crops have revolutionized insect pest control, but evolution of resistance by pests threatens their continued success. The primary strategy for combating pest resistance to crops producing insecticidal proteins from (Bt) uses refuges of non-Bt host plants to allow survival of susceptible insects. The prevailing paradigm is that refuges delay resistance that is rare and recessively inherited. However, we discovered refuges countered resistance to Bt cotton that was neither rare nor recessive. In a 15-year field study of the cotton bollworm, the frequency of a mutation conferring dominant resistance to Bt cotton surged 100-fold from 2006 to 2016 yet did not rise from 2016 to 2020. Computer simulations indicate the increased refuge percentage from 2016 to 2020 is sufficient to explain the observed halt in the evolution of resistance. The results also demonstrate the efficacy of a Bt crop can be sustained by non-Bt refuges of other crops.
• Guan, F., Dai, X., Yang, Y., Tabashnik, B. E., & Wu, Y. (2023). Population Genomics of Nonrecessive Resistance to Bt Toxin Cry1Ac in Helicoverpa armigera From Northern China. Journal of economic entomology, 116(2), 310-320.
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  Transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt) have provided control of some key pests since 1996. However, the evolution of resistance by pests reduces the benefits of Bt crops. Resistance to Bt crops that is not recessively inherited is especially challenging to manage. Here we analyzed nonrecessive resistance to Bt toxin Cry1Ac in eight field populations of Helicoverpa armigera sampled in 2018 from northern China, where this global pest has been exposed to Cry1Ac in Bt cotton since 1997. Bioassays revealed 7.5% of field-derived larvae were resistant to Cry1Ac of which 87% had at least one allele conferring nonrecessive resistance. To analyze this nonrecessive resistance, we developed and applied a variant of a genomic mapping approach called quantitative trait locus (QTL)-seq. This analysis identified a region on chromosome 10 associated with nonrecessive resistance to Cry1Ac in all 21 backcross families derived from field-collected moths. Individual sequencing revealed that all 21 field-collected resistant grandparents of the backcross families had a previously identified dominant point mutation in the tetraspanin gene HaTSPAN1 that occurs in the region of chromosome 10 identified by QTL-seq. QTL-seq also revealed a region on chromosome 26 associated with nonrecessive resistance in at most 14% of the backcross families. Overall, the results imply the point mutation in HaTSPAN1 is the primary genetic basis of nonrecessive resistance to Cry1Ac in field populations of H. armigera from northern China. Moreover, because nonrecessive resistance is predominant, tracking the frequency of this point mutation could facilitate resistance monitoring in the region.
• Jin, M., Shan, Y., Peng, Y., Wang, W., Zhang, H., Liu, K., Heckel, D. G., Wu, K., Tabashnik, B. E., & Xiao, Y. (2023). Downregulation of a transcription factor associated with resistance to Bt toxin Vip3Aa in the invasive fall armyworm. Proceedings of the National Academy of Sciences of the United States of America, 120(44), e2306932120.
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  Transgenic crops producing insecticidal proteins from (Bt) have revolutionized control of some major pests. However, more than 25 cases of field-evolved practical resistance have reduced the efficacy of transgenic crops producing crystalline (Cry) Bt proteins, spurring adoption of alternatives including crops producing the Bt vegetative insecticidal protein Vip3Aa. Although practical resistance to Vip3Aa has not been reported yet, better understanding of the genetic basis of resistance to Vip3Aa is urgently needed to proactively monitor, delay, and counter pest resistance. This is especially important for fall armyworm (), which has evolved practical resistance to Cry proteins and is one of the world's most damaging pests. Here, we report the identification of an association between downregulation of the transcription factor gene and resistance to Vip3Aa in . Results from a genome-wide association study, fine-scale mapping, and RNA-Seq identified this gene as a compelling candidate for contributing to the 206-fold resistance to Vip3Aa in a laboratory-selected strain. Experimental reduction of expression in a susceptible strain using RNA interference (RNAi) or CRISPR/Cas9 gene editing decreased susceptibility to Vip3Aa, confirming that reduced expression of this gene can cause resistance to Vip3Aa. Relative to the wild-type promoter for , the promoter in the resistant strain has deletions and lower activity. Data from yeast one-hybrid assays, genomics, RNA-Seq, RNAi, and proteomics identified genes that are strong candidates for mediating the effects of on Vip3Aa resistance. The results reported here may facilitate progress in understanding and managing pest resistance to Vip3Aa.
• Matheson, P., Parvizi, E., Fabrick, J. A., Siddiqui, H. A., Tabashnik, B. E., Walsh, T., & McGaughran, A. (2023). Genome-wide analysis reveals distinct global populations of pink bollworm (Pectinophora gossypiella). Scientific reports, 13(1), 11762.
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  The pink bollworm (Pectinophora gossypiella) is one of the world's most destructive pests of cotton. This invasive lepidopteran occurs in nearly all cotton-growing countries. Its presence in the Ord Valley of North West Australia poses a potential threat to the expanding cotton industry there. To assess this threat and better understand population structure of pink bollworm, we analysed genomic data from individuals collected in the field from North West Australia, India, and Pakistan, as well as from four laboratory colonies that originated in the United States. We identified single nucleotide polymorphisms (SNPs) using a reduced-representation, genotyping-by-sequencing technique (DArTseq). The final filtered dataset included 6355 SNPs and 88 individual genomes that clustered into five groups: Australia, India-Pakistan, and three groups from the United States. We also analysed sequences from Genbank for mitochondrial DNA (mtDNA) locus cytochrome c oxidase I (COI) for pink bollworm from six countries. We found low genetic diversity within populations and high differentiation between populations from different continents. The high genetic differentiation between Australia and the other populations and colonies sampled in this study reduces concerns about gene flow to North West Australia, particularly from populations in India and Pakistan that have evolved resistance to transgenic insecticidal cotton. We attribute the observed population structure to pink bollworm's narrow host plant range and limited dispersal between continents.
• Tabashnik, B. E. (2023). Cross-kingdom convergence provides promising proteins for pest control. Proceedings of the National Academy of Sciences of the United States of America, 120(45), e2316386120.
• Tabashnik, B. E., Carrière, Y., Wu, Y., & Fabrick, J. A. (2023). Global perspectives on field-evolved resistance to transgenic Bt crops: a special collection. Journal of economic entomology, 116(2), 269-274.
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  Crops genetically engineered to produce insect-killing proteins from Bacillus thuringiensis (Bt) have revolutionized management of some major pests, but their efficacy is reduced when pests evolve resistance. Practical resistance, which is field-evolved resistance that reduces the efficacy of Bt crops and has practical implications for pest management, has been reported in 26 cases in seven countries involving 11 pest species. This special collection includes six original papers that present a global perspective on field-evolved resistance to Bt crops. One is a synthetic review providing a comprehensive global summary of the status of the resistance or susceptibility to Bt crops of 24 pest species in 12 countries. Another evaluates the inheritance and fitness costs of resistance of Diabrotica virgifera virgifera to Gpp34/Tpp35Ab (formerly called Cry34/35Ab). Two papers describe and demonstrate advances in techniques for monitoring field-evolved resistance. One uses a modified F2 screen for resistance to Cry1Ac and Cry2Ab in Helicoverpa zea in the United States. The other uses genomics to analyze nonrecessive resistance to Cry1Ac in Helicoverpa armigera in China. Two papers provide multi-year monitoring data for resistance to Bt corn in Spain and Canada, respectively. The monitoring data from Spain evaluate responses to Cry1Ab of the corn borers Sesamia nonagrioides and Ostrinia nubilalis, whereas the data from Canada track responses of O. nubilalis to Cry1Ab, Cry1Fa, Cry1A.105, and Cry2Ab. We hope the new methods, results, and conclusions reported here will spur additional research and help to enhance the sustainability of current and future transgenic insecticidal crops.
• Tabashnik, B. E., Fabrick, J. A., & Carrière, Y. (2023). Global Patterns of Insect Resistance to Transgenic Bt Crops: The First 25 Years. Journal of economic entomology, 116(2), 297-309.
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  Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have improved pest management and reduced reliance on insecticide sprays. However, evolution of practical resistance by some pests has reduced the efficacy of Bt crops. We analyzed global resistance monitoring data for 24 pest species based on the first 25 yr of cultivation of Bt crops including corn, cotton, soybean, and sugarcane. Each of the 73 cases examined represents the response of one pest species in one country to one Bt toxin produced by one or more Bt crops. The cases of practical resistance rose from 3 in 2005 to 26 in 2020. Practical resistance has been documented in some populations of 11 pest species (nine lepidopterans and two coleopterans), collectively affecting nine widely used crystalline (Cry) Bt toxins in seven countries. Conversely, 30 cases reflect no decrease in susceptibility to Bt crops in populations of 16 pest species in 10 countries. The remaining 17 cases provide early warnings of resistance, which entail genetically based decreases in susceptibility without evidence of reduced field efficacy. The early warnings involve four Cry toxins and the Bt vegetative insecticidal protein Vip3Aa. Factors expected to favor sustained susceptibility include abundant refuges of non-Bt host plants, recessive inheritance of resistance, low resistance allele frequency, fitness costs, incomplete resistance, and redundant killing by multi-toxin Bt crops. Also, sufficiently abundant refuges can overcome some unfavorable conditions for other factors. These insights may help to increase the sustainability of current and future transgenic insecticidal crops.
• Fabrick, J. A., LeRoy, D. M., Mathew, L. G., Wu, Y., Unnithan, G. C., Yelich, A. J., Carrière, Y., Li, X., & Tabashnik, B. E. (2021). CRISPR-mediated mutations in the ABC transporter gene ABCA2 confer pink bollworm resistance to Bt toxin Cry2Ab. Scientific reports, 11(1), 10377.
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  Crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt) have many benefits and are important globally for managing insect pests. However, the evolution of pest resistance to Bt crops reduces their benefits. Understanding the genetic basis of such resistance is needed to better monitor, manage, and counter pest resistance to Bt crops. Previous work shows that resistance to Bt toxin Cry2Ab is associated with mutations in the gene encoding the ATP-binding cassette protein ABCA2 in lab- and field-selected populations of the pink bollworm (Pectinophora gossypiella), one of the world's most destructive pests of cotton. Here we used CRISPR/Cas9 gene editing to test the hypothesis that mutations in the pink bollworm gene encoding ABCA2 (PgABCA2) can cause resistance to Cry2Ab. Consistent with this hypothesis, introduction of disruptive mutations in PgABCA2 in a susceptible strain of pink bollworm increased the frequency of resistance to Cry2Ab and facilitated creation of a Cry2Ab-resistant strain. All Cry2Ab-resistant individuals tested in this study had disruptive mutations in PgABCA2. Overall, we found 17 different disruptive mutations in PgABCA2 gDNA and 26 in PgABCA2 cDNA, including novel mutations corresponding precisely to single-guide (sgRNA) sites used for CRISPR/Cas9. Together with previous results, these findings provide the first case of practical resistance to Cry2Ab where evidence identifies a specific gene in which disruptive mutations can cause resistance and are associated with resistance in field-selected populations.
• Qi, L., Dai, H., Jin, Z., Shen, H., Guan, F., Yang, Y., Tabashnik, B. E., & Wu, Y. (2021). Evaluating Cross-Resistance to Cry and Vip Toxins in Four Strains of Helicoverpa armigera with Different Genetic Mechanisms of Resistance to Bt Toxin Cry1Ac. Frontiers in microbiology, 12, 670402.
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  Evolution of resistance by pests has diminished the efficacy of transgenic crops producing insecticidal proteins from (Bt). In China, where transgenic cotton producing Bt toxin Cry1Ac has been planted since 1997, field control failures have not been reported but the frequency of resistance to Cry1Ac has increased in the cotton bollworm, This provides incentive to switch to multi-toxin Bt cotton, which is grown in many other countries. Previous work created four laboratory strains of with >100-fold resistance to Cry1Ac, with the genetic basis of resistance known in all but the LF256 strain. Here, we analyzed the genetic basis of resistance in Cry1Ac in LF256 and evaluated cross-resistance of all four strains to three toxins produced by widely planted multi-toxin Bt cotton: Cry1Fa, Cry2Ab, and Vip3Aa. DNA sequencing revealed that LF256 lacked the mutations in three genes (, , and ) that confer resistance to Cry1Ac in two other strains of we analyzed. Together with previous results, the data reported here show that each of the four strains examined has a different genetic basis of resistance to Cry1Ac. Significant positive cross-resistance occurred to Cry1Fa in three of the four strains tested but not to Cry2Ab or Vip3Aa in any strain. Thus, Cry2Ab and Vip3Aa are likely to be especially valuable for increasing the efficacy and durability of Bt cotton against populations that have some resistance to Cry1Ac.
• Tabashnik, B. E., Liesner, L. R., Ellsworth, P. C., Unnithan, G. C., Fabrick, J. A., Naranjo, S. E., Li, X., Dennehy, T. J., Antilla, L., Staten, R. T., & Carrière, Y. (2021). Transgenic cotton and sterile insect releases synergize eradication of pink bollworm a century after it invaded the United States. Proceedings of the National Academy of Sciences of the United States of America, 118(1).
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  Invasive organisms pose a global threat and are exceptionally difficult to eradicate after they become abundant in their new habitats. We report a successful multitactic strategy for combating the pink bollworm (), one of the world's most invasive pests. A coordinated program in the southwestern United States and northern Mexico included releases of billions of sterile pink bollworm moths from airplanes and planting of cotton engineered to produce insecticidal proteins from the bacterium (Bt). An analysis of computer simulations and 21 y of field data from Arizona demonstrate that the transgenic Bt cotton and sterile insect releases interacted synergistically to reduce the pest's population size. In Arizona, the program started in 2006 and decreased the pest's estimated statewide population size from over 2 billion in 2005 to zero in 2013. Complementary regional efforts eradicated this pest throughout the cotton-growing areas of the continental United States and northern Mexico a century after it had invaded both countries. The removal of this pest saved farmers in the United States $192 million from 2014 to 2019. It also eliminated the environmental and safety hazards associated with insecticide sprays that had previously targeted the pink bollworm and facilitated an 82% reduction in insecticides used against all cotton pests in Arizona. The economic and social benefits achieved demonstrate the advantages of using agricultural biotechnology in concert with classical pest control tactics.
• Van den Berg, J., Prasanna, B. M., Midega, C. A., Ronald, P. C., Carrière, Y., & Tabashnik, B. E. (2021). Managing Fall Armyworm in Africa: Can Bt Maize Sustainably Improve Control?. Journal of economic entomology, 114(5), 1934-1949.
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  The recent invasion of Africa by fall armyworm, Spodoptera frugiperda, a lepidopteran pest of maize and other crops, has heightened concerns about food security for millions of smallholder farmers. Maize genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) is a potentially useful tool for controlling fall armyworm and other lepidopteran pests of maize in Africa. In the Americas, however, fall armyworm rapidly evolved practical resistance to maize producing one Bt toxin (Cry1Ab or Cry1Fa). Also, aside from South Africa, Bt maize has not been approved for cultivation in Africa, where stakeholders in each nation will make decisions about its deployment. In the context of Africa, we address maize production and use; fall armyworm distribution, host range, and impact; fall armyworm control tactics other than Bt maize; and strategies to make Bt maize more sustainable and accessible to smallholders. We recommend mandated refuges of non-Bt maize or other non-Bt host plants of at least 50% of total maize hectares for single-toxin Bt maize and 20% for Bt maize producing two or more distinct toxins that are each highly effective against fall armyworm. The smallholder practices of planting more than one maize cultivar and intercropping maize with other fall armyworm host plants could facilitate compliance. We also propose creating and providing smallholder farmers access to Bt maize that produces four distinct Bt toxins encoded by linked genes in a single transgene cassette. Using this novel Bt maize as one component of integrated pest management could sustainably improve control of lepidopteran pests including fall armyworm.
• Yang, F., Kerns, D. L., Little, N. S., Santiago González, J. C., & Tabashnik, B. E. (2021). Early Warning of Resistance to Bt Toxin Vip3Aa in Helicoverpa zea. Toxins, 13(9).
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  Evolution of resistance by pests can reduce the benefits of crops genetically engineered to produce insecticidal proteins from (Bt). Because of the widespread resistance of to crystalline (Cry) Bt toxins in the United States, the vegetative insecticidal protein Vip3Aa is the only Bt toxin produced by Bt corn and cotton that remains effective against some populations of this polyphagous lepidopteran pest. Here we evaluated resistance to Vip3Aa using diet bioassays to test 42,218 larvae from three lab strains and 71 strains derived from the field during 2016 to 2020 in Arkansas, Louisiana, Mississippi, Tennessee, and Texas. Relative to the least susceptible of the three lab strains tested (BZ), susceptibility to Vip3Aa of the field-derived strains decreased significantly from 2016 to 2020. Relative to another lab strain (TM), 7 of 16 strains derived from the field in 2019 were significantly resistant to Vip3Aa, with up to 13-fold resistance. Susceptibility to Vip3Aa was significantly lower for strains derived from Vip3Aa plants than non-Vip3Aa plants, providing direct evidence of resistance evolving in response to selection by Vip3Aa plants in the field. Together with previously reported data, the results here convey an early warning of field-evolved resistance to Vip3Aa in that supports calls for urgent action to preserve the efficacy of this toxin.
• Carrière, Y., Brown, Z., Aglasan, S., Dutilleul, P., Carroll, M., Head, G., Tabashnik, B. E., Jørgensen, P. S., & Carroll, S. P. (2020). Crop rotation mitigates impacts of corn rootworm resistance to transgenic Bt corn. Proceedings of the National Academy of Sciences of the United States of America, 117(31), 18385-18392.
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  Transgenic crops that produce insecticidal proteins from (Bt) can suppress pests and reduce insecticide sprays, but their efficacy is reduced when pests evolve resistance. Although farmers plant refuges of non-Bt host plants to delay pest resistance, this tactic has not been sufficient against the western corn rootworm, In the United States, some populations of this devastating pest have rapidly evolved practical resistance to Cry3 toxins and Cry34/35Ab, the only Bt toxins in commercially available corn that kill rootworms. Here, we analyzed data from 2011 to 2016 on Bt corn fields producing Cry3Bb alone that were severely damaged by this pest in 25 crop-reporting districts of Illinois, Iowa, and Minnesota. The annual mean frequency of these problem fields was 29 fields (range 7 to 70) per million acres of Cry3Bb corn in 2011 to 2013, with a cost of $163 to $227 per damaged acre. The frequency of problem fields declined by 92% in 2014 to 2016 relative to 2011 to 2013 and was negatively associated with rotation of corn with soybean. The effectiveness of corn rotation for mitigating Bt resistance problems did not differ significantly between crop-reporting districts with versus without prevalent rotation-resistant rootworm populations. In some analyses, the frequency of problem fields was positively associated with planting of Cry3 corn and negatively associated with planting of Bt corn producing both a Cry3 toxin and Cry34/35Ab. The results highlight the central role of crop rotation for mitigating impacts of resistance to Bt corn.
• Carrière, Y., Degain, B. A., & Tabashnik, B. E. (2020). Effects of gene flow between Bt and non-Bt plants in a seed mixture of Cry1A.105 + Cry2Ab corn on performance of corn earworm in Arizona. Pest management science.
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  Using natural populations of Helicoverpa zea from Arizona, we tested the hypotheses that gene flow between Bt and non-Bt plants in a seed mixture of 10% non-Bt corn and 90% Bt corn producing Cry1A.105 and Cry2Ab reduces larval performance on ears from non-Bt plants, or increases performance on ears from Bt plants.
• Carrière, Y., Degain, B. A., Harpold, V. S., Unnithan, G. C., & Tabashnik, B. E. (2020). Gene Flow Between Bt and Non-Bt Plants in a Seed Mixture Increases Dominance of Resistance to Pyramided Bt Corn in Helicoverpa zea (Lepidoptera: Noctuidae). Journal of economic entomology, 113(5), 2041-2051.
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  For delaying evolution of pest resistance to transgenic corn producing Bacillus thuringiensis (Bt) toxins, limited data are available to compare the effectiveness of refuges of non-Bt corn planted in seed mixtures versus blocks. Here we addressed this issue in the ear-feeding pest Helicoverpa zea Boddie by measuring its survival and development in the laboratory on ears from field plots with 90% Cry1A.105 + Cry2Ab corn and 10% non-Bt corn planted in a seed mixture or blocks. We compared a strain of H. zea selected for resistance to Cry1Ac in the laboratory, its parent strain not selected in the laboratory, and their F1 progeny. The relative survival of the F1 progeny and dominance of resistance were higher on ears from Bt plants in the seed mixture than the block. Half of the kernels in ears from non-Bt plants in the seed mixture produced both Cry1A.105 and Cry2Ab. However, survival on ears from non-Bt plants did not differ between the block and seed mixture. In simulations based on the observed survival, resistance to Cry1A.105 + Cry2Ab corn evolved faster with the seed mixture than the blocks, because of the higher dominance of resistance in the seed mixture. Increasing the refuge percentage improved durability of Cry1A.105 + Cry2Ab corn more for the blocks than the seed mixture. These findings imply that, for a given percentage of non-Bt corn, resistance of H. zea and other ear-feeding pests to multi-toxin Bt corn is likely to evolve faster for seed mixtures than blocks.
• Fabrick, J. A., LeRoy, D. M., Unnithan, G. C., Yelich, A. J., Carrière, Y., Li, X., & Tabashnik, B. E. (2020). Shared and Independent Genetic Basis of Resistance to Bt Toxin Cry2Ab in Two Strains of Pink Bollworm. Scientific reports, 10(1), 7988.
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  Evolution of pest resistance threatens the benefits of crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt). Field populations of the pink bollworm (Pectinophora gossypiella), a global pest of cotton, have evolved practical resistance to transgenic cotton producing Bt toxin Cry2Ab in India, but not in the United States. Previous results show that recessive mutations disrupting an autosomal ATP-binding cassette gene (PgABCA2) are associated with pink bollworm resistance to Cry2Ab in field-selected populations from India and in one lab-selected strain from the United States (Bt4-R2). Here we discovered that an independently derived, lab-selected Cry2Ab-resistant pink bollworm strain from the United States (BX-R) also harbors mutations that disrupt PgABCA2. Premature stop codons introduced by mis-splicing of PgABCA2 pre-mRNA were prevalent in field-selected larvae from India and in both lab-selected strains. The most common mutation in field-selected larvae from India was also detected in both lab-selected strains. Results from interstrain crosses indicate BX-R has at least one additional mechanism of resistance to Cry2Ab that does not involve PgABCA2 and is not completely recessive or autosomal. We conclude that recessive mutations disrupting PgABCA2 are the primary, but not the only, mechanism of resistance to Cry2Ab in pink bollworm.
• Fritz, M. L., Nunziata, S. O., Guo, R., Tabashnik, B. E., & Carrière, Y. (2020). Mutations in a Novel Cadherin Gene Associated with Bt Resistance in. G3 (Bethesda, Md.), 10(5), 1563-1574.
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  Transgenic corn and cotton produce crystalline (Cry) proteins derived from the soil bacterium (Bt) that are toxic to lepidopteran larvae. , a key pest of corn and cotton in the U.S., has evolved widespread resistance to these proteins produced in Bt corn and cotton. While the genomic targets of Cry selection and the mutations that produce resistant phenotypes are known in other lepidopteran species, little is known about how selection by Cry proteins shape the genome of We scanned the genomes of Cry1Ac-selected and unselected lines, and identified twelve genes on five scaffolds that differed between lines, including (), a gene from a family that is involved in Cry1A resistance in other lepidopterans. Although this gene was expressed in the larval midgut, the protein it encodes has only 17 to 22% identity with cadherin proteins from other species previously reported to be involved in Bt resistance. An analysis of midgut-expressed cDNAs showed significant between-line differences in the frequencies of putative nonsynonymous substitutions (both SNPs and indels). Our results indicate that is a likely target of Cry1Ac selection in It remains unclear, however, whether genomic changes at this locus directly disrupt midgut binding of Cry1Ac and cause Bt resistance, or indirectly enhance fitness of in the presence of Cry1Ac by some other mechanism. Future work should investigate phenotypic effects of these nonsynonymous substitutions and their impact on fitness of larvae that ingest Cry1Ac.
• Huang, J., Xu, Y., Zuo, Y., Yang, Y., Tabashnik, B. E., & Wu, Y. (2020). Evaluation of five candidate receptors for three Bt toxins in the beet armyworm using CRISPR-mediated gene knockouts. Insect biochemistry and molecular biology, 121, 103361.
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  Insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) can provide safe and effective control of some major pests, but evolution of resistance by pests diminishes these benefits. Better understanding of the genetics and mechanisms of resistance is urgently needed to improve methods for monitoring, managing, and countering pest resistance to Bt toxins. Here we used CRISPR-mediated knockouts to evaluate the role of five genes encoding candidate Bt toxin receptors in Spodoptera exigua (beet armyworm), a devastating pest of vegetable, field and flower crops. We compared susceptibility to Bt toxins Cry1Ac, Cry1Fa, and Cry1Ca between the parent susceptible strain and each of five strains homozygous for the knockout of one of the candidate genes (SeAPN1, SeCad1, SeABCC1, SeABCC2 or SeABCC3). The results from the 15 pairwise comparisons reveal that SeABCC2 has a major role and SeCad1 a minor role in mediating toxicity of Cry1Ac and Cry1Fa. SeABCC2 also has a minor role in toxicity of Cry1Ca. In addition, the results imply little or no role for the other three candidate receptors in toxicity of Cry1Ac or Cry1Fa; or for the four candidate receptors other than SeABCC2 in toxicity of Cry1Ca.
• Li, J., Aidlin Harari, O., Doss, A. L., Walling, L. L., Atkinson, P. W., Morin, S., & Tabashnik, B. E. (2020). Can CRISPR gene drive work in pest and beneficial haplodiploid species?. Evolutionary applications, 13(9), 2392-2403.
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  Gene drives based on CRISPR/Cas9 have the potential to reduce the enormous harm inflicted by crop pests and insect vectors of human disease, as well as to bolster valued species. In contrast with extensive empirical and theoretical studies in diploid organisms, little is known about CRISPR gene drive in haplodiploids, despite their immense global impacts as pollinators, pests, natural enemies of pests, and invasive species in native habitats. Here, we analyze mathematical models demonstrating that, in principle, CRISPR homing gene drive can work in haplodiploids, as well as at sex-linked loci in diploids. However, relative to diploids, conditions favoring the spread of alleles deleterious to haplodiploid pests by CRISPR gene drive are narrower, the spread is slower, and resistance to the drive evolves faster. By contrast, the spread of alleles that impose little fitness cost or boost fitness was not greatly hindered in haplodiploids relative to diploids. Therefore, altering traits to minimize damage caused by harmful haplodiploids, such as interfering with transmission of plant pathogens, may be more likely to succeed than control efforts based on introducing traits that reduce pest fitness. Enhancing fitness of beneficial haplodiploids with CRISPR gene drive is also promising.
• Wang, J., Ma, H., Zhao, S., Huang, J., Yang, Y., Tabashnik, B. E., & Wu, Y. (2020). Functional redundancy of two ABC transporter proteins in mediating toxicity of Bacillus thuringiensis to cotton bollworm. PLoS pathogens, 16(3), e1008427.
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  Evolution of pest resistance reduces the efficacy of insecticidal proteins from the gram-positive bacterium Bacillus thuringiensis (Bt) used widely in sprays and transgenic crops. Better understanding of the genetic basis of resistance is needed to more effectively monitor, manage, and counter pest resistance to Bt toxins. Here we used CRISPR/Cas9 gene editing to clarify the genetics of Bt resistance and the associated effects on susceptibility to other microbial insecticides in one of the world's most damaging pests, the cotton bollworm (Helicoverpa armigera). We discovered that CRISPR-mediated knockouts of ATP-binding cassette (ABC) transporter genes HaABCC2 and HaABCC3 together caused >15,000-fold resistance to Bt toxin Cry1Ac, whereas knocking out either HaABCC2 or HaABCC3 alone had little or no effect. Inheritance of resistance was autosomal and recessive. Bioassays of progeny from interstrain crosses revealed that one wild type allele of either HaABCC2 or HaABCC3 is sufficient to sustain substantial susceptibility to Cry1Ac. In contrast with previous results, susceptibility to two insecticides derived from bacteria other than Bt (abamectin and spinetoram), was not affected by knocking out HaABCC2, HaABCC3, or both. The results here provide the first evidence that either HaABCC2 or HaABCC3 protein is sufficient to confer substantial susceptibility to Cry1Ac. The functional redundancy of these two proteins in toxicity of Cry1Ac to H. armigera is expected to reduce the likelihood of field-evolved resistance relative to disruption of a toxic process where mutations affecting a single protein can confer resistance.
• Wang, L., Ma, Y., Wei, W., Wan, P., Liu, K., Xu, M., Cong, S., Wang, J., Xu, D., Xiao, Y., Li, X., Tabashnik, B. E., & Wu, K. (2020). Cadherin repeat 5 mutation associated with Bt resistance in a field-derived strain of pink bollworm. Scientific reports, 10(1), 16840.
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  Evolution of resistance by pests reduces the benefits of transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Here we analyzed resistance to Bt toxin Cry1Ac in a field-derived strain of pink bollworm (Pectinophora gossypiella), a global pest of cotton. We discovered that the r14 allele of the pink bollworm cadherin gene (PgCad1) has a 234-bp insertion in exon 12 encoding a mutant PgCad1 protein that lacks 36 amino acids in cadherin repeat 5 (CR5). A strain homozygous for this allele had 237-fold resistance to Cry1Ac, 1.8-fold cross-resistance to Cry2Ab, and developed from neonate to adult on Bt cotton producing Cry1Ac. Inheritance of resistance to Cry1Ac was recessive and tightly linked with r14. PgCad1 transcript abundance in midgut tissues did not differ between resistant and susceptible larvae. Toxicity of Cry1Ac to transformed insect cells was lower for cells expressing r14 than for cells expressing wild-type PgCad1. Wild-type PgCad1 was transported to the cell membrane, whereas PgCad1 produced by r14 was not. In larval midgut tissue, PgCad1 protein occurred primarily on the brush border membrane only in susceptible larvae. The results imply r14 mediates pink bollworm resistance to Cry1Ac by reduced translation, increased degradation, and/or mislocalization of cadherin.
• Zhao, S., Jiang, D., Wang, F., Yang, Y., Tabashnik, B. E., & Wu, Y. (2020). Independent and Synergistic Effects of Knocking out Two ABC Transporter Genes on Resistance to Toxins Cry1Ac and Cry1Fa in Diamondback Moth. Toxins, 13(1).
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  Insecticidal proteins from (Bt) are used widely in sprays and transgenic crops to control insect pests. However, evolution of resistance by pests can reduce the efficacy of Bt toxins. Here we analyzed resistance to Bt toxins Cry1Ac and Cry1Fa in the diamondback moth (), one of the world's most destructive pests of vegetable crops. We used CRISPR/Cas9 gene editing to create strains with knockouts of the ATP-binding cassette (ABC) transporter genes , , or both. Bioassay results show that knocking out either gene alone caused at most 2.9-fold resistance but knocking out both caused >10,320-fold resistance to Cry1Ac and 380-fold resistance to Cry1Fa. Cry1Ac resistance in the double knockout strain was recessive and genetically linked with the loci. The results provide insight into the mechanism of cross-resistance to Cry1Fa in diamondback moth. They also confirm previous work with this pest showing that mutations disrupting both genes cause higher resistance to Cry1Ac than mutations affecting either or alone. Together with previous work, the results here highlight the value of using single and multiple gene knockouts to better understand the independent and synergistic effects of putative Bt toxin receptors on resistance to Bt toxins.
• Carriere et al., Y., & Tabashnik, B. E. (2019). Gossypol in cottonseed increases the fitness cost of resistance to Bt cotton in pink bollworm.. Crop Protection, 126. doi:https://www.sciencedirect.com/science/article/pii/S0261219419302601
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  Fitness costs of resistance to Bacillus thuringiensis (Bt) toxins can delay or counter the evolution of insect resistance to transgenic Bt crops. Here we tested the hypothesis that the plant defensive compound gossypol in cottonseed increases costs associated with resistance to Bt toxin Cry1Ac in pink bollworm (Pectinophora gossypiella), a cosmopolitan pest that feeds primarily on cottonseed. Previous work showed pink bollworm resistance to Cry1Ac produced by Bt cotton is associated with mutations disrupting a gene encoding a cadherin protein that binds Cry1Ac in susceptible larvae. We used larvae from two strains of pink bollworm, each harboring an intermediate frequency of a different cadherin allele linked with resistance. We tested larvae from both strains on two types of non-Bt cotton that differed nine-fold in the gossypol concentration in their seeds: a transgenic cultivar engineered for low gossypol production and its untransformed parental cultivar. After 10 days of larval feeding on bolls, the resistance allele frequency was significantly lower in larvae from the conventional cotton than transgenic cotton. These results imply the higher gossypol concentration in the conventional cottonseed increased the fitness cost affecting survival. Using a population genetics model, we estimated this increase in survival cost was at least 32%. We also detected a recessive fitness cost reducing larval weight in both strains of pink bollworm that did not differ between the two cotton cultivars. Designing insecticidal crops and refuge plants that exploit the vulnerability of resistant insects to plant defensive compounds could improve resistance management.
• Carrière, Y., Degain, B., Unnithan, G. C., Harpold, V. S., Li, X., & Tabashnik, B. E. (2019). Seasonal Declines in Cry1Ac and Cry2Ab Concentration in Maturing Cotton Favor Faster Evolution of Resistance to Pyramided Bt Cotton in Helicoverpa zea (Lepidoptera: Noctuidae). Journal of economic entomology, 112(6), 2907-2914.
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  Under ideal conditions, widely adopted transgenic crop pyramids producing two or more distinct insecticidal proteins from Bacillus thuringiensis (Bt) that kill the same pest can substantially delay evolution of resistance by pests. However, deviations from ideal conditions diminish the advantages of such pyramids. Here, we tested the hypothesis that changes in maturing cotton producing Cry1Ac and Cry2Ab affect evolution of resistance in Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), a pest with low inherent susceptibility to both toxins. In terminal leaves of field-grown Bt cotton, the concentration of both toxins was significantly higher for young, squaring plants than for old, fruiting plants. We used laboratory bioassays with plant material from field-grown cotton to test H. zea larvae from a strain selected for resistance to Cry1Ac in the laboratory, its more susceptible parent strain, and their F1 progeny. On young Bt cotton, no individuals survived to pupation. On old Bt cotton, survival to pupation was significantly higher for the lab-selected strain and the F1 progeny relative to the unselected parent strain, indicating dominant inheritance of resistance. Redundant killing, the extent to which insects resistant to one toxin are killed by another toxin in a pyramid, was complete on young Bt cotton, but not on old Bt cotton. No significant fitness costs associated with resistance were detected on young or old non-Bt cotton. Incorporation of empirical data into simulations indicates the observed increased selection for resistance on old Bt cotton could accelerate evolution of resistance to cotton producing Cry1Ac and Cry2Ab in H. zea.
• Fabrick, J. A., Mathew, L. G., LeRoy, D. M., Hull, J. J., Unnithan, G. C., Yelich, A. J., Carrière, Y., Li, X., & Tabashnik, B. E. (2019). Reduced cadherin expression associated with resistance to Bt toxin Cry1Ac in pink bollworm. Pest management science, 76(1), 67-74.
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  Better understanding of the molecular basis of resistance is needed to improve management of pest resistance to transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Here we analyzed resistance of the pink bollworm (Pectinophora gossypiella) to Bt toxin Cry1Ac, which is used widely in transgenic Bt cotton. Field-evolved practical resistance of pink bollworm to Cry1Ac is widespread in India, but not in China or the United States. Previous work with laboratory- and field-selected pink bollworm indicated that resistance to Cry1Ac is caused by changes in the amino acid sequence of a midgut cadherin protein (PgCad1) that binds Cry1Ac in susceptible larvae.
• Li, S., Hussain, F., Unnithan, G. C., Dong, S., UlAbdin, Z., Gu, S., Mathew, L. G., Fabrick, J. A., Ni, X., Carrière, Y., Tabashnik, B. E., & Li, X. (2019). A long non-coding RNA regulates cadherin transcription and susceptibility to Bt toxin Cry1Ac in pink bollworm, Pectinophora gossypiella. Pesticide biochemistry and physiology, 158, 54-60.
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  Extensive planting of transgenic crops producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has spurred increasingly rapid evolution of resistance in pests. In the pink bollworm, Pectinophora gossypiella, a devastating global pest, resistance to Bt toxin Cry1Ac produced by transgenic cotton is linked with mutations in a gene (PgCad1) encoding a cadherin protein that binds Cry1Ac in the larval midgut. We previously reported a long non-coding RNA (lncRNA) in intron 20 of cadherin alleles associated with both resistance and susceptibility to Cry1Ac. Here we tested the hypothesis that reducing expression of this lncRNA decreases transcription of PgCad1 and susceptibility to Cry1Ac. Quantitative RT-PCR showed that feeding susceptible neonates small interfering RNAs (siRNAs) targeting this lncRNA but not PgCad1 decreased the abundance of transcripts of both the lncRNA and PgCad1. Moreover, neonates fed the siRNAs had lower susceptibility to Cry1Ac. The results imply that the lncRNA increases transcription of PgCad1 and susceptibility of pink bollworm to Cry1Ac. The results suggest that disruption of lncRNA expression could be a novel mechanism of pest resistance to Bt toxins.
• Tabashnik, B. E., & Carrière, Y. (2019). Evaluating Cross-resistance Between Vip and Cry Toxins of Bacillus thuringiensis. Journal of economic entomology.
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  Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have revolutionized control of some major pests. Some recently introduced Bt crops make Vip3Aa, a vegetative insecticidal protein (Vip), which reportedly does not share binding sites or structural homology with the crystalline (Cry) proteins of Bt used widely in transgenic crops for more than two decades. Field-evolved resistance to Bt crops with practical consequences for pest control includes 21 cases that collectively reduce the efficacy of nine Cry proteins, but such practical resistance has not been reported yet for any Vip. Here, we review previously published data to evaluate cross-resistance between Vip and Cry toxins. We analyzed 31 cases based on 48 observations, with each case based on one to five observations assessing cross-resistance from pairwise comparisons between 21 resistant strains and 13 related susceptible strains of eight species of lepidopteran pests. Confirming results from previous analyses of smaller data sets, we found weak, statistically significant cross-resistance between Vip3 and Cry1 toxins, with a mean of 1.5-fold cross-resistance in 21 cases (range: 0.30-4.6-fold). Conversely, we did not detect significant positive cross-resistance between Vip3 toxins and Cry2Ab. Distinguishing between weak, significant cross-resistance, and no cross-resistance may be useful for better understanding mechanisms of resistance and effectively managing pest resistance to Bt crops.
• Tabashnik, B. E., & Carrière, Y. (2019). Global Patterns of Resistance to Bt Crops Highlighting Pink Bollworm in the United States, China, and India. Journal of economic entomology, 112(6), 2513-2523.
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  Crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt) have advanced pest control, but their benefits have been reduced by evolution of resistance in pests. The global monitoring data reviewed here reveal 19 cases of practical resistance to Bt crops, which is field-evolved resistance that reduces Bt crop efficacy and has practical consequences for pest control. Each case represents the responses of one pest species in one country to one Bt toxin. The results with pink bollworm (Pectinophora gossypiella) and Bt cotton differ strikingly among the world's three leading cotton-producing nations. In the southwestern United States, farmers delayed resistance by planting non-Bt cotton refuges from 1996 to 2005, then cooperated in a program that used Bt cotton, mass releases of sterile moths, and other tactics to eradicate this pest from the region. In China, farmers reversed low levels of pink bollworm resistance to Bt cotton by planting second-generation hybrid seeds from crosses between Bt and non-Bt cotton. This approach yields a refuge of 25% non-Bt cotton plants randomly interspersed within fields of Bt cotton. Farmers adopted this tactic voluntarily and unknowingly, not to manage resistance, but apparently because of its perceived short-term agronomic and economic benefits. In India, where non-Bt cotton refuges have been scarce and pink bollworm resistance to pyramided Bt cotton producing Cry1Ac and Cry2Ab toxins is widespread, integrated pest management emphasizing shortening of the cotton season, destruction of crop residues, and other tactics is now essential.
• Tabashnik, B. E., Li, X., Carrière, Y., Yelich, A. J., Unnithan, G. C., Hull, J. J., LeRoy, D. M., Mathew, L. G., & Fabrick, J. A. (2019). Reduced cadherin expression associated with resistance to Bt toxin Cry1Ac in pink bollworm.. Pest Manag Sci..
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  BACKGROUND:Better understanding of the molecular basis of resistance is needed to improve management of pest resistance to transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Here we analyzed resistance of the pink bollworm (Pectinophora gossypiella) to Bt toxin Cry1Ac, which is used widely in transgenic Bt cotton. Field-evolved practical resistance of pink bollworm to Cry1Ac is widespread in India, but not in China or the United States. Previous work with laboratory- and field-selected pink bollworm indicated that resistance to Cry1Ac is caused by changes in the amino acid sequence of a midgut cadherin protein (PgCad1) that binds Cry1Ac in susceptible larvae.RESULTS:Relative to a susceptible strain, the laboratory-selected APHIS-R strain had 530-fold resistance to Cry1Ac with autosomal recessive inheritance. Unlike previous results, resistance in this strain was not consistently associated with insertions or deletions in the expected amino acid sequence of PgCad1. However, this resistance was associated with 79- to 190-fold reduced transcription of the PgCad1 gene and markedly lower abundance of PgCad1 protein.CONCLUSION:The ability of pink bollworm and other major pests to evolve resistance to Bt toxins via both qualitative and quantitative changes in receptor proteins demonstrates their remarkable adaptability and presents challenges for monitoring and managing resistance to Bt crops.
• Wang, J., Xu, D., Wang, L., Cong, S., Wan, P., Lei, C., Fabrick, J. A., Li, X., Tabashnik, B. E., & Wu, K. (2019). Bt resistance alleles in field populations of pink bollworm from China: Similarities with the United States and decreased frequency from 2012 to 2015. Pest management science.
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  Although most monitoring of pest resistance to widely cultivated transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) relies on bioassays, DNA screening for alleles associated with resistance has some advantages, particularly for rare, recessively inherited resistance. In China's Yangtze River Valley, where farmers first planted transgenic cotton producing Bt toxin Cry1Ac in 2000, bioassays have been used to monitor the recessive resistance of pink bollworm (Pectinophora gossypiella). Previous bioassay results show a small but significant increase in resistance to Cry1Ac during 2008-2010, followed by a significant decrease in resistance during 2011-2015 associated with extensive planting of second-generation hybrid cotton seeds that boosted the percentage of non-Bt cotton. Here we screened DNA from 19 748 pink bollworm collected during 2012-2015 from the Yangtze River Valley for seven alleles associated with resistance to Cry1Ac. These alleles were previously identified from lab-selected strains; three from the U.S. and four from China.
• Wang, L., Ma, Y., Guo, X., Wan, P., Liu, K., Cong, S., Wang, J., Xu, D., Xiao, Y., Li, X., Tabashnik, B. E., & Wu, K. (2019). Pink Bollworm Resistance to Bt Toxin Cry1Ac Associated with an Insertion in Cadherin Exon 20. Toxins, 11(4).
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  Insecticidal proteins from (Bt) are widely used to control insect pests, but their efficacy is reduced when pests evolve resistance. We report on a novel allele () of the cadherin gene () in pink bollworm () associated with resistance to Bt toxin Cry1Ac, which is produced by transgenic cotton. The allele isolated from a field population in China has 1545 base pairs of a degenerate transposon inserted in exon 20 of , which generates a mis-spliced transcript containing a premature stop codon. A strain homozygous for had 300-fold resistance to Cry1Ac, 2.6-fold cross-resistance to Cry2Ab, and completed its life cycle on transgenic Bt cotton producing Cry1Ac. Inheritance of Cry1Ac resistance was recessive and tightly linked with . Compared with transfected insect cells expressing wild-type , cells expressing were less susceptible to Cry1Ac. Recombinant cadherin protein was transported to the cell membrane in cells transfected with the wild-type allele, but not in cells transfected with . Cadherin occurred on brush border membrane vesicles (BBMVs) in the midgut of susceptible larvae, but not resistant larvae. These results imply that the allele mediates Cry1Ac resistance in pink bollworm by interfering with the localization of cadherin.
• Wang, L., Wang, J., Ma, Y., Wan, P., Liu, K., Cong, S., Xiao, Y., Xu, D., Wu, K., Fabrick, J. A., Li, X., & Tabashnik, B. E. (2019). Transposon insertion causes cadherin mis-splicing and confers resistance to Bt cotton in pink bollworm from China. Scientific reports, 9(1), 7479.
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  Transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) are cultivated extensively, but rapid evolution of resistance by pests reduces their efficacy. We report a 3,370-bp insertion in a cadherin gene associated with resistance to Bt toxin Cry1Ac in the pink bollworm (Pectinophora gossypiella), a devastating global cotton pest. We found the allele (r15) harboring this insertion in a field population from China. The insertion is a miniature inverted repeat transposable element (MITE) that contains two additional transposons and produces two mis-spliced transcript variants (r15A and r15B). A strain homozygous for r15 had 290-fold resistance to Cry1Ac, little or no cross-resistance to Cry2Ab, and completed its life cycle on Bt cotton producing Cry1Ac. Inheritance of resistance was recessive and tightly linked with r15. For transformed insect cells, susceptibility to Cry1Ac was greater for cells producing the wild-type cadherin than for cells producing the r15 mutant proteins. Recombinant cadherin protein occurred on the cell surface in cells transformed with the wild-type or r15A sequences, but not in cells transformed with the r15B sequence. The similar resistance of pink bollworm to Cry1Ac in laboratory- and field-selected insects from China, India and the U.S. provides a basis for developing international resistance management practices.
• Wang, Y., Quan, Y., Yang, J., Shu, C., Wang, Z., Zhang, J., Gatehouse, A. M., Tabashnik, B. E., & He, K. (2019). Evolution of Asian Corn Borer Resistance to Bt Toxins Used Singly or in Pairs. Toxins, 11(8).
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  Transgenic crops producing insecticidal proteins from (Bt) have revolutionized pest control, but the benefits of this approach have been reduced by the evolution of resistance in pests. The widely adopted 'pyramid strategy' for delaying resistance entails transgenic crops producing two or more distinct toxins that kill the same pest. The limited experimental evidence supporting this strategy comes primarily from a model system under ideal conditions. Here we tested the pyramid strategy under nearly worst-case conditions, including some cross-resistance between the toxins in the pyramid. In a laboratory selection experiment with an artificial diet, we used Bt toxins Cry1Ab, Cry1F, and Cry1Ie singly or in pairs against , one of the most destructive pests of corn in Asia. Under the conditions evaluated, pairs of toxins did not consistently delay the evolution of resistance relative to single toxins.
• Wu, K., Tabashnik, B. E., Li, X., Fabrick, J. A., Lei, C., Wan, P., Cong, S., Xu, D., & Wang, J. (2019). Bt resistance alleles in field populations of pink bollworm from China: Similarities with the United States and decreased frequency from 2012 to 2015.. Pest Manag Sci..
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  BACKGROUND:Although most monitoring of pest resistance to widely cultivated transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) relies on bioassays, DNA screening for alleles associated with resistance has some advantages, particularly for rare, recessively inherited resistance. In China's Yangtze River Valley, where farmers first planted transgenic cotton producing Bt toxin Cry1Ac in 2000, bioassays have been used to monitor the recessive resistance of pink bollworm (Pectinophora gossypiella). Previous bioassay results show a small but significant increase in resistance to Cry1Ac during 2008-2010, followed by a significant decrease in resistance during 2011-2015 associated with extensive planting of second-generation hybrid cotton seeds that boosted the percentage of non-Bt cotton. Here we screened DNA from 19 748 pink bollworm collected during 2012-2015 from the Yangtze River Valley for seven alleles associated with resistance to Cry1Ac. These alleles were previously identified from lab-selected strains; three from the U.S. and four from China.RESULTS:The most common resistance allele was first identified from the U.S. and accounted for over 71% of all resistance alleles detected. Resistance was rare, with the total frequency of the seven resistance alleles showing a significant, 2.3-fold decrease from 0.0105 (95% CI: 0.0084-0.0132) in 2012 to 0.0046 (0.0031-0.0067) in 2015.CONCLUSIONS:The DNA screening data confirm results from bioassays showing pink bollworm resistance to Cry1Ac remained rare in the Yangtze River Valley from 2012-2015. The prevalence in China of the resistance allele identified from the U.S. implies a shared genetic basis of resistance that could facilitate molecular monitoring of resistance.
• Carriere, Y., Degain, B. A., Unnithan, G. C., Harpold, V. S., Li, X., & Tabashnik, B. E. (2018). Effects of seasonal changes in cotton plants on evolution of resistance to pyramided Cry1Ac + Cry1F cotton by Helicoverpa zea.. Pest Management Science, 74, 627-637.
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  BACKGROUND: In pests with inherently low susceptibility to Bacillus thuringiensis (Bt) toxins, seasonal declines in the concentration of Bt toxins in transgenic crops could accelerate evolution of resistance by increasing the dominance of resistance. Here, we evaluated Helicoverpa zea survival on young and old cotton plants that produced Cry1Ac + Cry1F or did not produce Bt toxins. RESULTS: Using a strain selected for resistance to Cry1Ac in the laboratory, its parent strain that was not selected in the laboratory, and their F1 progeny, we show that resistance to Cry1Ac + Cry1F cotton was partially dominant on young and old plants. On Cry1Ac + Cry1F cotton, redundant killing was incomplete on young plants but nearly complete on old plants. No significant fitness costs on non-Bt cotton occurred on young plants, but large recessive costs affected survival on old plants. Simulations models incorporating the empirical data show that the seasonal changes in fitness could delay resistance to Cry1Ac + Cry1F cotton by inducing low equilibrium frequencies of resistance alleles when refuges are sufficiently large. CONCLUSION: Our results suggest that including effects of seasonal changes in fitness of pests on Bt crops and refuge plants can enhance resistance risk assessment and resistance management.
• Carrière, Y., Williams, J. L., Crowder, D. W., & Tabashnik, B. E. (2018). Genotype-specific fitness cost of resistance to Bt toxin Cry1Ac in pink bollworm. Pest management science, 74(11), 2496-2503.
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  To improve resistance management strategies for Bacillus thuringiensis (Bt) crops, a better understanding of the relative fitness of pest genotypes with resistance alleles in the absence of Bt toxins is needed. Here, we evaluated the impact of costs of resistance to Bt toxin Cry1Ac on the relative fitness of specific pink bollworm (Pectinophora gossypiella) genotypes. We created two heterogeneous strains with an intermediate frequency of mutant cadherin alleles linked with resistance to Cry1Ac, reared the strains on diet without Bt and tracked the decline in frequency of resistant genotypes for 15-30 generations using polymerase chain reaction amplification. We used a population genetics model and sensitivity analyses to estimate the relative fitness of resistant genotypes.
• Gao, M., Wang, X., Yang, Y., Tabashnik, B. E., & Wu, Y. (2018). Epistasis confers resistance to Bt toxin Cry1Ac in the cotton bollworm. Evolutionary applications, 11(5), 809-819.
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  Evolution of resistance by insect pests reduces the benefits of extensively cultivated transgenic crops that produce insecticidal proteins from (Bt). Previous work showed that resistance to Bt toxin Cry1Ac, which is produced by transgenic cotton, can be conferred by mutations disrupting a cadherin protein that binds this Bt toxin in the larval midgut. However, the potential for epistatic interactions between the cadherin gene and other genes has received little attention. Here, we report evidence of epistasis conferring resistance to Cry1Ac in the cotton bollworm, , one of the world's most devastating crop pests. Resistance to Cry1Ac in strain LF256 originated from a field-captured male and was autosomal, recessive, and 220-fold relative to susceptible strain SCD. We conducted complementation tests for allelism by crossing LF256 with a strain in which resistance to Cry1Ac is conferred by a recessive allele at the cadherin locus . The resulting F offspring were resistant, suggesting that resistance to Cry1Ac in LF256 is also conferred by resistance alleles at this locus. However, the amino acid sequence in LF256 lacked insertions and deletions, and did not differ consistently between LF256 and a susceptible strain. In addition, most of the cadherin alleles in LF256 were not derived from the field-captured male. Moreover, Cry1Ac resistance was not genetically linked with the locus in LF256. Furthermore, LF256 and the susceptible strain were similar in levels of transcript, cadherin protein, and binding of Cry1Ac to cadherin. Overall, the results imply that epistasis between and an unknown second locus in LF256 yielded the observed resistance in the F progeny from the complementation test. The observed epistasis has important implications for interpreting results of the F screen used widely to monitor and analyze resistance, as well as the potential to accelerate evolution of resistance.
• Jin, L., Wang, J., Guan, F., Zhang, J., Yu, S., Liu, S., Xue, Y., Li, L., Wu, S., Wang, X., Yang, Y., Abdelgaffar, H., Jurat-Fuentes, J. L., Tabashnik, B. E., & Wu, Y. (2018). Dominant point mutation in a tetraspanin gene associated with field-evolved resistance of cotton bollworm to transgenic Bt cotton. Proceedings of the National Academy of Sciences of the United States of America, 115(46), 11760-11765.
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  Extensive planting of crops genetically engineered to produce insecticidal proteins from the bacterium (Bt) has suppressed some major pests, reduced insecticide sprays, enhanced pest control by natural enemies, and increased grower profits. However, rapid evolution of resistance in pests is reducing these benefits. Better understanding of the genetic basis of resistance to Bt crops is urgently needed to monitor, delay, and counter pest resistance. We discovered that a point mutation in a previously unknown tetraspanin gene in the cotton bollworm (), a devastating global pest, confers dominant resistance to Cry1Ac, the sole Bt protein produced by transgenic cotton planted in China. We found the mutation using a genome-wide association study, followed by fine-scale genetic mapping and DNA sequence comparisons between resistant and susceptible strains. CRISPR/Cas9 knockout of the tetraspanin gene restored susceptibility to a resistant strain, whereas inserting the mutation conferred 125-fold resistance in a susceptible strain. DNA screening of moths captured from 23 field sites in six provinces of northern China revealed a 100-fold increase in the frequency of this mutation, from 0.001 in 2006 to 0.10 in 2016. The correspondence between the observed trajectory of the mutation and the trajectory predicted from simulation modeling shows that the dominance of the mutation accelerated adaptation. Proactive identification and tracking of the tetraspanin mutation demonstrate the potential for genomic analysis, gene editing, and molecular monitoring to improve management of resistance.
• Mathew, L. G., Ponnuraj, J., Mallappa, B., Chowdary, L. R., Zhang, J., Tay, W. T., Walsh, T. K., Gordon, K. H., Heckel, D. G., Downes, S., Carrière, Y., Li, X., Tabashnik, B. E., & Fabrick, J. A. (2018). ABC transporter mis-splicing associated with resistance to Bt toxin Cry2Ab in laboratory- and field-selected pink bollworm. Scientific reports, 8(1), 13531.
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  Evolution of pest resistance threatens the benefits of genetically engineered crops that produce Bacillus thuringiensis (Bt) insecticidal proteins. Strategies intended to delay pest resistance are most effective when implemented proactively. Accordingly, researchers have selected for and analyzed resistance to Bt toxins in many laboratory strains of pests before resistance evolves in the field, but the utility of this approach depends on the largely untested assumption that laboratory- and field-selected resistance to Bt toxins are similar. Here we compared the genetic basis of resistance to Bt toxin Cry2Ab, which is widely deployed in transgenic crops, between laboratory- and field-selected populations of the pink bollworm (Pectinophora gossypiella), a global pest of cotton. We discovered that resistance to Cry2Ab is associated with mutations disrupting the same ATP-binding cassette transporter gene (PgABCA2) in a laboratory-selected strain from Arizona, USA, and in field-selected populations from India. The most common mutation, loss of exon 6 caused by alternative splicing, occurred in resistant larvae from both locations. Together with previous data, the results imply that mutations in the same gene confer Bt resistance in laboratory- and field-selected strains and suggest that focusing on ABCA2 genes may help to accelerate progress in monitoring and managing resistance to Cry2Ab.
• Peña-Cardeña, A., Grande, R., Sánchez, J., Tabashnik, B. E., Bravo, A., Soberón, M., & Gómez, I. (2018). The C-terminal protoxin domain of Bacillus thuringiensis Cry1Ab toxin has a functional role in binding to GPI-anchored receptors in the insect midgut. The Journal of biological chemistry.
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  Bacillus thuringiensis (Bt) Cry toxins are used worldwide for controlling insects. Cry1Ab is produced as a 130 kDa protoxin that is activated by proteolytic removal of an inert 500 amino-acids-long C-terminal region, enabling the activated toxin to bind to insect midgut receptor proteins and leading to its membrane insertion and pore formation. It has been proposed that the C-terminal region is only involved in toxin crystallization, but its role in receptor binding is undefined. Here we show that the C-terminal region of Cry1Ab protoxin provides additional binding sites to alkaline phosphatase (ALP) and aminopeptidase N (APN) insect receptors. ELISA, ligand blot, SPR and pull-down assays revealed that the Cry1Ab C-terminal region binds to both ALP and APN, but not to cadherin (CAD). Thus, the C-terminal region provides both higher binding affinity of the protoxin to the gut membrane that correlated with higher toxicity of protoxin than activated toxin. Moreover, Cry1Ab domain II loop 2 or 3 mutations reduced binding of the protoxin to CAD but not to ALP or APN, supporting the notion that protoxins have additional binding sites. These results imply that two different regions mediate the binding of Cry1Ab protoxin to membrane receptors, one located in domain II-III of the toxin and another in its C-terminal region, suggesting an active role of the C-terminal protoxin fragment in the mode of action of Cry toxins. These results suggest that future manipulations of the C-terminal protoxin region could alter specificity and increase toxicity of Bt proteins.
• Wang, L., Ma, Y., Wan, P., Liu, K., Xiao, Y., Wang, J., Cong, S., Xu, D., Wu, K., Fabrick, J. A., Li, X., & Tabashnik, B. E. (2018). Resistance to Bacillus thuringiensis linked with a cadherin transmembrane mutation affecting cellular trafficking in pink bollworm from China. Insect biochemistry and molecular biology, 94, 28-35.
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  Evolution of pest resistance reduces the efficacy of insecticidal proteins from the gram-positive bacterium Bacillus thuringiensis (Bt) used widely in sprays and transgenic crops. In some previously studied strains of three major lepidopteran pests, resistance to Bt toxin Cry1Ac is associated with mutations disrupting the extracellular or cytoplasmic domains of cadherin proteins that bind Cry1Ac in the midgut of susceptible larvae. Here we report the first case of a cadherin transmembrane mutation associated with insect resistance to Bt. We discovered this mutation in a strain of the devastating global cotton pest, the pink bollworm (Pectinophora gossypiella), derived from a field population in the Yangtze River Valley of China. The mutant allele analyzed here has a 207 base pair deletion and encodes a cadherin protein lacking its transmembrane domain. Relative to a susceptible strain, a strain homozygous for this allele had 220-fold resistance to Cry1Ac and 2.1-fold cross-resistance to Cry2Ab. On transgenic cotton plants producing Cry1Ac, no susceptible larvae survived, but the resistant strain completed its life cycle. Inheritance of resistance to Cry1Ac was autosomal, recessive and tightly linked with the cadherin gene. Transportation of cadherin protein to the cell membrane and susceptibility to Cry1Ac occurred in transfected insect cells expressing the wild type cadherin allele, but not in transfected insect cells expressing the mutant cadherin allele. The results imply that the mutant allele analyzed here confers resistance to Cry1Ac by disrupting cellular trafficking of cadherin.
• Zhang, M., Wei, J., Ni, X., Zhang, J., Jurat-Fuentes, J. L., Fabrick, J. A., Carrière, Y., Tabashnik, B. E., & Li, X. (2018). Decreased Cry1Ac activation by midgut proteases associated with Cry1Ac resistance in Helicoverpa zea. Pest management science.
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  Field-evolved resistance of Helicoverpa zea to Bacillus thuringiensis (Bt) toxin Cry1Ac was first reported more than a decade ago, yet the underlying mechanisms remain elusive. Towards understanding the mechanisms of resistance to Cry1Ac, we analyzed a susceptible (LAB-S) and two resistant (GA and GA-R) strains of H. zea. The GA strain was derived from Georgia and exposed to Bt toxins only in the field. The GA-R strain was derived from the GA strain and selected for increased resistance to Cry1Ac in the laboratory.
• Carrière, Y., Antilla, L., Liesner, L., & Tabashnik, B. E. (2017). Large-Scale Evaluation of Association Between Pheromone Trap Captures and Cotton Boll Infestation for Pink Bollworm (Lepidoptera: Gelechiidae). Journal of economic entomology, 110(3), 1345-1350.
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  Although transgenic cotton producing insecticidal proteins from Bacillus thuringiensis (Bt) is a cornerstone for pink bollworm control in some countries, integrated pest management remains important for bolstering sustainability of Bt cotton and is critical for controlling pink bollworm where Bt cotton is not available or where this pest has evolved resistance to Bt cotton. Here, we used data on moth captures in gossyplure-baited pheromone traps and boll infestations for 163 Bt and 152 non-Bt cotton fields from Arizona to evaluate accuracy of chemical control decisions relying on moth trapping data and capacity of Bt cotton to suppress survival of offspring produced by moths. Assuming an economic injury level of 12% boll infestation, the accuracy of decisions based on moth captures corresponding to economic thresholds of 6%, 8%, and 10% boll infestation increased from 44.7% to 67.1%. The association between moth captures and boll infestation was positive and significant for non-Bt cotton fields but was not significant for Bt cotton fields. Although chemical control decisions based on trapping data were only moderately accurate, pheromone traps could still be valuable for determining when moth populations are high enough to trigger boll sampling to more rigorously evaluate the need for insecticide sprays.
• Carrière, Y., Degain, B. A., Unnithan, G. C., Harpold, V. S., Heuberger, S., Li, X., & Tabashnik, B. E. (2017). Effects of seasonal changes in cotton plants on the evolution of resistance to pyramided cotton producing the Bt toxins Cry1Ac and Cry1F in Helicoverpa zea. Pest management science.
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  In pests with inherently low susceptibility to Bacillus thuringiensis (Bt) toxins, seasonal declines in the concentration of Bt toxins in transgenic crops could accelerate evolution of resistance by increasing the dominance of resistance. Here, we evaluated Helicoverpa zea survival on young and old cotton plants that produced the Bt toxins Cry1Ac and Cry1F or did not produce Bt toxins.
• Ni, M., Ma, W., Wang, X., Gao, M., Dai, Y., Wei, X., Zhang, L., Peng, Y., Chen, S., Ding, L., Tian, Y., Li, J., Wang, H., Wang, X., Xu, G., Guo, W., Yang, Y., Wu, Y., Heuberger, S., , Tabashnik, B. E., et al. (2017). Next-generation transgenic cotton: pyramiding RNAi and Bt counters insect resistance. Plant biotechnology journal, 15(9), 1204-1213.
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  Transgenic crops producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) are extensively cultivated worldwide. To counter rapidly increasing pest resistance to crops that produce single Bt toxins, transgenic plant 'pyramids' producing two or more Bt toxins that kill the same pest have been widely adopted. However, cross-resistance and antagonism between Bt toxins limit the sustainability of this approach. Here we describe development and testing of the first pyramids of cotton combining protection from a Bt toxin and RNA interference (RNAi). We developed two types of transgenic cotton plants producing double-stranded RNA (dsRNA) from the global lepidopteran pest Helicoverpa armigera designed to interfere with its metabolism of juvenile hormone (JH). We focused on suppression of JH acid methyltransferase (JHAMT), which is crucial for JH synthesis, and JH-binding protein (JHBP), which transports JH to organs. In 2015 and 2016, we tested larvae from a Bt-resistant strain and a related susceptible strain of H. armigera on seven types of cotton: two controls, Bt cotton, two types of RNAi cotton (targeting JHAMT or JHBP) and two pyramids (Bt cotton plus each type of RNAi). Both types of RNAi cotton were effective against Bt-resistant insects. Bt cotton and RNAi acted independently against the susceptible strain. In computer simulations of conditions in northern China, where millions of farmers grow Bt cotton as well as abundant non-transgenic host plants of H. armigera, pyramided cotton combining a Bt toxin and RNAi substantially delayed resistance relative to using Bt cotton alone.
• Ocelotl, J., Sánchez, J., Gómez, I., Tabashnik, B. E., Bravo, A., & Soberón, M. (2017). ABCC2 is associated with Bacillus thuringiensis Cry1Ac toxin oligomerization and membrane insertion in diamondback moth. Scientific reports, 7(1), 2386.
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  Cry1A insecticidal toxins bind sequentially to different larval gut proteins facilitating oligomerization, membrane insertion and pore formation. Cry1Ac interaction with cadherin triggers oligomerization. However, a mutation in an ABC transporter gene (ABCC2) is linked to Cry1Ac resistance in Plutella xylostella. Cry1AcMod, engineered to lack helix α-1, was able to form oligomers without cadherinbinding and effectively countered Cry1Ac resistance linked to ABCC2. Here we analyzed Cry1Ac and Cry1AcMod binding and oligomerization by western blots using brush border membrane vesicles (BBMV) from a strain of P. xylostella susceptible to Cry1Ac (Geneva 88) and a strain with resistance to Cry1Ac (NO-QAGE) linked to an ABCC2 mutation. Resistance correlated with lack of specific binding and reduced oligomerization of Cry1Ac in BBMV from NO-QAGE. In contrast, Cry1AcMod bound specifically and still formed oligomers in BBMV from both strains. We compared association of pre-formed Cry1Ac oligomer, obtained by incubating Cry1Ac toxin with a Manduca sexta cadherin fragment, with BBMV from both strains. Our results show that pre-formed oligomers associate more efficiently with BBMV from Geneva 88 than with BBMV from NO-QAGE, indicating that the ABCC2 mutation also affects the association of Cry1Ac oligomer with the membrane. These data indicate, for the first time, that ABCC2 facilitates Cry1Ac oligomerization and oligomer membrane insertion in P. xylostella.
• Tabashnik, B. E., & Carrière, Y. (2017). Surge in insect resistance to transgenic crops and prospects for sustainability. Nature biotechnology, 35(10), 926-935.
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  Transgenic crops have revolutionized insect pest control, but their effectiveness has been reduced by evolution of resistance in pests. We analyzed global monitoring data reported during the first two decades of transgenic crops, with each case representing the responses of one pest species in one country to one insecticidal protein from Bacillus thuringiensis (Bt). The cases of pest resistance to Bt crystalline (Cry) proteins produced by transgenic crops increased from 3 in 2005 to 16 in 2016. By contrast, in 17 other cases there was no decrease in pest susceptibility to Bt crops, including the recently introduced transgenic corn that produces a Bt vegetative insecticidal protein (Vip). Recessive inheritance of pest resistance has favored sustained susceptibility, but even when inheritance is not recessive, abundant refuges of non-Bt host plants have substantially delayed resistance. These insights may inform resistance management strategies to increase the durability of current and future transgenic crops.
• Wan, P., Xu, D., Cong, S., Jiang, Y., Huang, Y., Wang, J., Wu, H., Wang, L., Wu, K., Carrière, Y., Mathias, A., Li, X., & Tabashnik, B. E. (2017). Hybridizing transgenic Bt cotton with non-Bt cotton counters resistance in pink bollworm. Proceedings of the National Academy of Sciences of the United States of America, 114(21), 5413-5418.
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  Extensive cultivation of crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) has suppressed some major pests, reduced insecticide sprays, enhanced pest control by natural enemies, and increased grower profits. However, these benefits are being eroded by evolution of resistance in pests. We report a strategy for combating resistance by crossing transgenic Bt plants with conventional non-Bt plants and then crossing the resulting first-generation (F1) hybrid progeny and sowing the second-generation (F2) seeds. This strategy yields a random mixture within fields of three-quarters of plants that produce Bt toxin and one-quarter that does not. We hypothesized that the non-Bt plants in this mixture promote survival of susceptible insects, thereby delaying evolution of resistance. To test this hypothesis, we compared predictions from computer modeling with data monitoring pink bollworm (Pectinophora gossypiella) resistance to Bt toxin Cry1Ac produced by transgenic cotton in an 11-y study at 17 field sites in six provinces of China. The frequency of resistant individuals in the field increased before this strategy was widely deployed and then declined after its widespread adoption boosted the percentage of non-Bt cotton plants in the region. The correspondence between the predicted and observed outcomes implies that this strategy countered evolution of resistance. Despite the increased percentage of non-Bt cotton, suppression of pink bollworm was sustained. Unlike other resistance management tactics that require regulatory intervention, growers adopted this strategy voluntarily, apparently because of advantages that may include better performance as well as lower costs for seeds and insecticides.
• Carrière, Y., Fabrick, J. A., & Tabashnik, B. E. (2016). Can Pyramids and Seed Mixtures Delay Resistance to Bt Crops?. Trends in biotechnology, 34(4), 291-302.
• Tabashnik, B. E. (2016). Tips for battling billion-dollar beetles. Science (New York, N.Y.), 354(6312), 552-553.
• Tassone, E. E., Zastrow-Hayes, G., Mathis, J., Nelson, M. E., Wu, G., Flexner, J. L., Carrière, Y., Tabashnik, B. E., & Fabrick, J. A. (2016). Sequencing, de novo assembly and annotation of a pink bollworm larval midgut transcriptome. GigaScience, 5, 28.
• Wang, L., Wan, P., Cong, S., Wang, J., Huang, M., Tabashnik, B. E., Li, X., & Wu, K. (2016). Adult Exposure to Bt Toxin Cry1Ac Reduces Life Span and Reproduction of Resistant and Susceptible Pink Bollworm (Lepidoptera: Gelechiidae). Journal of economic entomology, 109, 1367-1373.
• Wei, J., Liang, G., Wang, B., Zhong, F., Chen, L., Khaing, M. M., Zhang, J., Guo, Y., Wu, K., & Tabashnik, B. E. (2016). Activation of Bt Protoxin Cry1Ac in Resistant and Susceptible Cotton Bollworm. PloS one, 11(6), e0156560.
• Xiao, Y., Liu, K., Zhang, D., Gong, L., He, F., Tabashnik, B. E., & Wu, K. (2016). Resistance to Bacillus thuringiensis Mediated by an ABC Transporter Mutation Increases Susceptibility to Toxins from Other Bacteria in an Invasive Insect. PLoS pathogens, 12(2), e1005450.
• Brévault, T., Tabashnik, B. E., & Carrière, Y. (2015). A seed mixture increases dominance of resistance to Bt cotton in Helicoverpa zea. Scientific reports, 5, 9807.
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  Widely grown transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) can benefit agriculture, but adaptation by pests threatens their continued success. Refuges of host plants that do not make Bt toxins can promote survival of susceptible insects and delay evolution of resistance, particularly if resistance is inherited as a recessive trait. However, data have been lacking to compare the dominance of resistance when Bt and non-Bt seeds are planted in random mixtures versus separate blocks. Here we report results from greenhouse experiments with transgenic cotton producing Bt toxin Cry1Ac and the bollworm, Helicoverpa zea, showing that the dominance of resistance was significantly higher in a seed mixture relative to a block of Bt cotton. The proportion of larvae on non-Bt cotton plants in the seed mixture was also significantly higher than expected under the null hypothesis of random distribution. In simulations based on observed survival, resistance evolved 2- to 4.5-fold faster in the seed mixture relative to separate blocks of Bt and non-Bt cotton. These findings support previous modelling results indicating that block refuges may be more effective than seed mixtures for delaying resistance in pests with mobile larvae and inherently low susceptibility to the toxins in Bt crops.
• Carrière, Y., Crickmore, N., & Tabashnik, B. E. (2015). Optimizing pyramided transgenic Bt crops for sustainable pest management. Nature biotechnology, 33(2), 161-8.
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  Transgenic crop pyramids producing two or more Bacillus thuringiensis (Bt) toxins that kill the same insect pest have been widely used to delay evolution of pest resistance. To assess the potential of pyramids to achieve this goal, we analyze data from 38 studies that report effects of ten Bt toxins used in transgenic crops against 15 insect pests. We find that compared with optimal low levels of insect survival, survival on currently used pyramids is often higher for both susceptible insects and insects resistant to one of the toxins in the pyramid. Furthermore, we find that cross-resistance and antagonism between toxins used in pyramids are common, and that these problems are associated with the similarity of the amino acid sequences of domains II and III of the toxins, respectively. This analysis should assist in future pyramid design and the development of sustainable resistance management strategies.
• Chen, W., Liu, C., Xiao, Y., Zhang, D., Zhang, Y., Li, X., Tabashnik, B. E., & Wu, K. (2015). A toxin-binding alkaline phosphatase fragment synergizes Bt toxin Cry1Ac against susceptible and resistant Helicoverpa armigera. PloS one, 10(4), e0126288.
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  Evolution of resistance by insects threatens the continued success of pest control using insecticidal crystal (Cry) proteins from the bacterium Bacillus thuringiensis (Bt) in sprays and transgenic plants. In this study, laboratory selection with Cry1Ac yielded five strains of cotton bollworm, Helicoverpa armigera, with resistance ratios at the median lethal concentration (LC50) of activated Cry1Ac ranging from 22 to 1700. Reduced activity and reduced transcription of an alkaline phosphatase protein that binds Cry1Ac was associated with resistance to Cry1Ac in the four most resistant strains. A Cry1Ac-binding fragment of alkaline phosphatase from H. armigera (HaALP1f) was not toxic by itself, but it increased mortality caused by Cry1Ac in a susceptible strain and in all five resistant strains. Although synergism of Bt toxins against susceptible insects by toxin-binding fragments of cadherin and aminopeptidase N has been reported previously, the results here provide the first evidence of synergism of a Bt toxin by a toxin-binding fragment of alkaline phosphatase. The results here also provide the first evidence of synergism of a Bt toxin by any toxin-binding peptide against resistant insects.
• Fabrick, J. A., Unnithan, G. C., Yelich, A. J., DeGain, B., Masson, L., Zhang, J., Carrière, Y., & Tabashnik, B. E. (2015). Multi-Toxin Resistance Enables Pink Bollworm Survival on Pyramided Bt Cotton. Scientific reports, 5, 16554.
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  Transgenic crops producing Bacillus thuringiensis (Bt) proteins kill key insect pests, providing economic and environmental benefits. However, the evolution of pest resistance threatens the continued success of such Bt crops. To delay or counter resistance, transgenic plant "pyramids" producing two or more Bt proteins that kill the same pest have been adopted extensively. Field populations of the pink bollworm (Pectinophora gossypiella) in the United States have remained susceptible to Bt toxins Cry1Ac and Cry2Ab, but field-evolved practical resistance to Bt cotton producing Cry1Ac has occurred widely in India. Here we used two rounds of laboratory selection to achieve 18,000- to 150,000-fold resistance to Cry2Ab in pink bollworm. Inheritance of resistance to Cry2Ab was recessive, autosomal, conferred primarily by one locus, and independent of Cry1Ac resistance. We created a strain with high resistance to both toxins by crossing the Cry2Ab-resistant strain with a Cry1Ac-resistant strain, followed by one selection with Cry2Ab. This multi-toxin resistant strain survived on field-collected Bt cotton bolls producing both toxins. The results here demonstrate the risk of evolution of resistance to pyramided Bt plants, particularly when toxins are deployed sequentially and refuges are scarce, as seen with Bt cotton and pink bollworm in India.
• Jin, L., Zhang, H., Lu, Y., Yang, Y., Wu, K., Tabashnik, B. E., & Wu, Y. (2015). Large-scale test of the natural refuge strategy for delaying insect resistance to transgenic Bt crops. Nature biotechnology, 33(2), 169-74.
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  The 'natural refuge strategy" for delaying insect resistance to transgenic cotton that produces insecticidal proteins from Bacillus thuringiensis (Bt) relies on refuges of host plants other than cotton that do not make Bt toxins. We tested this widely adopted strategy by comparing predictions from modeling with data from a four-year field study of cotton bollworm (Helicoverpa armigera) resistance to transgenic cotton producing Bt toxin Cry1Ac in six provinces of northern China. Bioassay data revealed that the percentage of resistant insects increased from 0.93% in 2010 to 5.5% in 2013. Modeling predicted that the percentage of resistant insects would exceed 98% in 2013 without natural refuges, but would increase to only 1.1% if natural refuges were as effective as non-Bt cotton refuges. Therefore, the results imply that natural refuges delayed resistance, but were not as effective as an equivalent area of non-Bt cotton refuges. The percentage of resistant insects with nonrecessive inheritance of resistance increased from 37% in 2010 to 84% in 2013. Switching to Bt cotton producing two or more toxins and integrating other control tactics could slow further increases in resistance.
• Ocelotl, J., Sánchez, J., Arroyo, R., García-Gómez, B. I., Gómez, I., Unnithan, G. C., Tabashnik, B. E., Bravo, A., & Soberón, M. (2015). Binding and Oligomerization of Modified and Native Bt Toxins in Resistant and Susceptible Pink Bollworm (20%). PloS one, 10(12), e0144086.
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  Insecticidal proteins from Bacillus thuringiensis (Bt) are used extensively in sprays and transgenic crops for pest control, but their efficacy is reduced when pests evolve resistance. Better understanding of the mode of action of Bt toxins and the mechanisms of insect resistance is needed to enhance the durability of these important alternatives to conventional insecticides. Mode of action models agree that binding of Bt toxins to midgut proteins such as cadherin is essential for toxicity, but some details remain unresolved, such as the role of toxin oligomers. In this study, we evaluated how Bt toxin Cry1Ac and its genetically engineered counterpart Cry1AcMod interact with brush border membrane vesicles (BBMV) from resistant and susceptible larvae of Pectinophora gossypiella (pink bollworm), a global pest of cotton. Compared with Cry1Ac, Cry1AcMod lacks 56 amino acids at the amino-terminus including helix α-1; previous work showed that Cry1AcMod formed oligomers in vitro without cadherin and killed P. gossypiella larvae harboring cadherin mutations linked with >1000-fold resistance to Cry1Ac. Here we found that resistance to Cry1Ac was associated with reduced oligomer formation and insertion. In contrast, Cry1AcMod formed oligomers in BBMV from resistant larvae. These results confirm the role of cadherin in oligomerization of Cry1Ac in susceptible larvae and imply that forming oligomers without cadherin promotes toxicity of Cry1AcMod against resistant P. gossypiella larvae that have cadherin mutations.
• Orpet, R. J., Degain, B. A., Tabashnik, B. E., & Carriere, Y. (2015). Effects of dietary protein to carbohydrate ratio on Bt toxicity and fitness costs of resistance in Helicoverpa zea.. Entomol. Exp. Appl., 156, 28-36.
• Orpet, R. J., Degain, B. A., Tabashnik, B. E., & Carrière, Y. (2015). Balancing Bt Toxin Avoidance and Nutrient Intake by Helicoverpa zea (Lepidoptera: Noctuidae) Larvae. Journal of economic entomology, 108(6), 2581-8.
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  To evaluate how the Cry1Ac Bacillus thuringiensis (Bt) toxin and the ratio of dietary protein to carbohydrate (P:C) independently and jointly affect Helicoverpa zea (Boddie) larval feeding performance and behavior, we conducted no-choice and binary choice experiments using chemically defined diets. We tested two related strains of this polyphagous pest: a strain selected for resistance to Cry1Ac in the laboratory (GA-R), and its field-derived parent strain (GA). In no-choice tests, feeding performance in GA and GA-R was superior on a non-Cry1Ac 80P:20C diet compared to a non-Cry1Ac 35P:65C diet or an 80P:20C diet containing Cry1Ac. Based on consumption and position measurements in binary choice tests, larvae preferred the non-Cry1Ac 80P:20C diet over the other two diets mentioned above. However, the association between preference and performance was weaker when comparing the Cry1Ac 80P:20C diet versus a Cry1Ac 35P:65C diet, and when comparing the Cry1Ac 80P:20C diet versus the non-Cry1Ac 35P:65C diet. In all choice situations, consumption preference occurred mainly by the percentage of larvae that fed almost entirely from one or the other diet, rather than from variation in the extent of diet mixing by individuals. Resistance to Cry1Ac affected the balance between toxin avoidance and nutrient intake: larvae from the more resistant GA-R strain consumed significantly more of the Cry1Ac 80P:20C diet when paired with non-Cry1Ac 35P:65C diet, while GA larvae consumed more of the latter, though not significantly. The results show that dietary P:C ratio, Cry1Ac, and resistance to Cry1Ac affected feeding behavior and performance of H. zea larvae.
• Tabashnik, B. E. (2015). ABCs of Insect Resistance to Bt. PLoS genetics, 11(11), e1005646.
• Tabashnik, B. E., Zhang, M., Fabrick, J. A., Wu, Y., Gao, M., Huang, F., Wei, J., Zhang, J., Yelich, A., Unnithan, G. C., Bravo, A., Soberón, M., Carrière, Y., & Li, X. (2015). Dual mode of action of Bt proteins: protoxin efficacy against resistant insects. Scientific reports, 5, 15107.
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  Transgenic crops that produce Bacillus thuringiensis (Bt) proteins for pest control are grown extensively, but insect adaptation can reduce their effectiveness. Established mode of action models assert that Bt proteins Cry1Ab and Cry1Ac are produced as inactive protoxins that require conversion to a smaller activated form to exert toxicity. However, contrary to this widely accepted paradigm, we report evidence from seven resistant strains of three major crop pests showing that Cry1Ab and Cry1Ac protoxins were generally more potent than the corresponding activated toxins. Moreover, resistance was higher to activated toxins than protoxins in eight of nine cases evaluated in this study. These data and previously reported results support a new model in which protoxins and activated toxins kill insects via different pathways. Recognizing that protoxins can be more potent than activated toxins against resistant insects may help to enhance and sustain the efficacy of transgenic Bt crops.
• Wei, J., Guo, Y., Liang, G., Wu, K., Zhang, J., Tabashnik, B. E., & Li, X. (2015). Cross-resistance and interactions between Bt toxins Cry1Ac and Cry2Ab against the cotton bollworm. Scientific reports, 5, 7714.
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  To delay evolution of pest resistance to transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt), the "pyramid" strategy uses plants that produce two or more toxins that kill the same pest. We conducted laboratory diet experiments with the cotton bollworm, Helicoverpa armigera, to evaluate cross-resistance and interactions between two toxins in pyramided Bt cotton (Cry1Ac and Cry2Ab). Selection with Cry1Ac for 125 generations produced 1000-fold resistance to Cry1Ac and 6.8-fold cross-resistance to Cry2Ab. Selection with Cry2Ab for 29 generations caused 5.6-fold resistance to Cry2Ab and 61-fold cross-resistance to Cry1Ac. Without exposure to Bt toxins, resistance to both toxins decreased. For each of the four resistant strains examined, 67 to 100% of the combinations of Cry1Ac and Cry2Ab tested yielded higher than expected mortality, reflecting synergism between these two toxins. Results showing minor cross-resistance to Cry2Ab caused by selection with Cry1Ac and synergism between these two toxins against resistant insects suggest that plants producing both toxins could prolong the efficacy of Bt cotton against this pest in China. Including toxins against which no cross-resistance occurs and integrating Bt cotton with other control tactics could also increase the sustainability of management strategies.
• Welch, K. L., Unnithan, G. C., Degain, B. A., Wei, J., Zhang, J., Li, X., Tabashnik, B. E., & Carrière, Y. (2015). Cross-resistance to toxins used in pyramided Bt crops and resistance to Bt sprays in Helicoverpa zea. Journal of invertebrate pathology, 132, 149-56.
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  To delay evolution of resistance by insect pests, farmers are rapidly increasing their use of transgenic crops producing two or more Bacillus thuringiensis (Bt) toxins that kill the same pest. A key condition favoring durability of these "pyramided" crops is the absence of cross-resistance between toxins. Here we evaluated cross-resistance in the major lepidopteran pest Helicoverpa zea (Boddie) to Bt toxins used in pyramids. In the laboratory, we selected a strain of this pest with Bt toxin Cry1Ac followed by selection with MVP II, a formulation containing a hybrid protoxin that is identical to Cry1Ac in the active portion of the toxin and 98.5% identical overall. We calculated the resistance ratio as the EC50 (concentration causing mortality or failure to develop beyond the first instar of 50% of larvae) for the laboratory-selected strain divided by the EC50 for its field-derived parent strain that was not selected in the laboratory. The resistance ratio was 20.0-33.9 (mean=27.0) for MVP II, 57.0 for Cry1Ac, 51.3 for Cry1A.105, 22.4 for Cry1Ab, 3.3 for Cry2Ab, 1.8 for Cry1Fa, and 1.6 for Vip3Aa. Resistance ratios were 2.9 for DiPel ES and 2.0 for Agree VG, which are commercial Bt spray formulations containing Cry1Ac, other Bt toxins, and Bt spores. By the conservative criterion of non-overlap of 95% fiducial limits, the EC50 was significantly higher for the selected strain than its parent strain for MVP II, Cry1Ac, Cry1A.105, Cry1Ab, Cry2Ab and DiPel ES. For Cry1Fa, Vip3Aa, and Agree VG, significantly lower susceptibility to a high concentration indicated low cross-resistance. The resistance ratio for toxins other than Cry1Ac was associated with their amino acid sequence similarity to Cry1Ac in domain II. Resistance to Cry1Ac and the observed cross-resistance to other Bt toxins could accelerate evolution of H. zea resistance to currently registered Bt sprays and pyramided Bt crops.
• Carroll, S. P., Jørgensen, P. S., Kinnison, M. T., Bergstrom, C. T., Denison, R. F., Gluckman, P., Smith, T. B., Strauss, S. Y., & Tabashnik, B. E. (2014). Applying evolutionary biology to address global challenges. Science (New York, N.Y.), 346(6207), doi:10.1126/science.1245993 (20%, last author).
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  Two categories of evolutionary challenges result from escalating human impacts on the planet. The first arises from cancers, pathogens, and pests that evolve too quickly and the second, from the inability of many valued species to adapt quickly enough. Applied evolutionary biology provides a suite of strategies to address these global challenges that threaten human health, food security, and biodiversity. This Review highlights both progress and gaps in genetic, developmental, and environmental manipulations across the life sciences that either target the rate and direction of evolution or reduce the mismatch between organisms and human-altered environments. Increased development and application of these underused tools will be vital in meeting current and future targets for sustainable development.
• Fabrick, J. A., Ponnuraj, J., Singh, A., Tanwar, R. K., Unnithan, G. C., Yelich, A. J., Li, X., Carrière, Y., & Tabashnik, B. E. (2014). Alternative splicing and highly variable cadherin transcripts associated with field-evolved resistance of pink bollworm to Bt cotton in India. PloS one, 9(5), e97900 (30%, last author).
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  Evolution of resistance by insect pests can reduce the benefits of insecticidal proteins from Bacillus thuringiensis (Bt) that are used extensively in sprays and transgenic crops. Despite considerable knowledge of the genes conferring insect resistance to Bt toxins in laboratory-selected strains and in field populations exposed to Bt sprays, understanding of the genetic basis of field-evolved resistance to Bt crops remains limited. In particular, previous work has not identified the genes conferring resistance in any cases where field-evolved resistance has reduced the efficacy of a Bt crop. Here we report that mutations in a gene encoding a cadherin protein that binds Bt toxin Cry1Ac are associated with field-evolved resistance of pink bollworm (Pectinophora gossypiella) in India to Cry1Ac produced by transgenic cotton. We conducted laboratory bioassays that confirmed previously reported resistance to Cry1Ac in pink bollworm from the state of Gujarat, where Bt cotton producing Cry1Ac has been grown extensively. Analysis of DNA from 436 pink bollworm from seven populations in India detected none of the four cadherin resistance alleles previously reported to be linked with resistance to Cry1Ac in laboratory-selected strains of pink bollworm from Arizona. However, DNA sequencing of pink bollworm derived from resistant and susceptible field populations in India revealed eight novel, severely disrupted cadherin alleles associated with resistance to Cry1Ac. For these eight alleles, analysis of complementary DNA (cDNA) revealed a total of 19 transcript isoforms, each containing a premature stop codon, a deletion of at least 99 base pairs, or both. Seven of the eight disrupted alleles each produced two or more different transcript isoforms, which implicates alternative splicing of messenger RNA (mRNA). This represents the first example of alternative splicing associated with field-evolved resistance that reduced the efficacy of a Bt crop.
• Jin, L., Zhang, H., Lu, Y., Yang, Y., Wu, K., Tabashnik, B. E., & Wu, Y. (2014). Large-scale test of the natural refuge strategy for delaying insect resistance to transgenic Bt crops. Nature Biotechnology, doi:10.1038/nbt.3100 (30%).
• Liu, C., Xiao, Y., Li, X., Oppert, B., Tabashnik, B. E., & Wu, K. (2014). Cis-mediated down-regulation of a trypsin gene associated with Bt resistance in cotton bollworm. Scientific reports, 4, 7219.
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  Transgenic plants producing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) are useful for pest control, but their efficacy is reduced when pests evolve resistance. Here we examined the mechanism of resistance to Bt toxin Cry1Ac in the laboratory-selected LF5 strain of the cotton bollworm, Helicoverpa armigera. This strain had 110-fold resistance to Cry1Ac protoxin and 39-fold resistance to Cry1Ac activated toxin. Evaluation of five trypsin genes revealed 99% reduced transcription of one trypsin gene (HaTryR) was associated with resistance. Silencing of this gene with RNA interference in susceptible larvae increased their survival on diets containing Cry1Ac. Bioassays of progeny from crosses revealed that resistance to Cry1Ac was genetically linked with HaTryR. We identified mutations in the promoter region of HaTryR in the resistant strain. In transfected insect cell lines, transcription was lower when driven by the resistant promoter compared with the susceptible promoter, implicating cis-mediated down-regulation of HaTryR transcription as a mechanism of resistance. The results suggest that H. armigera can adapt to Bt toxin Cry1Ac by decreased expression of trypsin. Because trypsin activation of protoxin is a critical step in toxicity, transgenic plants with activated toxins rather than protoxins might increase the durability of Bt crops.
• Tabashnik, B. E., Mota-Sanchez, D., Whalon, M. E., Hollingworth, R. M., & Carrière, Y. (2014). Defining terms for proactive management of resistance to Bt crops and pesticides. Journal of economic entomology, 107(2), 496-507.
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  Evolution of pest resistance to pesticides is an urgent global problem with resistance recorded in at least 954 species of pests, including 546 arthropods, 218 weeds, and 190 plant pathogens. To facilitate understanding and management of resistance, we provide definitions of 50 key terms related to resistance. We confirm the broad, long-standing definition of resistance, which is a genetically based decrease in susceptibility to a pesticide, and the definition of "field-evolved resistance," which is a genetically based decrease in susceptibility to a pesticide in a population caused by exposure to the pesticide in the field. The impact of field-evolved resistance on pest control can vary from none to severe. We define "practical resistance" as field-evolved resistance that reduces pesticide efficacy and has practical consequences for pest control. Recognizing that resistance is not "all or none" and that intermediate levels of resistance can have a continuum of effects on pest control, we describe five categories of field-evolved resistance and use them to classify 13 cases of field-evolved resistance to five Bacillus thuringiensis (Bt) toxins in transgenic corn and cotton based on monitoring data from five continents for nine major pest species. We urge researchers to publish and analyze their resistance monitoring data in conjunction with data on management practices to accelerate progress in determining which actions will be most useful in response to specific data on the magnitude, distribution, and impact of resistance.
• Tabashnik, B. E., Mota-Sanchez, D., Whalon, M., Hollingworth, R., & Carriere, Y. (2014). Defining terms for proactive management of resistance to Bt crops and pesticides. Journal of Economic Entomology, 496-507 (85%).
• Xiao, Y., Zhang, T., Liu, C., Heckel, D. G., Li, X., Tabashnik, B. E., & Wu, K. (2014). Mis-splicing of the ABCC2 gene linked with Bt toxin resistance in Helicoverpa armigera. Scientific reports, 4, 6184.
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  Toxins from the bacterium Bacillus thuringiensis (Bt) are used widely for insect control in sprays and transgenic plants, but their efficacy is reduced when pests evolve resistance. Previous work showed that mutations in a gene encoding the transporter protein ABCC2 are linked with resistance to Bt toxins Cry1Ab, Cry1Ac or both in four species of Lepidoptera. Here we compared the ABCC2 gene of Helicoverpa armigera (HaABCC2) between susceptible strains and a laboratory-selected strain with >1,000-fold resistance to Cry1Ac relative its susceptible parent strain. We discovered a 73-base pair (bp) insertion in the cDNA of the resistant strain that generates a premature stop codon expected to yield a truncated ABCC2 protein. Sequencing of genomic DNA revealed that this insertion is an intron that is not spliced out because of a 6-bp deletion at its splicing site. Analysis of progeny from crosses revealed tight genetic linkage between HaABCC2 and resistance to Cry1Ac. These results provide the first evidence that mis-splicing of a gene encoding an ABCC2 protein confers resistance to a Bt toxin.
• Xiao, Y., Zhang, T., Liu, C., Heckel, D. G., Li, X., Tabashnik, B. E., & Wu, K. (2014). Mis-splicing of the ABCC2 gene linked with Bt toxin resistance in Helicoverpa armigera.. Scientific Reports, 4(Article number: 6184), doi:10.1038/srep06184 (20%).
• Brévault, T., Heuberger, S., Zhang, M., Ellers-Kirk, C., Ni, X., Masson, L., Li, X., Tabashnik, B. E., & Carrière, Y. (2013). Potential shortfall of pyramided transgenic cotton for insect resistance management. Proceedings of the National Academy of Sciences of the United States of America, 110(15).
• Jin, L., Wei, Y., Zhang, L., Yang, Y., Tabashnik, B. E., & Wu, Y. (2013). Dominant resistance to Bt cotton and minor cross-resistance to Bt toxin Cry2Ab in cotton bollworm from China. Evolutionary applications, 6(8).
• Jin, L., Wei, Y., Zhang, L., Yang, Y., Tabashnik, B. E., & Yidong, W. u. (2013). Dominant resistance to Bt cotton and minor cross-resistance to Bt toxin Cry2Ab in cotton bollworm from China. Evolutionary Applications, 6(8), 1222-1235.
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  Abstract: Evolution of resistance by insect pests threatens the long-term benefits of transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt). Previous work has detected increases in the frequency of resistance to Bt toxin Cry1Ac in populations of cotton bollworm, Helicoverpa armigera, from northern China where Bt cotton producing Cry1Ac has been grown extensively for more than a decade. Confirming that trend, we report evidence from 2011 showing that the percentage of individuals resistant to a diagnostic concentration of Cry1Ac was significantly higher in two populations from different provinces of northern China (1.4% and 2.3%) compared with previously tested susceptible field populations (0%). We isolated two resistant strains: one from each of the two field-selected populations. Relative to a susceptible strain, the two strains had 460- and 1200-fold resistance to Cry1Ac, respectively. Both strains had dominant resistance to a diagnostic concentration of Cry1Ac in diet and to Bt cotton leaves containing Cry1Ac. Both strains had low, but significant cross-resistance to Cry2Ab (4.2- and 5.9-fold), which is used widely as the second toxin in two-toxin Bt cotton. Compared with resistance in other strains of H. armigera, the resistance in the two strains characterized here may be especially difficult to suppress. © 2013 The Authors. Evolutionary Applications published by John Wiley & Sons Ltd.
• Tabashnik, B. E., Brévault, T., & Carrière, Y. (2013). Insect resistance to Bt crops: Lessons from the first billion acres. Nature Biotechnology, 31(6), 510-521.
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  PMID: 23752438;Abstract: Evolution of resistance in pests can reduce the effectiveness of insecticidal proteins from Bacillus thuringiensis (Bt) produced by transgenic crops. We analyzed results of 77 studies from five continents reporting field monitoring data for resistance to Bt crops, empirical evaluation of factors affecting resistance or both. Although most pest populations remained susceptible, reduced efficacy of Bt crops caused by field-evolved resistance has been reported now for some populations of 5 of 13 major pest species examined, compared with resistant populations of only one pest species in 2005. Field outcomes support theoretical predictions that factors delaying resistance include recessive inheritance of resistance, low initial frequency of resistance alleles, abundant refuges of non-Bt host plants and two-toxin Bt crops deployed separately from one-toxin Bt crops. The results imply that proactive evaluation of the inheritance and initial frequency of resistance are useful for predicting the risk of resistance and improving strategies to sustain the effectiveness of Bt crops. © 2013 Nature America, Inc. All rights reserved.
• Tabashnik, B. E., Brévault, T., & Carrière, Y. (2013). Insect resistance to Bt crops: lessons from the first billion acres. Nature biotechnology, 31(6).
• Tabashnik, B. E., Fabrick, J. A., Unnithan, G. C., Yelich, A. J., Masson, L., Zhang, J., Bravo, A., & Soberón, M. (2013). Efficacy of genetically modified Bt toxins alone and in combinations against pink bollworm resistant to Cry1Ac and Cry2Ab. PLoS ONE, 8(11).
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  Abstract: Evolution of resistance in pests threatens the long-term efficacy of insecticidal proteins from Bacillus thuringiensis (Bt) used in sprays and transgenic crops. Previous work showed that genetically modified Bt toxins Cry1AbMod and Cry1AcMod effectively countered resistance to native Bt toxins Cry1Ab and Cry1Ac in some pests, including pink bollworm ( Pectinophora gossypiella ). Here we report that Cry1AbMod and Cry1AcMod were also effective against a laboratory-selected strain of pink bollworm resistant to Cry2Ab as well as to Cry1Ab and Cry1Ac. Resistance ratios based on the concentration of toxin killing 50% of larvae for the resistant strain relative to a susceptible strain were 210 for Cry2Ab, 270 for Cry1Ab, and 310 for Cry1Ac, but only 1.6 for Cry1AbMod and 2.1 for Cry1AcMod. To evaluate the interactions among toxins, we tested combinations of Cry1AbMod, Cry1Ac, and Cry2Ab. For both the resistant and susceptible strains, the net results across all concentrations tested showed slight but significant synergism between Cry1AbMod and Cry2Ab, whereas the other combinations of toxins did not show consistent synergism or antagonism. The results suggest that the modified toxins might be useful for controlling populations of pink bollworm resistant to Cry1Ac, Cry2Ab, or both. © 2013 Tabashnik et al.
• Tabashnik, B. E., Fabrick, J. A., Unnithan, G. C., Yelich, A. J., Masson, L., Zhang, J., Bravo, A., & Soberón, M. (2013). Efficacy of genetically modified Bt toxins alone and in combinations against pink bollworm resistant to Cry1Ac and Cry2Ab. PloS one, 8(11).
• , Y., Ellers-Kirk, C., Harthfield, K., Larocque, G., Degain, B., Dutilleul, P., Dennehy, T., Marsh, S., Crowder, D., Li, X., Ellsworth, P., Naranjo, S., Palumbo, J., Fournier, A., Antilla, L., & Tabashnik, B. (2012). Large-scale spatially explicit test of the refuge strategy for delaying insecticide resistance. Proc Natl Acad Sci USA, 109, 775-780.
• Crowder, D., Ellsworth, P., Naranjo, S., Tabashnik, B., & , Y. (2012). Modeling resistance to juvenile hormone analogs: Linking evolution ecology and management. Juvenile Hormones and Juvenoids: Modeling Biological Effects and Environmental Fate.
• Fabrick, J. A., & Tabashnik, B. E. (2012). Similar genetic basis of resistance to Bt toxin Cry1Ac in boll-selected and diet-selected strains of pink bollworm. PLoS ONE, 7(4).
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  PMID: 22530065;PMCID: PMC3329465;Abstract: Genetically engineered cotton and corn plants producing insecticidal Bacillus thuringiensis (Bt) toxins kill some key insect pests. Yet, evolution of resistance by pests threatens long-term insect control by these transgenic Bt crops. We compared the genetic basis of resistance to Bt toxin Cry1Ac in two independently derived, laboratory-selected strains of a major cotton pest, the pink bollworm (Pectinophora gossypiella [Saunders]). The Arizona pooled resistant strain (AZP-R) was started with pink bollworm from 10 field populations and selected with Cry1Ac in diet. The Bt4R resistant strain was started with a long-term susceptible laboratory strain and selected first with Bt cotton bolls and later with Cry1Ac in diet. Previous work showed that AZP-R had three recessive mutations (r1, r2, and r3) in the pink bollworm cadherin gene (PgCad1) linked with resistance to Cry1Ac and Bt cotton producing Cry1Ac. Here we report that inheritance of resistance to a diagnostic concentration of Cry1Ac was recessive in Bt4R. In interstrain complementation tests for allelism, F1 progeny from crosses between AZP-R and Bt4R were resistant to Cry1Ac, indicating a shared resistance locus in the two strains. Molecular analysis of the Bt4R cadherin gene identified a novel 15-bp deletion (r4) predicted to cause the loss of five amino acids upstream of the Cry1Ac-binding region of the cadherin protein. Four recessive mutations in PgCad1 are now implicated in resistance in five different strains, showing that mutations in cadherin are the primary mechanism of resistance to Cry1Ac in laboratory-selected strains of pink bollworm from Arizona.
• Fabrick, J., & Tabashnik, B. (2012). Similar genetic basis of resistance to Bt toxin Cry1Ac in boll-selected and diet-selected strains of pink bollworm. PLoS ONE, 7(4).
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  e35658
• Gassmann, A. J., Hannon, E. R., Sisterson, M. S., Stock, S. P., Carrire, Y., & Tabashnik, B. E. (2012). Effects of entomopathogenic nematodes on evolution of pink bollworm resistance to Bacillus thuringiensis Toxin Cry1Ac. Journal of Economic Entomology, 105(3), 994-1005.
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  PMID: 22812141;Abstract: The evolution of resistance by pests can reduce the efficacy of transgenic crops that produce insecticidal toxins from Bacillus thuringiensis (Bt). However, fitness costs may act to delay pest resistance to Bt toxins. Meta-analysis of results from four previous studies revealed that the entomopathogenic nematode Steinernema riobrave (Rhabditida: Steinernematidae) imposed a 20% fitness cost for larvae of pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), that were homozygous for resistance to Bt toxin Cry1Ac, but no significant fitness cost was detected for heterozygotes. We conducted greenhouse and laboratory selection experiments to determine whether S. riobrave would delay the evolution of pink bollworm resistance to Cry1Ac. We mimicked the high dose/refuge scenario in the greenhouse with Bt cotton (Gossypium hirsutum L.) plants and refuges of non-Bt cotton plants, and in the laboratory with diet containing Cry1Ac and refuges of untreated diet. In both experiments, half of the replicates were exposed to S. riobrave and half were not. In the greenhouse, S. riobrave did not delay resistance. In the laboratory, S. riobrave delayed resistance after two generations but not after four generations. Simulation modeling showed that an initial resistance allele frequency >0.015 and population bottlenecks can diminish or eliminate the resistance-delaying effects of fitness costs. We hypothesize that these factors may have reduced the resistance-delaying effects of S. riobrave in the selection experiments. The experimental and modeling results suggest that entomopathogenic nematodes could slow the evolution of pest resistance to Bt crops, but only under some conditions. © 2012 Entomological Society of America.
• Gassmann, A., Hannon, E., Sisterson, M., Stock, S., , Y., & Tabashnik, B. (2012). Effects of entomopathogenic nematodes on evolution of pink bollworm resistance to Bt toxin Cry1Ac. J Econ Entomol, 105, 994-1005.
• Meihls, L. N., Higdon, M. L., Ellersieck, M. R., Tabashnik, B. E., & Hibbard, B. E. (2012). Greenhouse-Selected Resistance to Cry3Bb1-Producing Corn in Three Western Corn Rootworm Populations. PLoS ONE, 7(12).
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  PMID: 23284656;PMCID: PMC3527414;Abstract: Transgenic corn producing the Bacillus thuringiensis (Bt) toxin Cry3Bb1 has been useful for controlling western corn rootworm, Diabrotica virgifera virgifera LeConte, one of the most economically important crop pests in the United States. However, rapid evolution of resistance by this beetle to Bt corn producing Cry3Bb1 has been reported previously from the laboratory, greenhouse, and field. Here we selected in the greenhouse for resistance to Cry3Bb1 corn in three colonies of WCR derived from Kansas, Minnesota, and Wisconsin, respectively. Three generations of rearing on Cry3Bb1 corn significantly increased larval survival on Cry3Bb1 corn, resulting in similar survival in the greenhouse for selected colonies on Cry3Bb1 corn and isoline corn that does not produce Bt toxin. After four to seven generations of rearing on Cry3Bb1 corn, survival in the field on Cry3Bb1 corn relative to isoline corn more than doubled for selected colonies (72%) compared with control colonies (33%). For both selected and control colonies, survival in the field was significantly lower on Cry3Bb1 corn than on isoline corn. On isoline corn, most fitness components were similar for selected colonies and control colonies. However, fecundity was significantly lower for selected colonies than control colonies, indicating a fitness cost associated with resistance. The rapid evolution of resistance by western corn rootworm to Bt corn reported here and previously underlines the importance of effective resistance management for this pest.
• Meihls, L., Higdon, M., Ellersieck, M., Tabashnik, B., & Hibbard, B. (2012). Greenhouse-selected resistance to Cry3Bb1-producing corn in three western corn rootworm populations. PLoS ONE, 12.
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  e51055
• Tabashnik, B. E., & Carrière, Y. (2012). Resistance to Transgenic Crops and Pest Outbreaks. Insect Outbreaks Revisited, 341-354.
• Tabashnik, B. E., & Gould, F. (2012). Delaying corn rootworm resistance to bt corn. Journal of Economic Entomology, 105(3), 767-776.
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  PMID: 22812111;Abstract: Transgenic crops producing Bacillus thuringiensis (Bt) toxins for insect control have been successful, but their efficacy is reduced when pests evolve resistance. To delay pest resistance to Bt crops, the U.S. Environmental Protection Agency (EPA) has required refuges of host plants that do not produce Bt toxins to promote survival of susceptible pests. Such refuges are expected to be most effective if the Bt plants deliver a dose of toxin high enough to kill nearly all hybrid progeny produced by matings between resistant and susceptible pests. In 2003, the EPA first registered corn, Zea mays L., producing a Bt toxin (Cry3Bb1) that kills western corn rootworm, Diabrotica virgifera virgifera LeConte, one of the most economically important crop pests in the United States. The EPA requires minimum refuges of 20% for Cry3Bb1 corn and 5% for corn producing two Bt toxins active against corn rootworms. We conclude that the current refuge requirements are not adequate, because Bt corn hybrids active against corn rootworms do not meet the high-dose standard, and western corn rootworm has rapidly evolved resistance to Cry3Bb1 corn in the laboratory, greenhouse, and field. Accordingly, we recommend increasing the minimum refuge for Bt corn targeting corn rootworms to 50% for plants producing one toxin active against these pests and to 20% for plants producing two toxins active against these pests. Increasing the minimum refuge percentage can help to delay pest resistance, encourage integrated pest management, and promote more sustainable crop protection. © 2012 Entomological Society of America.
• Tabashnik, B. E., Tabashnik, B. E., Kongming, W. u., Kongming, W. u., Yidong, W. u., & Yidong, W. u. (2012). Early detection of field-evolved resistance to Bt cotton in China: Cotton bollworm and pink bollworm. Journal of Invertebrate Pathology, 110(3), 301-306.
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  PMID: 22537835;Abstract: Transgenic crops producing . Bacillus thuringiensis (Bt) toxins kill some major insect pests, but pests can evolve resistance and thereby reduce the effectiveness of such Bt crops. The main approach for slowing pest adaptation to Bt crops uses non-Bt host plants as " refuges" to increase survival of susceptible pests. To delay evolution of pest resistance to cotton producing Bt toxin Cry1Ac, several countries have required refuges of non-Bt cotton, while farmers in China have relied on " natural" refuges of non-Bt host plants other than cotton. This strategy is designed for cotton bollworm (. Helicoverpa armigera), which attacks many crops and is the primary target of Bt cotton in China, but it does not apply to pink bollworm (. Pectinophora gossypiella), which feeds almost entirely on cotton in China. Here we review evidence of field-evolved resistance to Cry1Ac by cotton bollworm in northern China and by pink bollworm in the Yangtze River Valley of China. For both pests, results of laboratory diet bioassays reveal significantly decreased susceptibility of field populations to Cry1Ac, yet field control failures of Bt cotton have not been reported. The early detection of resistance summarized here may spur countermeasures such as planting Bt cotton that produces two or more distinct toxins, increased planting of non-Bt cotton, and integration of other management tactics together with Bt cotton. © 2012 Elsevier Inc..
• Tabashnik, B. E., Unnithan, G. C., Masson, L., Crowder, D. W., Li, X., & Carriere, Y. (2012). Asymmetrical cross-resistance between Bacillus thuringiensis toxins Cry1Ac and Cry2Ab in pink bollworm. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 106(29), 11889-11894.
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  Transgenic crops producing Bacillus thuringiensis (Bt) toxins kill some key insect pests and can reduce reliance on insecticide sprays. Sustainable use of such crops requires methods for delaying evolution of resistance by pests. To thwart pest resistance, some transgenic crops produce 2 different Bt toxins targeting the same pest. This "pyramid'' strategy is expected to work best when selection for resistance to 1 toxin does not cause cross-resistance to the other toxin. The most widely used pyramid is transgenic cotton producing Bt toxins Cry1Ac and Cry2Ab. Cross-resistance between these toxins was presumed unlikely because they bind to different larval midgut target sites. Previous results showed that laboratory selection with Cry1Ac caused little or no cross-resistance to Cry2A toxins in pink bollworm (Pectinophora gossypiella), a major cotton pest. We show here, however, that laboratory selection of pink bollworm with Cry2Ab caused up to 420-fold cross-resistance to Cry1Ac as well as 240-fold resistance to Cry2Ab. Inheritance of resistance to high concentrations of Cry2Ab was recessive. Larvae from a laboratory strain resistant to Cry1Ac and Cry2Ab in diet bioassays survived on cotton bolls producing only Cry1Ac, but not on cotton bolls producing both toxins. Thus, the asymmetrical cross-resistance seen here does not threaten the efficacy of pyramided Bt cotton against pink bollworm. Nonetheless, the results here and previous evidence indicate that cross-resistance occurs between Cry1Ac and Cry2Ab in some key cotton pests. Incorporating the potential effects of such cross-resistance in resistance management plans may help to sustain the efficacy of pyramided Bt crops.
• Tabashnik, B., & , Y. (2012). Resistance to transgenic crops and pest outbreaks. Insect Outbreaks Revisited, 341-254.
• Tabashnik, B., & Gould, F. (2012). Delaying corn rootworm resistance to Bt corn. J Econ Entomol, 105, 767-776.
• Tabashnik, B., Carrière, Y., Ellers-Kirk, C., Hartfield, K., Larocque, G., Degain, B., Dutilleul, P., Dennehy, T. J., Marsh, S. E., Crowder, D. W., Li, X., Ellsworth, P. C., Naranjo, S. E., Palumbo, J. C., Fournier, A., Antilla, L., & Tabashnik, B. E. (2012). Large-scale, spatially-explicit test of the refuge strategy for delaying insecticide resistance. Proceedings of the National Academy of Sciences of the United States of America, 109(3).
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  The refuge strategy is used worldwide to delay the evolution of pest resistance to insecticides that are either sprayed or produced by transgenic Bacillus thuringiensis (Bt) crops. This strategy is based on the idea that refuges of host plants where pests are not exposed to an insecticide promote survival of susceptible pests. Despite widespread adoption of this approach, large-scale tests of the refuge strategy have been problematic. Here we tested the refuge strategy with 8 y of data on refuges and resistance to the insecticide pyriproxyfen in 84 populations of the sweetpotato whitefly (Bemisia tabaci) from cotton fields in central Arizona. We found that spatial variation in resistance to pyriproxyfen within each year was not affected by refuges of melons or alfalfa near cotton fields. However, resistance was negatively associated with the area of cotton refuges and positively associated with the area of cotton treated with pyriproxyfen. A statistical model based on the first 4 y of data, incorporating the spatial distribution of cotton treated and not treated with pyriproxyfen, adequately predicted the spatial variation in resistance observed in the last 4 y of the study, confirming that cotton refuges delayed resistance and treated cotton fields accelerated resistance. By providing a systematic assessment of the effectiveness of refuges and the scale of their effects, the spatially explicit approach applied here could be useful for testing and improving the refuge strategy in other crop-pest systems.
• Tabashnik, B., Morin, S., Unnithan, G., Yelich, A., Ellers-Kirk, C., Harpold, V., Sisterson, M., Ellsworth, P., Dennehy, T., Antilla, L., Liesner, L., Whitlow, M., Staten, R., Fabrick, J., Li, X., & , Y. (2012). Sustained susceptibility of pink bollworm to Bt cotton in the United States. GM Crops and Food: Biotechnology in Agriculture and the Food Chain, 3(3), 194-200.
• Tabashnik, B., Wu, K., & Wu, Y. (2012). Early detection of field evolved resistance to Bt cotton in China: Cotton bollworm and pink bollworm. J Invert Pathol, 110, 301-306.
• Thomas, M. B., Charles, H., Read, A. F., van, H., Tabashnik, B. E., C., J., Waage, J. K., & Takken, W. (2012). Lessons from agriculture for the sustainable management of Malaria vectors. PLoS Medicine, 9(7).
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  PMID: 22802742;PMCID: PMC3393651;
• Thomas, M., Godfray, C., Read, A., van, d., Tabashnik, B., van, L. J., Waage, J., & Takken, W. (2012). Lessons from agriculture for the sustainable management of malaria vectors. PLoS Medicine, 9(7), e1001262.
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  e1001262; doi:10.1371/journal.pmed.1001262
• Wan, P., Huang, Y., Huaiheng, W. u., Huang, M., Cong, S., Tabashnik, B. E., & Kongming, W. u. (2012). Increased frequency of pink bollworm resistance to Bt toxin Cry1Ac in China. PLoS ONE, 7(1).
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  PMID: 22238687;PMCID: PMC3251611;Abstract: Transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) kill some key insect pests, but evolution of resistance by pests can reduce their efficacy. The main approach for delaying pest adaptation to Bt crops uses non-Bt host plants as "refuges" to increase survival of susceptible pests. To delay evolution of pest resistance to transgenic cotton producing Bt toxin Cry1Ac, the United States and some other countries have required refuges of non-Bt cotton, while farmers in China have relied on "natural" refuges of non-Bt host plants other than cotton. The "natural" refuge strategy focuses on cotton bollworm (Helicoverpa armigera), the primary target of Bt cotton in China that attacks many crops, but it does not apply to another major pest, pink bollworm (Pectinophora gossypiella), which feeds almost entirely on cotton in China. Here we report data showing field-evolved resistance to Cry1Ac by pink bollworm in the Yangtze River Valley of China. Laboratory bioassay data from 51 field-derived strains show that the susceptibility to Cry1Ac was significantly lower during 2008 to 2010 than 2005 to 2007. The percentage of field populations yielding one or more survivors at a diagnostic concentration of Cry1Ac increased from 0% in 2005-2007 to 56% in 2008-2010. However, the median survival at the diagnostic concentration was only 1.6% from 2008 to 2010 and failure of Bt cotton to control pink bollworm has not been reported in China. The early detection of resistance reported here may promote proactive countermeasures, such as a switch to transgenic cotton producing toxins distinct from Cry1A toxins, increased planting of non-Bt cotton, and integration of other management tactics together with Bt cotton. © 2012 Wan et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
• Wan, P., Huang, Y., Tabashnik, B. E., Huang, M., & Kongming, W. u. (2012). The halo effect: Suppression of pink bollworm on non-Bt cotton by Bt cotton in China. PLoS ONE, 7(7).
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  PMID: 22848685;PMCID: PMC3407057;Abstract: In some previously reported cases, transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) have suppressed insect pests not only in fields planted with such crops, but also regionally on host plants that do not produce Bt toxins. Here we used 16 years of field data to determine if Bt cotton caused this "halo effect" against pink bollworm (Pectinophora gossypiella) in six provinces of the Yangtze River Valley of China. In this region, the percentage of cotton hectares planted with Bt cotton increased from 9% in 2000 to 94% in 2009 and 2010. We found that Bt cotton significantly decreased the population density of pink bollworm on non-Bt cotton, with net decreases of 91% for eggs and 95% for larvae on non-Bt cotton after 11 years of Bt cotton use. Insecticide sprays targeting pink bollworm and cotton bollworm (Helicoverpa armigera) decreased by 69%. Previously reported evidence of the early stages of evolution of pink bollworm resistance to Bt cotton in China has raised concerns that if unchecked, such resistance could eventually diminish or eliminate the benefits of Bt cotton. The results reported here suggest that it might be possible to find a percentage of Bt cotton lower than the current level that causes sufficient regional pest suppression and reduces the risk of resistance. © 2012 Wan et al.
• Wan, P., Huang, Y., Tabashnik, B., Huang, M., & Wu, K. (2012). The halo effect: Suppression of pink bollworm on non-Bt cotton by Bt-cotton in China. PLoS ONE, 7(7).
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  e42004 doi:10.1371/journal.pone.0042004
• Wan, P., Huang, Y., Wu, H., Huang, M., Cong, S., Tabashnik, B., & Wu, K. (2012). Increased frequency of pink bollworm resistance to Bt toxin Cry1Ac. PLoS ONE, 7(1).
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  e29975 doi:10.1371/journal.pone.0029975
• Zhang, H., Shuwen, W. u., Yang, Y., Tabashnik, B. E., & Yidong, W. u. (2012). Non-Recessive Bt Toxin Resistance Conferred by an Intracellular Cadherin Mutation in Field-Selected Populations of Cotton Bollworm. PLoS ONE, 7(12).
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  PMID: 23285292;PMCID: PMC3532162;Abstract: Transgenic crops producing Bacillus thuringiensis (Bt) toxins have been planted widely to control insect pests, yet evolution of resistance by the pests can reduce the benefits of this approach. Recessive mutations in the extracellular domain of toxin-binding cadherin proteins that confer resistance to Bt toxin Cry1Ac by disrupting toxin binding have been reported previously in three major lepidopteran pests, including the cotton bollworm, Helicoverpa armigera. Here we report a novel allele from cotton bollworm with a deletion in the intracellular domain of cadherin that is genetically linked with non-recessive resistance to Cry1Ac. We discovered this allele in each of three field-selected populations we screened from northern China where Bt cotton producing Cry1Ac has been grown intensively. We expressed four types of cadherin alleles in heterologous cell cultures: susceptible, resistant with the intracellular domain mutation, and two complementary chimeric alleles with and without the mutation. Cells transfected with each of the four cadherin alleles bound Cry1Ac and were killed by Cry1Ac. However, relative to cells transfected with either the susceptible allele or the chimeric allele lacking the intracellular domain mutation, cells transfected with the resistant allele or the chimeric allele containing the intracellular domain mutation were less susceptible to Cry1Ac. These results suggest that the intracellular domain of cadherin is involved in post-binding events that affect toxicity of Cry1Ac. This evidence is consistent with the vital role of the intracellular region of cadherin proposed by the cell signaling model of the mode of action of Bt toxins. Considered together with previously reported data, the results suggest that both pore formation and cell signaling pathways contribute to the efficacy of Bt toxins. © 2012 Zhang et al.
• Zhang, H., T., W. T., Zhao, J., Jin, L., Yang, J., Liu, C., Yang, Y., Wu, S., Wu, K., Cui, J., Tabashnik, B., & Wu, Y. (2012). Diverse genetic basis of field-evolved resistance to Bt cotton in cotton bollworm from China. Proc Nat l Acad Sci, 109(10), 275-10, 280.
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• Zhang, H., Tian, W., Zhao, J., Jin, L., Yang, J., Liu, C., Yang, Y., Shuwen, W. u., Kongming, W. u., Cui, J., Tabashnik, B. E., & Yidong, W. u. (2012). Diverse genetic basis of field-evolved resistance to Bt cotton in cotton bollworm from China. Proceedings of the National Academy of Sciences of the United States of America, 109(26), 10275-10280.
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  PMID: 22689968;PMCID: PMC3387040;Abstract: Evolution of pest resistance reduces the efficacy of insecticidal proteins from Bacillus thuringiensis (Bt) used in sprays or in transgenic crops. Although several pests have evolved resistance to Bt crops in the field, information about the genetic basis of fieldevolved resistance to Bt crops has been limited. In particular, laboratory- selected resistance to Bt toxin Cry1Ac based on recessive mutations in a gene encoding a toxin-binding cadherin protein has been identified in three major cotton pests, but previous work has not determined if such mutations are associated with fieldselected resistance to Bt cotton. Here we show that the most common resistance alleles in field populations of cotton bollworm, Helicoverpa armigera, selected with Bt cotton in northern China, had recessive cadherin mutations, including the deletion mutation identified via laboratory selection. However, unlike all previously studied cadherin resistance alleles, one field-selected cadherin resistance allele conferred nonrecessive resistance. We also detected nonrecessive resistance that was not genetically linked with the cadherin locus. In field-selected populations, recessive cadherin alleles accounted for 75-84% of resistance alleles detected. However, most resistance alleles occurred in heterozygotes and 59-94% of resistant individuals carried at least one nonrecessive resistance allele. The results suggest that resistance management strategies must account for diverse resistance alleles in field-selected populations, including nonrecessive alleles.
• Zhang, H., Wu, S., Yang, Y., Tabashnik, B., & Wu, Y. (2012). Non-recessive Bt toxin resistance conferred by an intracellular cadherin mutation in field-selected populations of cotton bollworm. PLoS ONE, 7(12).
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  e53418 doi:10.1371/journal.pone.0053418
• , Y., Ellers-Kirk, C., Harthfield, K., Larocque, G., Degain, B., Dutilleul, P., Dennehy, T., Marsh, S., Crowder, D., Li, X., Ellsworth, P., Naranjo, S., Palumbo, J., Fournier, A., Antilla, L., & Tabashnik, B. (2011). Large-scale spatially explicit test of the refuge strategy for delaying insecticide resistance. Proceedings of the National Academy of Sciences USA.
• Fabrick, J., Mathew, L., Tabashnik, B., & Li, X. (2011). Insertion of an intact CR1 retrotransposon in a cadherin gene linked with Bt resistance in the pink bollworm, Pectinophora gossypiella. Insect Mol Biol, 20, 651-665.
• Heuberger, S., Crowder, D. W., Brévault, T., Tabashnik, B. E., & Carrière, Y. (2011). Modeling the effects of plant-to-plant gene flow, larval behavior, and refuge size on pest resistance to bt cotton. Environmental Entomology, 40(2), 484-495.
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  Abstract: Growers of Bacillus thuringiensis (Bt) crops often use refuges of non-Bt plants to delay pest resistance, but plant-to-plant gene flow between Bt and non-Bt crops could affect this strategy. Here we used simulation modeling to explore the consequences of pollen-and seed-mediated gene flow in cotton fields on the evolution of resistance in a generic pest. We modeled a landscape of 0.5-ha fields where growers used farm-saved seed, as could often occur in the developing world. Specifically, we examined the effects of moderate and high gene flow rates, larval feeding behavior, dominance of resistance, refuge type and abundance, and the interactions among these factors. With either completely dominant or completely recessive inheritance of resistance, gene flow among plants and larval feeding behavior had limited practical impact on resistance evolution. With intermediate dominance, however, moderate or high gene flow among plants substantially accelerated resistance evolution in some simulations where non-Bt cotton refuges were 5 or 20% of the cotton acreage. The acceleration was usually greater when larvae moved and fed indiscriminately among Bt and non-Bt cotton plants than when larvae were sedentary or discriminated among plant types. Adding alternative host plant refuges to the landscape delayed resistance, while increasing the non-Bt cotton refuge from 20 to 50% of the cotton acreage had positive, negative, or neutral effects, depending on dominance, the amount of alternative host plant refuges, and larval feeding behavior. The results suggest that, under certain conditions, reducing gene flow between refuges and Bt crops could help delay pest resistance. © 2011 Entomological Society of America.
• Himler, A. G., Adachi-Hagimori, T., Bergen, J. E., Kozuch, A., Kelly, S. E., Tabashnik, B. E., Chiel, E., Duckworth, V. E., Dennehy, T. J., Zchori-Fein, E., & Hunter, M. S. (2011). Rapid Spread of a Bacterial Symbiont in an Invasive Whitefly Is Driven by Fitness Benefits and Female Bias. SCIENCE, 332(6026), 254-256.
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  Maternally inherited bacterial symbionts of arthropods are common, yet symbiont invasions of host populations have rarely been observed. Here, we show that Rickettsia sp. nr. bellii swept into a population of an invasive agricultural pest, the sweet potato whitefly, Bemisia tabaci, in just 6 years. Compared with uninfected whiteflies, Rickettsia-infected whiteflies produced more offspring, had higher survival to adulthood, developed faster, and produced a higher proportion of daughters. The symbiont thus functions as both mutualist and reproductive manipulator. The observed increased performance and sex-ratio bias of infected whiteflies are sufficient to explain the spread of Rickettsia across the southwestern United States. Symbiont invasions such as this represent a sudden evolutionary shift for the host, with potentially large impacts on its ecology and invasiveness.
• Himler, A., Adachi-Hagimori, T., Bergen, J., Kozuch, A., Kelly, S., Tabashnik, B., Chiel, E., Duckworth, V., Dennehy, T., Zchori-Fein, E., & Hunter, M. (2011). Rapid spread of a bacterial symbiont in an invasive whitefly is driven by fitness benefits and female bias. Science, 332, 254-256.
• Simmons, G. S., McKemey, A. R., Morrison, N. I., O'Connell, S., Tabashnik, B. E., Claus, J., Guoliang, F. u., Tang, G., Sledge, M., Walker, A. S., Phillips, C. E., Miller, E. D., Rose, R. I., Staten, R. T., Donnelly, C. A., & Alphey, L. (2011). Field performance of a genetically engineered strain of pink bollworm. PLoS ONE, 6(9).
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  PMID: 21931649;PMCID: PMC3172240;Abstract: Pest insects harm crops, livestock and human health, either directly or by acting as vectors of disease. The Sterile Insect Technique (SIT) - mass-release of sterile insects to mate with, and thereby control, their wild counterparts - has been used successfully for decades to control several pest species, including pink bollworm, a lepidopteran pest of cotton. Although it has been suggested that genetic engineering of pest insects provides potential improvements, there is uncertainty regarding its impact on their field performance. Discrimination between released and wild moths caught in monitoring traps is essential for estimating wild population levels. To address concerns about the reliability of current marking methods, we developed a genetically engineered strain of pink bollworm with a heritable fluorescent marker, to improve discrimination of sterile from wild moths. Here, we report the results of field trials showing that this engineered strain performed well under field conditions. Our data show that attributes critical to SIT in the field - ability to find a mate and to initiate copulation, as well as dispersal and persistence in the release area - were comparable between the genetically engineered strain and a standard strain. To our knowledge, these represent the first open-field experiments with a genetically engineered insect. The results described here provide encouragement for the genetic control of insect pests.
• Simmons, G., McKemey, A., Morrison, N., O'Connell, S., Tabashnik, B., Claus, J., Fu, G., Tang, G., Sledge, M., Walker, A., Phillips, C., Miller, E., Rose, R., Staten, R., Donnelly, C., & Alphey, L. (2011). Field performance of a genetically engineeered strain of pink bollworm. PLoS ONE, 6(96).
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  e24110 doi:10.1371/journal.pone.0024110
• Tabashnik, B. E., Huang, F., Ghimire, M. N., Leonard, B. R., Siegfried, B. D., Rangasamy, M., Yang, Y., Yidong, W. u., Gahan, L. J., Heckel, D. G., Bravo, A., & Soberón, M. (2011). Efficacy of genetically modified Bt toxins against insects with different genetic mechanisms of resistance. Nature Biotechnology, 29(12), 1128-1131.
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  PMID: 21983521;Abstract: Transgenic crops that produce Bacillus thuringiensis (Bt) toxins are grown widely for pest control, but insect adaptation can reduce their efficacy. The genetically modified Bt toxins Cry1AbMod and Cry1AcMod were designed to counter insect resistance to native Bt toxins Cry1Ab and Cry1Ac. Previous results suggested that the modified toxins would be effective only if resistance was linked with mutations in genes encoding toxin-binding cadherin proteins. Here we report evidence from five major crop pests refuting this hypothesis. Relative to native toxins, the potency of modified toxins was >350-fold higher against resistant strains of Plutella xylostella and Ostrinia nubilalis in which resistance was not linked with cadherin mutations. Conversely, the modified toxins provided little or no advantage against some resistant strains of three other pests with altered cadherin. Independent of the presence of cadherin mutations, the relative potency of the modified toxins was generally higher against the most resistant strains. © 2011 Nature America, Inc. All rights reserved.
• Tabashnik, B. E., Van Rensburg, J. B., & Carriere, Y. (2011). Field-Evolved Insect Resistance to Bt Crops: Definition, Theory, and Data. JOURNAL OF ECONOMIC ENTOMOLOGY, 102(6), 2011-2025.
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  Transgenic crops producing Bacillus thuringiensis (Bt) toxins for insect pest control have been successful, but their efficacy is reduced when pests evolve resistance. Here we review the definition of field-evolved resistance, the relationship between resistance and field control problems, the theory underlying strategies for delaying resistance, and resistance monitoring methods. We also analyze resistance monitoring data from five continents reported in 41 studies that evaluate responses of field populations of 11 lepidopteran pests to four Bt toxins produced by Bt corn and cotton. After more than a decade since initial commercialization of Bt crops, most target pest populations remain susceptible, whereas field-evolved resistance has been documented in some populations of three noctuid moth species: Spodoptera frugiperda (J.E. Smith) to Cry1F in Bt corn in Puerto Rico, Busseola fusca (Fuller) to Cry1Ab in Bt corn in South Africa, and Helicoverpa zea (Boddie) to Cry1Ac and Cry2Ab in Bt cotton in the southeastern United States. Field outcomes are consistent with predictions from theory, suggesting that factors delaying resistance include recessive inheritance of resistance, abundant refuges of non-Bt host plants, and two-toxin Bt crops deployed separately from one-toxin Bt crops. The insights gained from systematic analyses of resistance monitoring data may help to enhance the durability of transgenic insecticidal crops. We recommend continued use of the long-standing definition of resistance cited here and encourage discussions about which regulatory actions, if any, should be triggered by specific data on the magnitude, distribution, and impact of field-evolved resistance.
• Tabashnik, B., Huang, F., Ghimire, M., Leonard, B., Siegfried, B., Rangasamy, M., Yang, Y., Wu, Y., Gahan, L., Heckel, D., Bravo, A., & Soberon, M. (2011). Efficacy of genetically modified Bt toxins against insects with different mechanisms of resistance. Nature Biotechnology, 29, 1128-1131.
• Wan, P., Huang, Y., Wu, H., Huang, M., Cong, S., Tabashnik, B., & Wu, K. (2011). Increased frequency of pink bollworm resistance to Bt toxin Cry1Ac. PLoS ONE.
• Wang, J., Simmons, G. S., Miller, T. A., Tabashnik, B. E., & Park, Y. (2011). Global variation in the piggyBac-like element of pink bollworm, Pectinophora gossypiella. Journal of Asia-Pacific Entomology, 14(1), 131-135.
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  Abstract: The piggyBac transposable element, originally discovered in the cabbage looper, Trichoplusia ni, has been used widely in genetic engineering of insects including the pink bollworm, Pectinophora gossypiella, a major lepidopteran pest of cotton. Previously, we identified an intact copy of a piggyBac-like element (PLE) in pink bollworm, designated as PgPLE1.1. Here we report global variation in the occurrence and sequence of PgPLE1.1 and its flanking sequences. Low to high frequency of the PgPLE1.1 insertion was observed in populations from USA, Mexico, China, India, and Israel, while there is no PgPLE1.1 insertion in the populations from Australia. Investigation of the five haplotypes of PgPLE1.1, their frequency, and the flanking sequences of PgPLE1.1 revealed significant differences of the populations from Australia and China compared to other global populations, although recent occurrences of extensive gene flows among global populations were evident. © 2010 Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society.
• Wang, J., Simmons, G., Miller, T., Tabashnik, B., & Park, Y. (2011). Global variation in the piggyBac-like element of the pink bollworm, Pectinophora gossypiella. Journal of Asia Pacific Entomology, 14, 131-135.
• Williams, J. L., Ellers-Kirk, C., Orth, R. G., Gassmann, A. J., Head, G., Tabashnik, B. E., & Carrière, Y. (2011). Fitness cost of resistance to bt cotton linked with increased gossypol content in pink bollworm larvae. PLoS ONE, 6(6).
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  PMID: 21738799;PMCID: PMC3128109;Abstract: Fitness costs of resistance to Bacillus thuringiensis (Bt) crops occur in the absence of Bt toxins, when individuals with resistance alleles are less fit than individuals without resistance alleles. As costs of Bt resistance are common, refuges of non-Bt host plants can delay resistance not only by providing susceptible individuals to mate with resistant individuals, but also by selecting against resistance. Because costs typically vary across host plants, refuges with host plants that magnify costs or make them less recessive could enhance resistance management. Limited understanding of the physiological mechanisms causing fitness costs, however, hampers attempts to increase costs. In several major cotton pests including pink bollworm (Pectinophora gossypiella), resistance to Cry1Ac cotton is associated with mutations altering cadherin proteins that bind this toxin in susceptible larvae. Here we report that the concentration of gossypol, a cotton defensive chemical, was higher in pink bollworm larvae with cadherin resistance alleles than in larvae lacking such alleles. Adding gossypol to the larval diet decreased larval weight and survival, and increased the fitness cost affecting larval growth, but not survival. Across cadherin genotypes, the cost affecting larval growth increased as the gossypol concentration of larvae increased. These results suggest that increased accumulation of plant defensive chemicals may contribute to fitness costs associated with resistance to Bt toxins. © 2011 Williams et al.
• Zhang, H., Yin, W., Zhao, J., Jin, L., Yang, Y., Shuwen, W. u., Tabashnik, B. E., & Yidong, W. u. (2011). Early warning of cotton bollworm resistance associated with intensive planting of Bt cotton in China. PLoS ONE, 6(8).
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  PMID: 21857961;PMCID: PMC3153483;Abstract: Transgenic crops producing Bacillus thuringiensis (Bt) toxins kill some key insect pests, but evolution of resistance by pests can reduce their efficacy. The predominant strategy for delaying pest resistance to Bt crops requires refuges of non-Bt host plants to promote survival of susceptible pests. To delay pest resistance to transgenic cotton producing Bt toxin Cry1Ac, farmers in the United States and Australia planted refuges of non-Bt cotton, while farmers in China have relied on "natural" refuges of non-Bt host plants other than cotton. Here we report data from a 2010 survey showing field-evolved resistance to Cry1Ac of the major target pest, cotton bollworm (Helicoverpa armigera), in northern China. Laboratory bioassay results show that susceptibility to Cry1Ac was significantly lower in 13 field populations from northern China, where Bt cotton has been planted intensively, than in two populations from sites in northwestern China where exposure to Bt cotton has been limited. Susceptibility to Bt toxin Cry2Ab did not differ between northern and northwestern China, demonstrating that resistance to Cry1Ac did not cause cross-resistance to Cry2Ab, and implying that resistance to Cry1Ac in northern China is a specific adaptation caused by exposure to this toxin in Bt cotton. Despite the resistance detected in laboratory bioassays, control failures of Bt cotton have not been reported in China. This early warning may spur proactive countermeasures, including a switch to transgenic cotton producing two or more toxins distinct from Cry1A toxins. © 2011 Zhang et al.
• Zhang, H., Yin, W., Zhao, J., Yang, Y., Wu, S., Tabashnik, B., & Wu, Y. (2011). Early warning of cotton bollworm resistance associated with intensive planting of Bt cotton in China. PLoS ONE, 6(8).
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  e22874 doi: 10.1371/journal.pone.0022874
• Carrière, Y., Crowder, D. W., & Tabashnik, B. E. (2010). Evolutionary ecology of insect adaptation to Bt crops. Evolutionary Applications, 3(5-6), 561-573.
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  Abstract: Transgenic crops producing Bacillus thuringiensis (Bt) toxins are used worldwide to control major pests of corn and cotton. Development of strategies to delay the evolution of pest resistance to Bt crops requires an understanding of factors affecting responses to natural selection, which include variation in survival on Bt crops, heritability of resistance, and fitness advantages associated with resistance mutations. The two main strategies adopted for delaying resistance are the refuge and pyramid strategies. Both can reduce heritability of resistance, but pyramids can also delay resistance by reducing genetic variation for resistance. Seasonal declines in the concentration of Bt toxins in transgenic cultivars, however, can increase the heritability of resistance. The fitness advantages associated with resistance mutations can be reduced by agronomic practices, including increasing refuge size, manipulating refuges to increase fitness costs, and manipulating Bt cultivars to reduce fitness of resistant individuals. Manipulating costs and fitness of resistant individuals on transgenic insecticidal crops may be especially important for thwarting evolution of resistance in haplodiploid and parthenogenetic pests. Field-evolved resistance to Bt crops in only five pests during the last 14years suggests that the refuge strategy has successfully delayed resistance, but the accumulation of resistant pests could accelerate. © 2010 Blackwell Publishing Ltd.
• Fernández-Luna, M. T., Fernández-Luna, M. T., Tabashnik, B. E., Tabashnik, B. E., Lanz-Mendoza, H., Lanz-Mendoza, H., Bravo, A., Bravo, A., Soberón, M., Soberón, M., Miranda-Ríos, J., & Miranda-Ríos, J. (2010). Single concentration tests show synergism among Bacillus thuringiensis subsp. israelensis toxins against the malaria vector mosquito Anopheles albimanus. Journal of Invertebrate Pathology, 104(3), 231-233.
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  PMID: 20361977;Abstract: Bioassays of insecticidal proteins from Bacillus thuringiensis subsp. israelensis with larvae of the malaria vector mosquito Anopheles albimanus showed that the cytolytic protein Cyt1Aa was not toxic alone, but it increased the toxicity of the crystalline proteins Cry4Ba and Cry11Aa. Synergism also occurred between Cry4Ba and Cry11Aa toxins. Whereas many previous analyses of synergism have been based on a series of toxin concentrations leading to comparisons between expected and observed values for the concentration killing 50% of insects tested (LC50), we describe and apply a method here that enables testing for synergism based on single concentrations of toxins. © 2010 Elsevier Inc.
• Gassmann, A. J., Stock, S. P., Tabashnik, B. E., & Singer, M. S. (2010). Tritrophic effects of host plants on an herbivore-pathogen interaction. Annals of the Entomological Society of America, 103(3), 371-378.
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  Abstract: Tritrophic interactions may include directly harmful effects of host plants on herbivores, and directly or indirectly harmful effects of host plants on the natural enemies of herbivores. Tritrophic interactions involving parasitoids and predators have received considerable attention but less is known about how host plants affect entomopathogens. We compared growth and resistance to entomopathogenic nematodes for "woolly bear" caterpillars, Grammia incorrupta (=geneura) (Hy. Edwards) (Lepidoptera: Arctiidae) fed lettuce, Lactuca sativa L. (Asteraceae), versus threadleaf groundsel, Senecio longilobus Benth. (Asteraceae). Both plants are members of the Asteraceae, but only S. longilobus contains pyrrolizidine alkaloids. Caterpillars gained more mass when fed L. sativa compared with S. longilobus; yet, in one of four cases studied, resistance to nematodes was higher when caterpillars ate S. longilobus. Caterpillar resistance to nematodes did not differ between host plants in the other cases. In addition, nematode reproduction was higher in cadavers of G. incorrupta that had been fed L. sativa instead of S. longilobus, suggesting that S. longilobus had indirectly detrimental effects on entomopathogenic nematodes. Our results illustrate how trade-offs may arise in tritrophic interactions involving entomopathogens, as the cost of decreased growth imposed by S. longilobus was accompanied by the benefit of greater resistance to entomopathogenic nematodes. © 2010 Entomological Society of America.
• Hannon, E. R., Sisterson, M. S., Stock, S. P., Carrire, Y., Tabashnik, B. E., & Gassmann, A. J. (2010). Effects of four nematode species on fitness costs of pink bollworm resistance to Bacillus thuringiensis toxin CrylAc. Journal of Economic Entomology, 103(5), 1821-1831.
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  PMID: 21061986;Abstract: Evolution of resistance by pests can reduce the efficacy of transgenic crops that produce insecticidal toxins from the bacterium Bacillus thuringiensis Berliner (Bt). In conjunction with refuges of non-Bt host plants, fitness costs can delay the evolution of resistance. Furthermore, fitness costs often vary with ecological conditions, suggesting that agricultural landscapes can be manipulated to magnify fitness costs and thereby prolong the efficacy of Bt crops. In the current study, we tested the effects of four species of entomopathogenic nematodes (Steinernematidae and Heterorhabditidae) on the magnitude and dominance of fitness costs of resistance to Bt toxin CrylAc in pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae). For more than a decade, field populations of pink bollworm in the United States have remained susceptible to Bt cotton Gossypium hirsutum L. producing CrylAc; however, we used laboratory strains that had a mixture of susceptible and resistant individuals. In laboratory experiments, dominant fitness costs were imposed by the nematode Steinernema riobrave Cabanillas, Poinar, and Raulston but no fitness costs were imposed by Steinernema carpocapsae Weiser, Steinernema sp. (ML18 strain), or Heterorhabditis sonorensis Stock, Rivera-Orduo, and Flores-Lara. In computer simulations, evolution of resistance to CrylAc by pink bollworm was substantially delayed by treating some non-Bt cotton refuge fields with nematodes that imposed a dominant fitness cost, similar to the cost observed in laboratory experiments with S. riobrave. Based on the results here and in related studies, we conclude that entomopathogenic nematodes could bolster insect resistance management, but the success of this approach will depend on selecting the appropriate species of nematode and environment, as fitness costs were magnified by only two of five species evaluated and also depended on environmental factors. © 2010 Entomological Society of America.
• Heuberger, S., Crowder, D., Brevault, T., Tabashnik, B., & , Y. (2010). Modeling the effects of plant-to-plant gene flow, larval behavior, and refuge size on pest resistance to Bt cotton. Environ Entomol, 40, 484-495.
• Heuberger, S., Ellers-Kirk, C., Tabashnik, B. E., & Carrière, Y. (2010). Pollen- and seed-mediated transgene flow in commercial cotton seed production fields. PLoS ONE, 5(11).
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  PMID: 21152426;PMCID: PMC2994710;Abstract: Background: Characterizing the spatial patterns of gene flow from transgenic crops is challenging, making it difficult to design containment strategies for markets that regulate the adventitious presence of transgenes. Insecticidal Bacillus thuringiensis (Bt) cotton is planted on millions of hectares annually and is a potential source of transgene flow. Methodology/Principal Findings: Here we monitored 15 non-Bt cotton (Gossypium hirsutum, L.) seed production fields (some transgenic for herbicide resistance, some not) for gene flow of the Bt cotton cry1Ac transgene. We investigated seedmediated gene flow, which yields adventitious Bt cotton plants, and pollen-mediated gene flow, which generates outcrossed seeds. A spatially-explicit statistical analysis was used to quantify the effects of nearby Bt and non-Bt cotton fields at various spatial scales, along with the effects of pollinator abundance and adventitious Bt plants in fields, on pollenmediated gene flow. Adventitious Bt cotton plants, resulting from seed bags and planting error, comprised over 15% of plants sampled from the edges of three seed production fields. In contrast, pollen-mediated gene flow affected less than 1% of the seed sampled from field edges. Variation in outcrossing was better explained by the area of Bt cotton fields within 750 m of the seed production fields than by the area of Bt cotton within larger or smaller spatial scales. Variation in outcrossing was also positively associated with the abundance of honey bees. Conclusions/Significance: A comparison of statistical methods showed that our spatially-explicit analysis was more powerful for understanding the effects of surrounding fields than customary models based on distance. Given the low rates of pollen-mediated gene flow observed in this study, we conclude that careful planting and screening of seeds could be more important than field spacing for limiting gene flow. © 2010 Heuberger et al.
• Tabashnik, B. E. (2010). Communal benefits of transgenic corn. Science, 330(6001), 189190-.
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  Abstract: Genetically engineered corn plants can reduce pest damage on neighboring, unmodified plants.
• Tabashnik, B. E. (2010). Plant science. Communal benefits of transgenic corn.. Science (New York, N.Y.), 330(6001), 189-190.
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  PMID: 20929767;
• Tabashnik, B. E., & Carrire, Y. (2010). Field-evolved resistance to Bt cotton: Bollworm in the U.S. and pink bollworm in India. Southwestern Entomologist, 35(3), 417-424.
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  Abstract: Some field populations of bollworm, Helicoverpa zea (Boddie), in the southeastern U.S. have evolved resistance to Bacillus thuringiensis Berliner (Bt) toxins Cry1Ac and Cry2Ab produced by transgenic cotton, Gossypium hirsutum L. Decreased susceptibility to Cry1Ac, the toxin in first-generation Bt cotton (Bollgard), was detected in laboratory diet bioassays and was associated with increased survival on Bollgard plant tissues and control problems in the field. Bollgard cotton is no longer registered in the U.S. and has been replaced primarily by Bt cotton that produces Cry1Ac and Cry2Ab (Bollgard II). Laboratory diet bioassay results showed that the percentage of field populations resistant to Cry2Ab increased from 0% in 2002 to 50% in 2005. In bioassays from 2003 to 2006, 14 field-derived strains of bollworm had
• Tabashnik, B. E., Sisterson, M. S., Ellsworth, P. C., Dennehy, T. J., Antilla, L., Liesner, L., Whitlow, M., Staten, R. T., Fabrick, J. A., Unnithan, G. C., Yelich, A. J., Ellers-Kirk, C., Harpold, V. S., Li, X., & Carriere, Y. (2010). Suppressing resistance to Bt cotton with sterile insect releases. NATURE BIOTECHNOLOGY, 28(12), 1304-U119.
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  Genetically engineered crops that produce insecticidal toxins from Bacillus thuringiensis (Bt) are grown widely for pest control(1). However, insect adaptation can reduce the toxins' efficacy(2-5). The predominant strategy for delaying pest resistance to Bt crops requires refuges of non-Bt host plants to provide susceptible insects to mate with resistant insects(2-7). Variable farmer compliance is one of the limitations of this approach. Here we report the benefits of an alternative strategy where sterile insects are released to mate with resistant insects and refuges are scarce or absent. Computer simulations show that this approach works in principle against pests with recessive or dominant inheritance of resistance. During a large-scale, four-year field deployment of this strategy in Arizona, resistance of pink bollworm (Pectinophora gossypiella) to Bt cotton did not increase. A multitactic eradication program that included the release of sterile moths reduced pink bollworm abundance by >99%, while eliminating insecticide sprays against this key invasive pest.
• Tabashnik, B., & Tabashnik, B. E. (2010). Plant science. Communal benefits of transgenic corn. Science (New York, N.Y.), 330(6001).
• Wang, J., Miller, E. D., Simmons, G. S., Miller, T. A., Tabashnik, B. E., & Park, Y. (2010). Piggybac-like elements in the pink bollworm, pectinophora gossypiella. Insect Molecular Biology, 19(2), 177-184.
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  PMID: 20017756;Abstract: A transgenic line of the pink bollworm, Pectinophora gossypiella, a key lepidopteran cotton pest, was generated previously using the piggyBac transposon IFP2 from Trichoplusia ni. Here we identified an endogenous piggyBac-like element (PLE), designated as PgPLE1, in the pink bollworm. A putatively intact copy of PgPLE1 (PgPLE1.1) presents the canonical features of PLE: inverted terminal repeats with three C/G residues at the extreme ends, inverted subterminal repeats, TTAA target site and an open reading frame encoding transposase with 68% similarity to IFP2. Vectorette PCR revealed large variation in the insertion sites of PgPLE1 amongst worldwide populations, indicating the potential mobility of PgPLE1. The PgPLE1 was undetectable in the genome of Pectinophora endema, implying the recent invasion of PgPLE1 after the divergence of these two closely related species. © 2009 The Authors.
• Zhang, H., Yin, W., Zhao, J., Jin, L., Yang, Y., Wu, S., Tabashnik, B. E., & Wu, Y. (2010). Early Warning of Cotton Bollworm Resistance Associated with Intensive Planting of Bt Cotton in China. PLOS ONE, 6(8).
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  Transgenic crops producing Bacillus thuringiensis (Bt) toxins kill some key insect pests, but evolution of resistance by pests can reduce their efficacy. The predominant strategy for delaying pest resistance to Bt crops requires refuges of non-Bt host plants to promote survival of susceptible pests. To delay pest resistance to transgenic cotton producing Bt toxin Cry1Ac, farmers in the United States and Australia planted refuges of non-Bt cotton, while farmers in China have relied on "natural" refuges of non-Bt host plants other than cotton. Here we report data from a 2010 survey showing field-evolved resistance to Cry1Ac of the major target pest, cotton bollworm (Helicoverpa armigera), in northern China. Laboratory bioassay results show that susceptibility to Cry1Ac was significantly lower in 13 field populations from northern China, where Bt cotton has been planted intensively, than in two populations from sites in northwestern China where exposure to Bt cotton has been limited. Susceptibility to Bt toxin Cry2Ab did not differ between northern and northwestern China, demonstrating that resistance to Cry1Ac did not cause cross-resistance to Cry2Ab, and implying that resistance to Cry1Ac in northern China is a specific adaptation caused by exposure to this toxin in Bt cotton. Despite the resistance detected in laboratory bioassays, control failures of Bt cotton have not been reported in China. This early warning may spur proactive countermeasures, including a switch to transgenic cotton producing two or more toxins distinct from Cry1A toxins.
• Carrière, Y., Showalter, A. M., Fabrick, J. A., Sollome, J., Ellers-Kirk, C., & Tabashnik, B. E. (2009). Cadherin gene expression and effects of Bt resistance on sperm transfer in pink bollworm. Journal of Insect Physiology, 55(11), 1058-1064.
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  PMID: 19666026;Abstract: Cadherin proteins bind Bacillus thuringiensis (Bt) toxins in lepidopteran midguts but their inherent function remains unclear. In pink bollworm, Pectinophora gossypiella, three recessive mutations in a cadherin gene (BtR) are tightly linked with resistance to Bt toxin Cry1Ac. Here we examined patterns of transcription of this gene and the association between cadherin genotype and sperm transfer in pink bollworm. Cadherin RNA was most abundant in larvae, but was also found in adults and embryos. In fourth instar larvae, cadherin RNA was most abundant in the gut, yet its presence in the testes indicates a potential role in sperm production. Previously, we found reduced first-male paternity in pink bollworm males homozygous for cadherin mutations conferring resistance to Bt, when a resistant and susceptible male competed for access to a female. However, the number of offspring sired by resistant and susceptible males was similar without competition. Male Lepidoptera produce both fertile eupyrene sperm and anucleate, non-fertile apyrene sperm, suggesting that apyrene sperm may contribute to male reproductive success when sperm competition occurs. Accordingly, we hypothesized that cadherin-based resistance to Bt entails fitness costs that reduce apyrene sperm transfer. To test this hypothesis, we compared apyrene and eupyrene sperm transfer in males from four strains of pink bollworm. Transfer of apyrene and eupyrene sperm was lower in homozygous resistant than in susceptible males. Furthermore, homozygous resistant males weighed less than susceptible males, which could have diminished sperm transfer by resistant males directly, or via a positive association between male weight, spermatophore weight and sperm transfer. While data suggest that cadherin mutations induced a recessive fitness cost affecting apyrene sperm transfer, these mutations also generated recessive costs that affected other traits and could have lowered first-male paternity. © 2009 Elsevier Ltd.
• Crowder, D. W., Ellers-Kirk, C., Tabashnik, B. E., & Carrière, Y. (2009). Lack of fitness costs associated with pyriproxyfen resistance in the B biotype of Bemisia tabaci. Pest Management Science, 65(3), 235-240.
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  PMID: 19097024;Abstract: BACKGROUND: The insect growth regulator pyriproxyfen has provide deffective control of the whitefly Bemisiatabaci Gennadius in many countries. Here, whether or not fitness costs were associated with pyriproxyfen resistance in a laboratory-selected resistant strain (QC02-R) of the B biotype was determined. RESULTS: Mortality caused by pyriproxyfen and fitness traits over time were measured in unselected and selected hybrid strains, which were created by crossing individuals of the resistant strain with individuals of a susceptible strain. Fitness costs were not associated with resistance in QC02-R, as mortality caused by pyriproxyfen did not increase over time in unselected hybrid strains and fitness traitswere similar in unselected and selected hybrid strains. Using a new method to examine the inheritance of resistance, based on data from fitness cost experiments, it was estimated that pyriproxyfen resistance is controlled by two loci in the QC02-R strain. CONCLUSION: The lack of fitness costs associated with pyriproxyfen resistance could promote the evolution of resistance in field populations with similar traits to QC02-R. © 2008 Society of Chemical Industry.
• Crowder, D. W., Horowitz, A. R., Tabashnik, B. E., Dennehy, T. J., Denholm, I., Gorman, K., & Carrire, Y. (2009). Analyzing haplodiploid inheritance of insecticide resistance in whitefly biotypes. Bulletin of Entomological Research, 99(3), 307-315.
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  PMID: 19159498;Abstract: We developed new methods for analyzing inheritance of insecticide resistance in haplodiploid arthropods and applied them to elucidate resistance of the whitefly Bemisia tabaci (Gennadius) to an insect growth regulator, pyriproxyfen. Two invasive biotypes of this devastating crop pest, the B biotype in Arizona and the Q biotype in Israel, have evolved resistance to pyriproxyfen. Here, we incorporated data from laboratory bioassays and crossing procedures exploiting haplodiploidy into statistical and analytical models to estimate the number of loci affecting pyriproxyfen resistance in strains of both biotypes. In tests with models of one to ten loci, the best fit between expected and observed mortality occurred with a two-locus model for the B biotype strain (QC-02) and for one- and two-locus models for the Q biotype strain (Pyri-R). The estimated minimum number of loci affecting resistance was 1.6 for the B biotype strain and 1.0 for the Q biotype strain. The methods used here can be applied to insecticide resistance and other traits in haplodiploid arthropods.
• Franklin, M. T., Nieman, C. L., Janmaat, A. F., Soberón, M., Bravo, A., Tabashnik, B. E., & Myers, J. H. (2009). Modified Bacillus thuringiensis toxins and a hybrid B. thuringiensis strain counter greenhouse-selected resistance in Trichoplusia ni. Applied and Environmental Microbiology, 75(17), 5739-5741.
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  PMID: 19592525;PMCID: PMC2737904;Abstract: Resistance of greenhouse-selected strains of the cabbage looper, Trichoplusia ni, to Bacillus thuringiensis subsp. kurstaki was countered by a hybrid strain of B. thuringiensis and genetically modified toxins Cry1AbMod and Cry1AcMod, which lack helix α-1. Resistance to Cry1AbMod and Cry1AcMod was >100-fold less than resistance to native toxins Cry1Ab and Cry1Ac. Copyright © 2009, American Society for Microbiology. All Rights Reserved.
• Gassmann, A. J., Carrière, Y., & Tabashnik, B. E. (2009). Fitness costs of insect resistance to bacillus thuringiensis. Annual Review of Entomology, 54, 147-163.
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  PMID: 19067630;Abstract: Evolution of resistance by insect pests threatens the continued effectiveness of Bacillus thuringiensis (Bt) toxins in sprays and transgenic crops. Fitness costs of Bt resistance occur when, in the absence of Bt toxins, fitness is lower for resistant insects than for susceptible insects. Modeling results show that fitness costs can delay resistance by selecting against Bt-resistant genotypes in refuges where insects are not exposed to Bt toxins. In 77 studies including 18 species, fitness costs were detected in 62% of experiments testing for declines in resistance and in 34% of fitness component comparisons. Mean fitness costs were 15.5% for survival, 7.4% for development time, and 2.5% for mass. Although most fitness costs were recessive, nonrecessive costs can select more strongly against resistance. Because fitness costs vary with ecological conditions, refuges designed to increase the dominance or magnitude of fitness costs could be especially useful for delaying pest resistance. Copyright © 2009 by Annual Reviews. All rights reserved.
• Gassmann, A. J., Fabrick, J. A., Sisterson, M. S., Hannon, E. R., Stock, S. P., Carriere, Y., & Tabashnik, B. E. (2009). Effects of pink bollworm resistance to bacillus thuringiensis on phenoloxidase activity and susceptibility to entomopathogenic nematodes. Journal of Economic Entomology, 102(3), 1224-1232.
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  PMID: 19610442;Abstract: Widespread planting of crops genetically engineered to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) imposes selection on many key agricultural pests to evolve resistance to Bt. Fitness costs can slow the evolution of Bt resistance. We examined effects of entomopathogenic nematodes on fitness costs of Bt resistance in the pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), a major pest of cotton, Gossypium hirsutum L., in the southwestern United States that is currently controlled by transgenic cotton that produces Bt toxin CrylAc. We tested whether the entomopathogenic nematodes Steinernema riobrave Cabanillas, Poinar, and Raulston (Rhabditida: Steinernematidae) and Heterorhabditis bacteriophora Poinar (Rh-abditida: Heterorhabditidae) affected fitness costs of resistance to CrylAc in two laboratory-selected hybrid strains of pink bollworm reared on non-Bt cotton bolls. The nematode S. riobrave imposed a recessive fitness cost for one strain, and H. bacteriophora imposed a fitness cost affecting heterozygous resistant individuals for the other strain. Activity of phenoloxidase, an important component of insects' immune response, did not differ between Bt-resistant and Bt-susceptible families. This suggests phenoloxidase does not affect susceptibility to entomopathogenic nematodes in Bt-resistant pink bollworm. Additionally, phenoloxidase activity does not contribute to Bt resistance, as has been found in some species. We conclude that other mechanisms cause higher nematode-imposed mortality for pink bollworm with Bt resistance genes. Incorporation of nematode-imposed fitness costs into a spatially explicit simulation model suggests that entomopathogenic nematodes in non-Bt refuges could delay resistance by pink bollworm to Bt cotton. © 2009 Entomological Society of America.
• Showalter, A. M., Heuberger, S., Tabashnik, B. E., & Carrière, Y. (2009). A primer for using transgenic insecticidal cotton in developing countries. Journal of Insect Science, 9.
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  PMID: 19613464;PMCID: PMC3011844;Abstract: Many developing countries face the decision of whether to approve the testing and commercial use of insecticidal transgenic cotton and the task of developing adequate regulations for its use. In this review, we outline concepts and provide information to assist farmers, regulators and scientists in making decisions concerning this technology. We address seven critical topics: 1) molecular and breeding techniques used for the development of transgenic cotton cultivars, 2) properties of transgenic cotton cultivars and their efficacy against major insect pests, 3) agronomic performance of transgenic cotton in developing countries, 4) factors affecting transgene expression, 5) impact of gene flow between transgenic and non-transgenic cotton, 6) non-target effects of transgenic cotton, and 7) management of pest resistance to transgenic cotton. © This is an open access paper. We use the Creative Commons Attribution 3.0 license that permits unrestricted use, provided that the paper is properly attributed.
• Tabashnik, B. E., & Carrière, Y. (2009). Insect resistance to genetically modified crops. Environmental Impact of Genetically Modified Crops, 74-100.
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  Abstract: Crops genetically modified to produce crystal (Cry) toxins from Bacillus thuringiensis (Bt) for insect control can reduce reliance on conventional insecticides. Evolution of resistance to Bt toxins by insect populations is the primary threat to the continued success of this approach. Resistance of lepidopteran insects to Bt toxins in the Cry1A family commonly entails recessive inheritance and reduced toxin binding to midgut membrane target sites. Analysis of more than a decade of data from studies monitoring resistance to Bt maize and Bt cotton shows that field-evolved resistance was detected in some US populations of Helicoverpa zea, but not in populations of five other major lepidopteran pests from Australia, China, Spain and the USA: Helicoverpa armigera, Heliothis virescens, Ostrinia nubilalis, Pectinophora gossypiella and Sesamia nonagrioides. The resistance of H. zea to the Cry1Ac toxin in Bt cotton has not caused widespread crop failures, in part because insecticide sprays and two-toxin cotton producing Cry2Ab and Cry1Ac have been used to control this pest. Field-evolved resistance also has been reported recently to Bt corn producing Cry1Ab in Busseola fusca in South Africa and to Bt corn producing Cry1P in Spodoptera frugiperda in Puerto Rico.The documented field outcomes are consistent with projections from modelling based on the population genetic principles underlying the refuge strategy. In particular, H. zea was expected to evolve resistance faster than other pests because it has non-recessive inheritance of resistance to Cry1Ac. In other words, the concentration of Cry1Ac is not sufficient to kill a high percentage of hybrid progeny from matings between resistant and susceptible moths. The results suggest that refuges of non-Bt host plants have helped to delay resistance. © CAB International 2009.
• Tabashnik, B. E., Huang, F., Ghimire, M. N., Leonard, B. R., Siegfried, B. D., Rangasamy, M., Yang, Y., Wu, Y., Gahan, L. J., Heckel, D. G., Bravo, A., & Soberon, M. (2009). Efficacy of genetically modified Bt toxins against insects with different genetic mechanisms of resistance. NATURE BIOTECHNOLOGY, 29(12), 1128-U98.
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  Transgenic crops that produce Bacillus thuringiensis (Bt) toxins are grown widely for pest control(1), but insect adaptation can reduce their efficacy(2-6). The genetically modified Bt toxins Cry1AbMod and Cry1AcMod were designed to counter insect resistance to native Bt toxins Cry1Ab and Cry1Ac(7). Previous results suggested that the modified toxins would be effective only if resistance was linked with mutations in genes encoding toxin-binding cadherin proteins(7). Here we report evidence from five major crop pests refuting this hypothesis. Relative to native toxins, the potency of modified toxins was >350-fold higher against resistant strains of Plutella xylostella and Ostrinia nubilalis in which resistance was not linked with cadherin mutations. Conversely, the modified toxins provided little or no advantage against some resistant strains of three other pests with altered cadherin. Independent of the presence of cadherin mutations, the relative potency of the modified toxins was generally higher against the most resistant strains.
• Tabashnik, B. E., Rensburg, J. V., & Carríre, Y. (2009). Field-evolved insect resistance to Bt crops: Definition, theory, and data. Journal of Economic Entomology, 102(6), 2011-2025.
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  PMID: 20069826;Abstract: Transgenic crops producing Bacillus thuringiensis (Bt) toxins for insect pest control have been successful, but their efficacy is reduced when pests evolve resistance. Here we review the definition of field-evolved resistance, the relationship between resistance and field control problems, the theory underlying strategies for delaying resistance, and resistance monitoring methods. We also analyze resistance monitoring data from five continents reported in 41 studies that evaluate responses of field populations of 11 lepidopteran pests to four Bt toxins produced by Bt corn and cotton. After more than a decade since initial commercialization of Bt crops, most target pest populations remain susceptible, whereas field-evolved resistance has been documented in some populations of three noctuid moth species: Spodoptera frugiperda (J. E. Smith) to in Bt corn in Puerto Rico, Busseola fusca (Fuller) to Cry1Ab in Bt corn in South Africa, and Helicoverpa zea (Boddie) to Cry1Ac and Cry2Ab in Bt cotton in the southeastern United States. Field outcomes are consistent with predictions from theory, suggesting that factors delaying resistance include recessive inheritance of resistance, abundant refuges of non-Bt host plants, and two-toxin Bt crops deployed separately from one-toxin Bt crops. The insights gained from systematic analyses of resistance monitoring data may help to enhance the durability of transgenic insecticidal crops. We recommend continued use of the long-standing definition of resistance cited here and encourage discussions about which regulatory actions, if any, should be triggered by specific data on the magnitude, distribution, and impact of field-evolved resistance. © 2009 Entomological Society of America.
• Tabashnik, B., Carrière, Y., Showalter, A. M., Fabrick, J. A., Sollome, J., Ellers-Kirk, C., & Tabashnik, B. E. (2009). Cadherin gene expression and effects of Bt resistance on sperm transfer in pink bollworm. Journal of insect physiology, 55(11).
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  Cadherin proteins bind Bacillus thuringiensis (Bt) toxins in lepidopteran midguts but their inherent function remains unclear. In pink bollworm, Pectinophora gossypiella, three recessive mutations in a cadherin gene (BtR) are tightly linked with resistance to Bt toxin Cry1Ac. Here we examined patterns of transcription of this gene and the association between cadherin genotype and sperm transfer in pink bollworm. Cadherin RNA was most abundant in larvae, but was also found in adults and embryos. In fourth instar larvae, cadherin RNA was most abundant in the gut, yet its presence in the testes indicates a potential role in sperm production. Previously, we found reduced first-male paternity in pink bollworm males homozygous for cadherin mutations conferring resistance to Bt, when a resistant and susceptible male competed for access to a female. However, the number of offspring sired by resistant and susceptible males was similar without competition. Male Lepidoptera produce both fertile eupyrene sperm and anucleate, non-fertile apyrene sperm, suggesting that apyrene sperm may contribute to male reproductive success when sperm competition occurs. Accordingly, we hypothesized that cadherin-based resistance to Bt entails fitness costs that reduce apyrene sperm transfer. To test this hypothesis, we compared apyrene and eupyrene sperm transfer in males from four strains of pink bollworm. Transfer of apyrene and eupyrene sperm was lower in homozygous resistant than in susceptible males. Furthermore, homozygous resistant males weighed less than susceptible males, which could have diminished sperm transfer by resistant males directly, or via a positive association between male weight, spermatophore weight and sperm transfer. While data suggest that cadherin mutations induced a recessive fitness cost affecting apyrene sperm transfer, these mutations also generated recessive costs that affected other traits and could have lowered first-male paternity.
• Tabashnik, B., Gassmann, A. J., Fabrick, J. A., Sisterson, M. S., Hannon, E. R., Stock, S. P., Carrière, Y., & Tabashnik, B. E. (2009). Effects of pink bollworm resistance to Bacillus thuringiensis on phenoloxidase activity and susceptibility to entomopathogenic nematodes. Journal of economic entomology, 102(3).
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  Widespread planting of crops genetically engineered to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) imposes selection on many key agricultural pests to evolve resistance to Bt. Fitness costs can slow the evolution of Bt resistance. We examined effects of entomopathogenic nematodes on fitness costs of Bt resistance in the pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), a major pest of cotton, Gossypium hirsutum L., in the southwestern United States that is currently controlled by transgenic cotton that produces Bt toxin Cry1Ac. We tested whether the entomopathogenic nematodes Steinernema riobrave Cabanillas, Poinar, and Raulston (Rhabditida: Steinernematidae) and Heterorhabditis bacteriophora Poinar (Rhabditida: Heterorhabditidae) affected fitness costs of resistance to Cry1Ac in two laboratory-selected hybrid strains of pink bollworm reared on non-Bt cotton bolls. The nematode S. riobrave imposed a recessive fitness cost for one strain, and H. bacteriophora imposed a fitness cost affecting heterozygous resistant individuals for the other strain. Activity of phenoloxidase, an important component of insects' immune response, did not differ between Bt-resistant and Bt-susceptible families. This suggests phenoloxidase does not affect susceptibility to entomopathogenic nematodes in Bt-resistant pink bollworm. Additionally, phenoloxidase activity does not contribute to Bt resistance, as has been found in some species. We conclude that other mechanisms cause higher nematode-imposed mortality for pink bollworm with Bt resistance genes. Incorporation of nematode-imposed fitness costs into a spatially explicit simulation model suggests that entomopathogenic nematodes in non-Bt refuges could delay resistance by pink bollworm to Bt cotton.
• Zhang, H., Tian, W., Zhao, J., Jin, L., Yang, J., Liu, C., Yang, Y., Wu, S., Wu, K., Cui, J., Tabashnik, B. E., & Wu, Y. (2009). Diverse genetic basis of field-evolved resistance to Bt cotton in cotton bollworm from China. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 109(26), 10275-10280.
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  Evolution of pest resistance reduces the efficacy of insecticidal proteins from Bacillus thuringiensis (Bt) used in sprays or in transgenic crops. Although several pests have evolved resistance to Bt crops in the field, information about the genetic basis of field-evolved resistance to Bt crops has been limited. In particular, laboratory-selected resistance to Bt toxin Cry1Ac based on recessive mutations in a gene encoding a toxin-binding cadherin protein has been identified in three major cotton pests, but previous work has not determined if such mutations are associated with field-selected resistance to Bt cotton. Here we show that the most common resistance alleles in field populations of cotton bollworm, Helicoverpa armigera, selected with Bt cotton in northern China, had recessive cadherin mutations, including the deletion mutation identified via laboratory selection. However, unlike all previously studied cadherin resistance alleles, one field-selected cadherin resistance allele conferred nonrecessive resistance. We also detected nonrecessive resistance that was not genetically linked with the cadherin locus. In field-selected populations, recessive cadherin alleles accounted for 75-84% of resistance alleles detected. However, most resistance alleles occurred in heterozygotes and 59-94% of resistant individuals carried at least one nonrecessive resistance allele. The results suggest that resistance management strategies must account for diverse resistance alleles in field-selected populations, including nonrecessive alleles.
• Crowder, D. W., Ellers-Kirk, C., Yafuso, C. M., Dennehy, T. J., Degain, B. A., Harpold, V. S., Tabashnik, B. E., & Carrière, Y. (2008). Inheritance of resistance to pyriproxyfen in Bemisia tabaci (Hemiptera: Aleyrodidae) males and females (B biotype). Journal of Economic Entomology, 101(3), 927-932.
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  PMID: 18613596;Abstract: We evaluated effects of the insect growth regulator pyriproxyfen on Bemisia tabaci (Gennadius) (Bbiotype) (Hemiptera Aleyrodidae) males and females in laboratory bioassays. Insects were treated with pyriproxyfen as either eggs or nymphs. In all tests, the LC50 for a laboratory-selected resistant strain was at least 620 times greater than for an unselected susceptible strain. When insects were treated as eggs, survival did not differ between males and females of either strain. When insects were treated as nymphs, survival did not differ between susceptible males and susceptible females, but resistant males had higher mortality than resistant females. The dominance of resistance decreased as pyriproxyfen concentration increased. Resistance was partially or completely dominant at the lowest concentration tested and completely recessive at the highest concentration tested. Hybrid female progeny from reciprocal crosses between the susceptible and resistant strains responded alike in bioassays; thus, maternal effects were not evident. Rapid evolution of resistance to pyriproxyfen could occur if individuals in field populations had resistance with traits similar to those of the laboratory-selected strain examined here. © 2008 Entomological Society of America.
• Gassmann, A. J., Stock, S. P., Sisterson, M. S., Carrière, Y., & Tabashnik, B. E. (2008). Synergism between entomopathogenic nematodes and Bacillus thuringiensis crops: Integrating biological control and resistance management. Journal of Applied Ecology, 45(3), 957-966.
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  Abstract: 1. The past decade has witnessed a continual increase in the use of crops genetically modified to produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). This presents the challenge of designing agricultural systems to manage pests and the evolution of resistance to Bt. 2. We tested whether entomopathogenic nematodes might act synergistically with Bt crops by killing pests in non-Bt refuges and by increasing the fitness costs of resistance to Bt. We also tested whether insect mortality and fitness costs were affected by the cotton phytochemical gossypol. 3. The entomopathogenic nematode Steinernema riobrave increased the fitness cost of Bt resistance, indicating that its presence in refuges may slow pest adaptation to Bt crops. No effect on fitness costs was detected for the nematode Heterorhabditis bacteriophora. Gossypol did not alter nematode-imposed fitness costs. 4. Simulation modelling supported the hypothesis that nematodes in refuges may slow resistance evolution. 5. The effects of gossypol on insect mortality from nematodes and nematode reproduction differed between nematode species. Gossypol increased insect mortality caused by H. bacteriophora but did not affect mortality caused by S. riobrave. Gossypol enhanced reproduction of H. bacteriophora and decreased reproduction of S. riobrave. 6. Synthesis and applications. Our results point to the value of developing integrated pest management strategies for Bt crops that include non-Bt refuges in which entomopathogenic nematodes are used as a pest-management agent. Because entomopathogenic nematodes can magnify fitness costs of Bt resistance, their presence in refuges may delay resistance by pests to Bt crops. Moreover, entomopathogenic nematodes can serve as biological control agents thereby decreasing dependence on conventional insecticides to manage pest populations in refuges. © 2008 The Authors.
• Heuberger, S., Ellers-Kirk, C., Yafuso, C., Gassmann, A. J., Tabashnik, B. E., Dennehy, T. J., & Carrière, Y. (2008). Effects of refuge contamination by transgenes on Bt resistance in pink bollworm (Lepidoptera: Gelechiidae). Journal of Economic Entomology, 101(2), 504-514.
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  PMID: 18459418;Abstract: Refuges of non-Bacillus thuringiensis (Bt) cotton, Gossypium hirsutum L., are used to delay Bt resistance in pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), a pest that eats cotton seeds. Contamination of refuges by Bt transgenes could reduce the efficacy of this strategy. Previously, three types of contamination were identified in refuges: 1 ) homozygous Bt cotton plants, with 100% of their seeds producing the Bt toxin Cry1Ac; 2) hemizygous Bt plants with 70-80% of their seeds producing Cry1Ac; and 3) non-Bt plants that outcrossed with Bt plants, resulting in bolls with Cry1Ac in 12-17% of their seeds. Here, we used laboratory bioassays to examine the effects of Bt contamination on feeding behavior and survival of pink bollworm that were resistant (rr), susceptible (ss), or heterozygous for resistance (rs) to Cry1Ac. In choice tests, rr and rs larvae did not differ from ss in preference for non-Bt versus Bt seeds. Survival of rr and rs also did not differ from ss on artificial outcrossed bolls (a mixture of 20% Bt and 80% non-Bt cotton seeds). On artificial hemizygous Bt bolls (70% Bt seeds) and homozygous Bt bolls (100% Bt seeds), rr had higher survival than ss, although rs and ss did not differ. In a simulation model, levels of refuge contamination observed in the field had negligible effects on resistance evolution in pink bollworm. However, in hypothetical simulations where contamination conferred a selective advantage to rs over ss individuals in refuges, resistance evolution was accelerated. © 2008 Entomological Society of America.
• Heuberger, S., Yafuso, C., Degrandi-Hoffman, G., Tabashnik, B. E., Carrière, Y., & Dennehy, T. J. (2008). Outcrossed cottonseed and adventitious Bt plants in Arizona refuges. Environmental Biosafety Research, 7(2), 87-96.
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  PMID: 18549770;Abstract: Outcrossing of non-Bt cotton (Gossypium hirsutum (L.)) in refuges by transgenic cultivars could reduce the efficacy of refuges for delaying resistance in seed-feeding pests. Based on reports that outcrossing decreased as distance from Bt cotton increased in small-scale studies, we hypothesized that increasing refuge width or distance from Bt fields would reduce outcrossing. In a large-scale study in Arizona, we quantified Bt seed in refuges of experimental and commercial fields, comparing outcrossing between in-field (narrow) and external (wide) refuges and among rows of refuges at various distances from Bt fields. Some refuges, including those in tightly controlled experimental plots, contained up to 8% adventitious Bt plants. Some, but not all, Bt plants likely resulted Bt from seed in the non-Bt seed bags. We did not detect a difference in outcrossing between in-field and external refuges. However, statistical power was low because outcrossing was low ( 0.4% of seeds) in both treatments. Higher outcrossing levels ( 4.6% of seeds) were observed in the studies measuring outcrossing at various distances from Bt fields, yet outcrossing did not decrease as the distance from Bt fields increased. We hypothesize that Bt plants in refuges cross-pollinated surrounding non- plants, overshadowing the expected association between distance from Bt fields and outcrossing. © 2008 ISBR EDP Sciences.
• Marvier, M., Carrière, Y., Ellstrand, N., Gepts, P., Kareiva, P., Rosi-Marshall, E., Tabashnik, B. E., & Wolfenbarger, L. L. (2008). Harvesting data from genetically engineered crops. Science, 320(5875), 452-453.
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  PMID: 18436759;Abstract: More than a billion acres have been planted with genetically engineered crops in the USA since 1996, but we do not fully know their ecological costs and benefits.
• Tabashnik, B. (2008). Insect shows resistance to Bt crops. Appropriate Technology, 35(1), 49-50.
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  Abstract: A genetically modified cotton and maize crops were developed in 1996, that incorporates bacillus thuringiensis (Bt) to control pests like cotton bollworm. However, a team of University of Arizona entomologists discovered in recent study showed that the bollworms have now resistance to Bt crops. It is found that major pests attacking Bt crops have not evolved resistance. Addressing this issue, the UA researchers delay the resistance, where non-Bt crops are planted near Bt crops to provide "refuges" for susceptible pests. As a result, it showed that the bollworm resistance evolved fastest in the states with lowest abundance of refuges.
• Tabashnik, B. E. (2008). Delaying insect resistance to transgenic crops. Proceedings of the National Academy of Sciences of the United States of America, 105(49), 19029-19030.
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  PMID: 19057017;PMCID: PMC2614707;
• Tabashnik, B. E., & Carriére, Y. (2008). Evolution of insect resistance to transgenic plants. Specialization, Speciation, and Radiation: The Evolutionary Biology of Herbivorous Insects, 267-279.
• Tabashnik, B. E., Gassman, A. J., Crowder, D. W., & Carrière, Y. (2008). Reply to Field-evolved resistance to Bt toxins (Nature Biotechnology (2008) 26 (1074-1076)). Nature Biotechnology, 26(12), 1383-.
• Tabashnik, B. E., Gassmann, A. J., Crowder, D. W., & Carrière, Y. (2008). Insect resistance to Bt crops: Evidence versus theory. Nature Biotechnology, 26(2), 199-202.
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  PMID: 18259177;Abstract: Evolution of insect resistance threatens the continued success of transgenic crops producing Bacillus thuringiensis (Bt) toxins that kill pests. The approach used most widely to delay insect resistance to Bt crops is the refuge strategy, which requires refuges of host plants without Bt toxins near Bt crops to promote survival of susceptible pests. However, large-scale tests of the refuge strategy have been problematic. Analysis of more than a decade of global monitoring data reveals that the frequency of resistance alleles has increased substantially in some field populations of Helicoverpa zea, but not in five other major pests in Australia, China, Spain and the United States. The resistance of H. zea to Bt toxin Cry1Ac in transgenic cotton has not caused widespread crop failures, in part because other tactics augment control of this pest. The field outcomes documented with monitoring data are consistent with the theory underlying the refuge strategy, suggesting that refuges have helped to delay resistance. © 2008 Nature Publishing Group.
• Tabashnik, B., & Tabashnik, B. E. (2008). Delaying insect resistance to transgenic crops. Proceedings of the National Academy of Sciences of the United States of America, 105(49).
• Carrière, Y., Ellers-Kirk, C., Biggs, R. W., Sims, M. A., Dennehy, T. J., & Tabashnik, B. E. (2007). Effects of resistance to Bt cotton on diapause in the pink bollworm, Pectinophora gossypiella. Journal of Insect Science, 7.
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  PMID: 20345287;PMCID: PMC2999450;Abstract: Fitness costs associated with resistance to Bacillus thuringiensis (Bt) crops are expected to delay the evolution of resistance. In a previous study where pink bollworm, Pectinophora gossypiella (Lepidoptera: Gelechiidae), larvae overwintered in outdoor insectaries, individuals from Bt-resistant strains had lower survival than individuals from Bt-susceptible strains or F1 progeny from crosses between resistant and susceptible adults. To investigate the physiological basis of such recessive cost, diapause duration was experimentally manipulated in the laboratory. Compared to a Bt-susceptible strain and F1 progeny, we hypothesized that Bt-resistant strains could exhibit a lower propensity or intensity of diapause, faster weight loss during overwintering, lower initial weight of diapausing larvae, and reduced longevity of moths emerging from diapause. Results were as expected for initial weight of diapausing larvae and longevity of overwintered male moths or female moths remaining in diapause for a short period. However, a higher diapause induction and intensity and slower weight loss occurred in F1 progeny and Bt-resistant strains than in a Bt-susceptible strain. Moreover, F1 progeny had greater overwintering survival than the Bt-resistant and Bt-susceptible strains, and F1 female moths had the greatest longevity after sustaining long diapausing periods. All of these unexpected results may be explained by pleiotropic effects of resistance to Bt cotton that increased the strength of diapause in the F1 progeny and Bt-resistant strains. Incomplete resistance was reflected in disadvantages suffered by Bt-resistant individuals feeding on a Bt diet instead of a non-Bt diet, including lower diapause propensity, lower diapause intensity and reduced longevity of overwintered male moths. While this study suggests that the evolution of resistance to Bt cotton and feeding on a Bt diet in Bt-resistant individuals have pervasive effects on several traits associated with diapause, further field experiments are needed to elucidate the basis of the overwintering cost in the pink bollworm.
• Fabrick, J. A., & Tabashnik, B. E. (2007). Binding of Bacillus thuringiensis toxin Cry1Ac to multiple sites of cadherin in pink bollworm. Insect Biochemistry and Molecular Biology, 37(2), 97-106.
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  PMID: 17244539;Abstract: Toxins from Bacillus thuringiensis (Bt) are widely used for pest control. In particular, Bt toxin Cry1Ac produced by transgenic cotton kills some key lepidopteran pests. We found that Cry1Ac binds to recombinant peptides corresponding to extracellular regions of a cadherin protein (BtR) in a major cotton pest, pink bollworm (Pectinophora gossypiella) (PBW). In conjunction with previous results showing that PBW resistance to Cry1Ac is linked with mutations in the BtR gene, the results reported here support the hypothesis that BtR is a receptor for Cry1Ac in PBW. Similar to other lepidopteran cadherins that bind Bt toxins, BtR has at least two Cry1Ac-binding domains in cadherin-repeat regions 10 and 11, which are immediately adjacent to the membrane proximal region. However, unlike cadherins from Manduca sexta and Bombyx mori, toxin binding was not seen in regions more distal from the membrane proximal region. We also found that both the protoxin and activated toxin forms of Cry1Ac bound to recombinant BtR fragments, suggesting that Cry1Ac activation may occur either before or after receptor binding. © 2006 Elsevier Ltd. All rights reserved.
• Heckel, D. G., Heckel, D. G., Gahan, L. J., Gahan, L. J., Baxter, S. W., Baxter, S. W., Zhao, J., Zhao, J., Shelton, A. M., Shelton, A. M., Gould, F., Gould, F., Tabashnik, B. E., & Tabashnik, B. E. (2007). The diversity of Bt resistance genes in species of Lepidoptera. Journal of Invertebrate Pathology, 95(3), 192-197.
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  PMID: 17482643;Abstract: Although the mode of action of Cry1A toxins produced by Bacillus thuringiensis is fairly well understood, knowledge of the molecular mechanisms by which lepidopteran species have evolved resistance to them is still in its infancy. The most common type of resistance has been called "Mode 1" and is characterized by recessive inheritance, >500-fold resistance to and reduced binding by at least one Cry1A toxin, and negligible cross-resistance to Cry1C. In three lepidopteran species, Heliothis virescens, Pectinophora gossypiella, and Helicoverpa armigera, Mode 1 resistance is caused by mutations in a toxin-binding 12-cadherin-domain protein expressed in the larval midgut. These mutations all interrupt the primary sequence of the protein and prevent its normal localization in the membrane, presumably removing a major toxic binding target of the Cry1A toxins. In Plutella xylostella, however, Mode 1 resistance appears to be caused by a different genetic mechanism, as Cry1A resistance is unlinked to the cadherin gene. Mapping studies in H. virescens have detected an additional major Cry1A resistance gene, which on the basis of comparative linkage mapping is distinct from the one in P. xylostella. An additional resistance mechanism supported by genetic data involves a protoxin-processing protease in Plodia interpunctella, and this is likely to be different from the genes mapped in Plutella and Heliothis. Thus, resistance to Cry1A toxins in species of Lepidoptera has a complex genetic basis, with at least four distinct, major resistance genes of which three are mapped in one or more species. The connection between resistance genes and the mechanisms they encode remains a challenging task to elucidate. © 2007.
• Sisterson, M. S., Biggs, R. W., Manhardt, N. M., Carrière, Y., Dennehy, T. J., & Tabashnik, B. E. (2007). Effects of transgenic Bt cotton on insecticide use and abundance of two generalist predators. Entomologia Experimentalis et Applicata, 124(3), 305-311.
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  Abstract: Considerable effort has been expended to determine if crops genetically engineered to produce Bacillus thuringiensis (Bt) toxins harm non-target arthropods. However, if Bt crops kill target pests and thereby reduce insecticide use, this could benefit some non-target arthropods. We analyzed data from 21 commercial cotton fields in Arizona to test the effects of Bt cotton on insecticide use and abundance of two non-target arthropods, the generalist predators Chrysoperla carnea Stephens (Neuroptera: Chrysopidae) and Orius tristicolor (White) (Heteroptera: Anthocoridae). The number of insecticide sprays was more than double for non-Bt cotton compared with Bt cotton that produced Cry1Ac. The abundance of both predators was negatively associated with the number of insecticide sprays, although significantly so for only one of two sampling periods for each species tested. With the effects of insecticides statistically removed, field type (Bt or non-Bt cotton) did not affect the abundance of either predator. Accordingly, without adjusting for the effects of insecticide sprays, the abundance of C. carnea was higher in Bt cotton fields than in non-Bt cotton fields, but significantly so during only one of two sampling periods. The abundance of O. tristicolor did not differ between field types, even without adjusting for effects of insecticide sprays. The results indicate that Bt crops can affect insecticide use, which in turn can affect the relative abundance of non-target arthropods in Bt and non-Bt fields. Thus, environmental impact assessment should incorporate analysis of the effects of transgenic crops on management practices, as well as evaluation of the direct effects of such crops. © 2007 The Authors.
• Sisterson, M. S., Carrière, Y., Dennehy, T. J., & Tabashnik, B. E. (2007). Nontarget effects of transgenic insecticidal crops: Implications of source-sink population dynamics. Environmental Entomology, 36(1), 121-127.
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  PMID: 17349125;Abstract: Widespread planting of transgenic insecticidal (TI) crops for pest control has raised concerns about potential harm to nontarget arthropods. Because the first generation of TI crops produce single Bacillus thuringiensis (Bt) toxins causing little or no harm to most nontarget arthropods, they are not likely to cause such negative effects. However, varieties of transgenic crops with multiple Bt toxins or novel toxins might be more harmful to nontarget arthropods. Field studies assessing nontarget effects typically compare the relative abundance of nontarget arthropods in TI crop fields to non-TI crop fields. However, for nontarget arthropods that are killed by TI crops, such analyses may miss important effects. Results from simulations of a spatially explicit population dynamics model show that large-scale planting of TI crops could cause three types of negative effects on nontarget arthropods that suffer mortality caused by TI crops: (1) lower abundance in TI fields than non-TI fields with little or no effect on abundance in non-TI fields, (2) lower abundance in TI fields than non-TI fields and decreased abundance in non-TI fields, and (3) loss of the arthropod from TI and non-TI fields. Simulation results show that factors increasing the potential for negative effects of TI crops on nontarget arthropods in non-TI fields are low reproduction, high emigration, high adoption of TI crops, high mortality in TI fields, insecticide sprays, and rotation of TI and non-TI fields. The results suggest that risk assessment should consider the regional distribution of transgenic crops and the life history traits of nontarget arthropods to identify the most vulnerable regions and nontarget species.
• Soberón, M., Pardo-López, L., López, I., Gómez, I., Tabashnik, B. E., & Bravo, A. (2007). Engineering modified Bt toxins to counter insect resistance. Science, 318(5856), 1640-1642.
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  PMID: 17975031;Abstract: The evolution of insect resistance threatens the effectiveness of Bacillus thuringiensis (Bt) toxins that are widely used in sprays and transgenic crops. Resistance to Bt toxins in some insects is linked with mutations that disrupt a toxin-binding cadherin protein. We show that susceptibility to the Bt toxin Cry1Ab was reduced by cadherin gene silencing with RNA interference in Manduca sexta, confirming cadherin's role in Bt toxicity. Native Cry1A toxins required cadherin to form oligomers, but modified Cry1A toxins lacking one a-helix did not. The modified toxins killed cadherin-silenced M. sexta and Bt-resistant Pectinophora gossypiella that had cadherin deletion mutations. Our findings suggest that cadherin promotes Bt toxicity by facilitating toxin oligomerization and demonstrate that the modified Bt toxins may be useful against pests resistant to standard Bt toxins.
• Tabashnik, B., Carrière, Y., Ellers-Kirk, C., Biggs, R. W., Sims, M. A., Dennehy, T. J., & Tabashnik, B. E. (2007). Effects of resistance to Bt cotton on diapause in the pink bollworm, Pectinophora gossypiella. Journal of insect science (Online), 7.
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  Fitness costs associated with resistance to Bacillus thuringiensis (Bt) crops are expected to delay the evolution of resistance. In a previous study where pink bollworm, Pectinophora gossypiella (Lepidoptera: Gelechiidae), larvae overwintered in outdoor insectaries, individuals from Bt-resistant strains had lower survival than individuals from Bt-susceptible strains or F1 progeny from crosses between resistant and susceptible adults. To investigate the physiological basis of such recessive cost, diapause duration was experimentally manipulated in the laboratory. Compared to a Bt-susceptible strain and F1 progeny, we hypothesized that Bt-resistant strains could exhibit a lower propensity or intensity of diapause, faster weight loss during overwintering, lower initial weight of diapausing larvae, and reduced longevity of moths emerging from diapause. Results were as expected for initial weight of diapausing larvae and longevity of overwintered male moths or female moths remaining in diapause for a short period. However, a higher diapause induction and intensity and slower weight loss occurred in F1 progeny and Bt-resistant strains than in a Bt-susceptible strain. Moreover, F1 progeny had greater overwintering survival than the Bt-resistant and Bt-susceptible strains, and F1 female moths had the greatest longevity after sustaining long diapausing periods. All of these unexpected results may be explained by pleiotropic effects of resistance to Bt cotton that increased the strength of diapause in the F1 progeny and Bt-resistant strains. Incomplete resistance was reflected in disadvantages suffered by Bt-resistant individuals feeding on a Bt diet instead of a non-Bt diet, including lower diapause propensity, lower diapause intensity and reduced longevity of overwintered male moths. While this study suggests that the evolution of resistance to Bt cotton and feeding on a Bt diet in Bt-resistant individuals have pervasive effects on several traits associated with diapause, further field experiments are needed to elucidate the basis of the overwintering cost in the pink bollworm.
• Tabashnik, B., Sisterson, M. S., Carrière, Y., Dennehy, T. J., & Tabashnik, B. E. (2007). Nontarget effects of transgenic insecticidal crops: implications of source-sink population dynamics. Environmental entomology, 36(1).
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  Widespread planting of transgenic insecticidal (TI) crops for pest control has raised concerns about potential harm to nontarget arthropods. Because the first generation of TI crops produce single Bacillus thuringiensis (Bt) toxins causing little or no harm to most nontarget arthropods, they are not likely to cause such negative effects. However, varieties of transgenic crops with multiple Bt toxins or novel toxins might be more harmful to nontarget arthropods. Field studies assessing nontarget effects typically compare the relative abundance of nontarget arthropods in TI crop fields to non-TI crop fields. However, for nontarget arthropods that are killed by TI crops, such analyses may miss important effects. Results from simulations of a spatially explicit population dynamics model show that large-scale planting of TI crops could cause three types of negative effects on nontarget arthropods that suffer mortality caused by TI crops: (1) lower abundance in TI fields than non-TI fields with little or no effect on abundance in non-TI fields, (2) lower abundance in TI fields than non-TI fields and decreased abundance in non-TI fields, and (3) loss of the arthropod from TI and non-TI fields. Simulation results show that factors increasing the potential for negative effects of TI crops on nontarget arthropods in non-TI fields are low reproduction, high emigration, high adoption of TI crops, high mortality in TI fields, insecticide sprays, and rotation of TI and non-TI fields. The results suggest that risk assessment should consider the regional distribution of transgenic crops and the life history traits of nontarget arthropods to identify the most vulnerable regions and nontarget species.
• Alves, A. P., Spencer, T. A., Tabashnik, B. E., & Siegfried, B. D. (2006). Inheritance of resistance to the Cry1Ab Bacillus thuringiensis toxin in Ostrinia nubilalis (Lepidoptera: Crambidae). Journal of Economic Entomology, 99(2), 494-501.
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  PMID: 16686152;Abstract: Laboratory selection with Cry1Ab, the predominant Bacillus thuringiensis (Bt) toxin in transgenic corn, Zea mays L., produced >1000-fold resistance in two laboratory strains of European corn borer, Ostrinia nubilalis (Hübner). We tested the offspring of various crosses to determine the mode of inheritance of resistance to Cry1Ab. Patterns of inheritance of resistance were similar in the two resistant strains. The progeny of reciprocal F 1 crosses (resistant male x susceptible female and vice versa) responded alike in bioassays, indicating autosomal inheritance. The median lethal concentrations (LC50 values) of F1 were intermediate between the resistant and susceptible parents, indicating approximately additive inheritance. However, the dominance of resistance increased as the concentration of Cry1Ab decreased. Analysis of progeny from backcrosses (F1 x susceptible strain) suggests that resistance was controlled by more than one locus. In particular, the fit of observed to expected mortality improved as the number of putative loci increased from 1 to 10. The polygenic nature of resistance in these two laboratory strains suggests that major genes for resistance to Cry1Ab were not common in the founding populations of O. nubilalis. A low initial frequency of major genes for Cry1Ab resistance might be an important factor in delaying evolution of resistance to Bt corn in this pest. © 2006 Entomological Society of America.
• Carrière, Y., Ellers-Kirk, C., Biggs, R. W., Nyboer, M. E., Unnithan, G. C., Dennehy, T. J., & Tabashnik, B. E. (2006). Cadherin-based resistance to Bacillus thuringiensis cotton in hybrid strains of pink bollworm: Fitness costs and incomplete resistance. Journal of Economic Entomology, 99(6), 1925-1935.
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  PMID: 17195656;Abstract: Recessive resistance to Bacillus thuringiensis (Bt) cotton, Gossypium hirsutum L., in laboratory-selected strains of pink bollworm, Pectinophora gossypiella (Saunders), is associated with three resistance alleles (r1, r2, and r3) of a cadherin gene. Previous experiments based on measurement of fitness components in Bt-resistant and Bt-susceptible strains revealed that fitness costs and incomplete resistance are associated with resistance. Here, we used two hybrid strains of pink bollworm, each containing a mixture of susceptible and resistant individuals, and polymerase chain reaction (PCR) amplifications to test the association between cadherin genotype and fitness components for individuals sharing a common genetic background. All survivors on Bt cotton had two r alleles, confirming that recessive cadherin alleles are tightly linked with resistance to Bt cotton. On non-Bt cotton, significantly greater developmental time for rr than ss larvae indicated a recessive fitness cost, but costs did not affect survival or pupal weight. Incomplete resistance was manifested as longer developmental time, lower survival, and smaller pupal weight in rr individuals developing on Bt cotton compared with non-Bt cotton. As in previous experiments, no significant variation in performance on Bt cotton was detected among rr genotypes. However, a meta-analysis of data from seven experiments revealed that survival on Bt cotton relative to non-Bt cotton was lower in r2r3 and higher in r1r2 compared with the other rr genotypes. Assessment of fitness components associated with cadherin genotypes in hybrid strains of pink bollworm confirms that recessive resistance to Bt cotton is associated with recessive fitness costs and incomplete resistance. © 2006 Entomological Society of America.
• Carrière, Y., Nyboer, M. E., Ellers-Kirk, C., Sollome, J., Colletto, N., Antilla, L., Dennehy, T. J., Staten, R. T., & Tabashnik, B. E. (2006). Effect of resistance to Bacillus thuringiensis cotton on pink bollworm (Lepidoptera: Gelechiidae) response to sex pheromone. Journal of Economic Entomology, 99(3), 946-953.
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  PMID: 16813335;Abstract: Fitness costs associated with resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) could reduce male response to pheromone traps. Such costs would cause underestimation of resistance frequency if monitoring was based on analysis of males caught in pheromone traps. To develop a DNA-based resistance monitoring program for pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), we compared the response to pheromone traps of males with and without cadherin alleles associated with resistance to Bt cotton (Gossypium hirsutum L.). When irradiated males from two hybrid laboratory strains with an inter-mediate frequency of resistance alleles were released in large field cages, the probability of capture in pheromone traps was not lower for males with resistance alleles than for males without resistance alleles. These results suggest that analysis of trapped males would not underestimate the frequency of resistance. As the time males spent in traps in the field increased from 3 to 15 d, the success of DNA amplification declined from 100 to 30%. Thus, the efficiency of a DNA-based resistance monitoring program would be improved by analyzing males remaining in traps for 3 d or less. © 2006 Entomological Society of America.
• Ferry, N., Mulligan, E. A., Stewart, C. N., Tabashnik, B. E., Port, G. R., & M., A. (2006). Prey-mediated effects of transgenic canola on a beneficial, non-target, carabid beetle. Transgenic Research, 15(4), 501-514.
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  PMID: 16906450;Abstract: Transgenic plants producing insecticidal proteins from Bacillus thuringiensis (Bt) can control some major insect pests and reduce reliance on sprayed insecticides. However, large scale adoption of this technology has raised concerns about potential negative effects, including evolution of pest resistance to Bt toxins, transgene flow from Bt crops to other plants, and harm to non-target beneficial organisms. Furthermore, concern has also been expressed over the effects this technology may have on biodiversity in general. Ecologically relevant risk assessment is therefore required (Risk = Hazard x Exposure). Transgenic plants that produce Bt toxins to kill insect pests could harm beneficial predators. This might occur directly by transmission of toxin via prey, or indirectly by toxin-induced reduction in prey quality (Hazard). To test these hypotheses, we determined the effects of Bt-producing canola on a predatory ground beetle (Pterostichus madidus) fed larvae of diamondback moth (Plutella xylostella) that were either susceptible or resistant to the Bt toxin. Survival, weight gain, and adult reproductive fitness did not differ between beetles fed prey reared on Bt-producing plants and those fed prey from control plants. Furthermore, while Bt-resistant prey was shown to deliver high levels of toxin to the beetle when they were consumed, no significant impact upon the beetle was observed. Subsequent investigation showed that in choice tests (Exposure), starved and partially satiated female beetles avoided Bt-fed susceptible prey, but not Bt-fed resistant prey. However, in the rare cases when starved females initially selected Bt-fed susceptible prey, they rapidly rejected them after beginning to feed. This prey type was shown to provide sufficient nutrition to support reproduction in the bioassay suggesting that Bt-fed susceptible prey is acceptable in the absence of alternative prey, however adults possess a discrimination ability based on prey quality. These results suggest that the direct effects of Bt-producing canola on predator life history was minimal, and that predators' behavioural preferences may mitigate negative indirect effects of reduced quality of prey caused by consumption of Bt-producing plants. The results presented here therefore suggest that cultivation of Bt canola may lead to conservation of non-target predatory and scavenging organisms beneficial in pest control, such as carabids, and may therefore provide more sustainable agricultural systems than current practices. In addition, minimal impacts on beneficial carabids in agro-ecosystems suggest that Bt canola crops are likely to be compatible with integrated pest management (IPM) systems. © Springer Science+Business Media B.V. 2006.
• Gassmann, A. J., Stock, S. P., Carrière, Y., & Tabashnik, B. E. (2006). Effect of entomopathogenic nematodes on the fitness cost of resistance to Bt toxin Cry1Ac in pink bollworm (Lepidoptera: Gelechiidae). Journal of Economic Entomology, 99(3), 920-926.
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  PMID: 16813331;Abstract: The widespread use of crop plants genetically engineered to produce toxins from the bacterium Bacillus thuringiensis (Bt) imposes selection on insect populations to evolve resistance. The pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), is a major pest of cotton in the southwestern United States that is currently controlled with transgenic cotton that produces Bt toxin Cry1Ac. Previously reported theoretical work suggests that, in conjunction with a high dose/refuge strategy, fitness costs of Bt resistance can slow or prevent the evolution of resistance. We report here that the entomopathogenic nematode Steinernema riobrave (Rhabditida: Steinernematidae) increased the fitness cost of resistance to Cry1Ac in P. gossypiella. Mortality of P. gossypiella from fourth instar to adult eclosion was significantly higher for a Bt-resistant strain than a susceptible strain in tests with two to 14 infective juveniles of S. riobrave per larva, but it did not differ between strains when nematodes were absent. Nematodes established in P. gossypiella larvae at all concentrations tested, and nematode reproduction in infected P. gossypiella larvae occurred at nematode concentrations of four to 14 infective juveniles per larva. Our results suggest that incorporation of entomopathogenic nematodes into an integrated resistance management strategy could help to delay pest resistance to Bt toxins. © 2006 Entomological Society of America.
• Tabashnik, B. E., Biggs, R. W., Fabrick, J. A., Gassmann, A. J., Dennehy, T. J., Carrière, Y., & Morin, S. (2006). High-level resistance to Bacillus thuringiensis toxin CrylAc and cadherin genotype in pink bollworm. Journal of Economic Entomology, 99(6), 2125-2131.
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  PMID: 17195682;Abstract: Resistance to transgenic cotton, Gossypium hirsutum L., producing Bacillus thuringiensis (Bt) toxin CrylAc is linked with three recessive alleles of a cadherin gene in laboratory-selected strains of pink bollworm, Pectinophora gossypiella (Saunders), a major cotton pest. Here, we analyzed a strain (MOV97-R) with a high frequency of cadherin resistance alleles, a high frequency of resistance to 10 μg of CrylAc per milliliter of diet, and an intermediate frequency of resistance to 1,000 μg of CrylAc per ml of diet. We selected two strains for increased resistance by exposing larvae from MOV97-R to diet with 1,000 μg of CrylAc per ml of diet. In both selected strains, two to three rounds of selection increased survival at 1,000 μg of CrylAc per ml of diet to at least 76%, indicating genetic variation in survival at this high concentration and yielding >4,300-fold resistance relative to a susceptible strain. Variation in cadherin genotype did not explain variation in survival at 1,000 μg of CrylAc per ml of diet, implying that one or more other loci affected survival at this concentration. This conclusion was confirmed with results showing that when exposure to CrylAc stopped, survival at 1,000 μg of CrylAc per ml of diet dropped substantially, but survival at 10 μg CrylAc per ml of diet remained close to 100% and all survivors had two cadherin resistance alleles. Although survival at 1,000 μg of CrylAc per ml of diet is not required for resistance to Bt cotton, understanding how genes other than cadherin confer increased survival at this high concentration may reveal novel mechanisms of resistance. © 2006 Entomological Society of America.
• Tabashnik, B. E., Fabrick, J. A., Henderson, S., Biggs, R. W., Yafuso, C. M., Nyboer, M. E., Manhardt, N. M., Coughlin, L. A., Sollome, J., Carrière, Y., Dennehy, T. J., & Morin, S. (2006). DNA screening reveals pink bollworm resistance to Bt cotton remains rare after a decade of exposure. Journal of Economic Entomology, 99(5), 1525-1530.
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  PMID: 17066779;Abstract: Transgenic crops producing toxins from the bacterium Bacillus thuringiensis (Bt) kill insect pests and can reduce reliance on insecticide sprays. Although Bt cotton (Gossypium hirsutum L.) and Bt corn (Zea mays L.) covered 26 million ha worldwide in 2005, their success could be cut short by evolution of pest resistance. Monitoring the early phases of pest resistance to Bt crops is crucial, but it has been extremely difficult because bioassays usually cannot detect heterozygotes harboring one allele for resistance. We report here monitoring of resistance to Bt cotton with DNA-based screening, which detects single resistance alleles in heterozygotes. We used polymerase chain reaction primers that specifically amplify three mutant alleles of a cadherin gene linked with resistance to Bt cotton in pink bollworm, Pectinophora gossypiella (Saunders), a major pest. We screened DNA of 5,571 insects derived from 59 cotton fields in Arizona, California, and Texas during 2001-2005. No resistance alleles were detected despite a decade of exposure to Bt cotton. In conjunction with data from bioassays and field efficacy tests, the results reported here contradict predictions of rapid pest resistance to Bt crops. © 2006 Entomological Society of America.
• Tabashnik, B., Gassmann, A. J., Stock, S. P., Carrière, Y., & Tabashnik, B. E. (2006). Effect of entomopathogenic nematodes on the fitness cost of resistance to Bt toxin crylac in pink bollworm (Lepidoptera: Gelechiidae). Journal of economic entomology, 99(3).
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  The widespread use of crop plants genetically engineered to produce toxins from the bacterium Bacillus thuringiensis (Bt) imposes selection on insect populations to evolve resistance. The pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), is a major pest of cotton in the southwestern United States that is currently controlled with transgenic cotton that produces Bt toxin Cry1Ac. Previously reported theoretical work suggests that, in conjunction with a high dose/refuge strategy, fitness costs of Bt resistance can slow or prevent the evolution of resistance. We report here that the entomopathogenic nematode Steinernema riobrave (Rhabditida: Steinernematidae) increased the fitness cost of resistance to Cry1Ac in P. gossypiella. Mortality of P. gossypiella from fourth instar to adult eclosion was significantly higher for a Bt-resistant strain than a susceptible strain in tests with two to 14 infective juveniles of S. riobrave per larva, but it did not differ between strains when nematodes were absent. Nematodes established in P. gossypiella larvae at all concentrations tested, and nematode reproduction in infected P. gossypiella larvae occurred at nematode concentrations of four to 14 infective juveniles per larva. Our results suggest that incorporation of entomopathogenic nematodes into an integrated resistance management strategy could help to delay pest resistance to Bt toxins.
• Baxter, S. W., Zhao, J. -., Gahan, L. J., Shelton, A. M., Tabashnik, B. E., & Heckel, D. G. (2005). Novel genetic basis of field-evolved resistance to Bt toxins in Plutella xylostella. Insect Molecular Biology, 14(3), 327-334.
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  PMID: 15926902;Abstract: Insecticidal toxins from Bacillus thuringiensis (Bt) are widely used to control pest insects, but evolution of resistance threatens their continued efficacy. The most common type of Bt resistance ('Mode 1') is characterized by recessive inheritance, > 500-fold resistance to at least one Cry1A toxin, negligible cross-resistance to Cry1C, and reduced binding of Bt toxins to midgut membrane target sites. Mutations affecting a Cry1A-binding midgut cadherin protein are linked to laboratory-selected Mode 1 resistance in Heliothis virescens and Pectinophora gossypiella. Here we show that field-evolved Mode 1 resistance in the diamondback moth, Plutella xylostella, has a different genetic basis, indicating that screening for resistance in the field should not be restricted to a previously proposed DNA-based search for cadherin mutations. © 2005 The Royal Entomological Society.
• Carrière, Y., Ellers-Kirk, C., Biggs, R., Degain, B., Holley, D., Yafuso, C., Evans, P., Dennehy, T. J., & Tabashnik, B. E. (2005). Effects of cotton cultivar on fitness costs associated with resistance of pink bollworm (Lepidoptera: Gelechiidae) to Bt cotton. Journal of Economic Entomology, 98(3), 947-954.
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  PMID: 16022327;Abstract: Fitness costs associated with insect resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) reduce the fitness on non-Bt refuge plants of resistant individuals relative to susceptible individuals. Because costs may vary among host plants, choosing refuge cultivars that increase the dominance or magnitude of costs could help to delay resistance. Specifically, cultivars with high concentrations of toxic phytochemicals could magnify costs. To test this hypothesis, we compared life history traits of three independent sets of pink bollworm, Pectinophora gossypiella (Saunders), populations on two cotton cultivars that differed in antibiosis against this cotton pest. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. Confirming previous findings with pink bollworm feeding on cotton, costs primarily affected survival and were recessive on both cultivars. The magnitude of the survival cost did not differ between cultivars. Although the experimental results did not reveal differences between cultivars in the magnitude or dominance of costs, modeling results suggest that differences between cultivars in pink bollworm survival could affect resistance evolution. Thus, knowledge of the interaction between host plants and fitness costs associated with resistance to Bt crops could be helpful in guiding the choice of refuge cultivars. © 2005 Entomological Society of America.
• Carrière, Y., Ellers-Kirk, C., Kumar, K., Heuberger, S., Whitlow, M., Antilla, L., Dennehy, T. J., & Tabashnik, B. E. (2005). Long-term evaluation of compliance with refuge requirements for Bt cotton. Pest Management Science, 61(4), 327-330.
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  PMID: 15714465;Abstract: The success of the refuge strategy for delaying pest resistance to Bt crops depends on compliance by farmers. However, the accuracy of previous estimates of compliance has been questioned. We have applied a novel approach based on the use of Geographical Information System (GIS) technology to measure compliance with refuge requirements for Bt cotton in six Arizona regions from 1998 to 2003. Although compliance varied among regions, overall compliance was above 88% in five of six years. With the cooperation of farmers, our approach allows precise and economical assessment of compliance with the refuge strategy. © 2005 Society of Chemical Industry.
• Higginson, D. M., Morin, S., Nyboer, M. E., Biggs, R. W., Tabashnik, B. E., & Carrière, Y. (2005). Evolutionary trade-offs of insect resistance to Bacillus thuringiensis crops: Fitness cost affecting paternity. Evolution, 59(4), 915-920.
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  PMID: 15926701;Abstract: Evolution of resistance to insecticides provides a useful model for examining fitness trade-offs associated with adaptation to stress. Here, we examined male reproductive costs in pink bollworm (Pectinophora gossypiella) resistant to an insecticidal protein of Bacillus thuringiensis (Bt) produced by transgenic cotton, using contrasts between two pairs of related susceptible and resistant strains. Without competition for access to females, no costs affecting reproductive success of resistant males were observed. Resistant and susceptible males had similar mating frequency and fertility. Additionally, fecundity of females mated to resistant and susceptible males was comparable. In competition for matings with virgin females, resistant and susceptible males had comparable success in one strain, whereas susceptible males tended to mate more often than resistant males in the other. However, irrespective of strain origin, resistant males that mated first sired significantly less offspring than susceptible males that mated first. The reduced first-male paternity in resistant males may involve reduced sperm precedence caused by mutations in a cadherin gene linked with resistance to Bt cotton. © 2005 The Society for the Study of Evolution. All rights reserved.
• Sisterson, M. S., & Tabashnik, B. E. (2005). Simulated effects of transgenic Bt crops on specialist parasitoids of target pests. Environmental Entomology, 34(4), 733-742.
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  Abstract: Large-scale adoption of transgenic crops that produce Bacillus thuringiensis (Bt) toxins could greatly reduce populations of target pests and their specialist parasitoids. We used a spatially explicit model of host-parasitoid population dynamics to examine effects of Bt crops on parasitoid persistence. We modeled a 9,000-ha region with 900 fields of 10 ha, each planted with either a Bt crop or a non-Bt crop refuge. In the 13,524 simulations we conducted, the pest (i.e., host) was never lost from the region before the parasitoid. The parasitoid was lost from the region in 36% of the simulations. The host was lost after the parasitoid in 12% of all simulations. The probability of regional parasitoid loss increased as the percentage of Bt fields and rotation of field types (Bt and non-Bt) increased and as host reproductive rate and parasitoid attack rate decreased. Regional parasitoid loss also increased as the insecticide spray threshold for target pests in non-Bt fields decreased and as parasitoid susceptibility to insecticide increased. With field types fixed, regional parasitoid loss increased as parasitoid movement increased. However, with field types rotated, parasitoid movement did not influence regional parasitoid loss. Host movement did not influence regional parasitoid loss. Factors not modeled that would reduce risk of regional parasitoid loss include use of alternate hosts by the parasitoid, use of alternate host plants by the target pest, and evolution of resistance to the Bt crop by the pest. The results suggest that risk of regional parasitoid loss can be assessed from the parasitoid life history traits and reduced by increasing the percentage of refuge fields, fixing refuge locations, and increasing the insecticide spray threshold for target pests in refuges. © 2005 Entomological Society of America.
• Sisterson, M. S., Carrière, Y., Dennehy, T. J., & Tabashnik, B. E. (2005). Evolution of resistance to transgenic crops: Interactions between insect movement and field distribution. Journal of Economic Entomology, 98(6), 1751-1762.
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  PMID: 16539091;Abstract: The refuge strategy is designed to delay evolution of pest resistance to transgenic crops producing Bacillus thuringiensis Berliner (Bt) toxins. Movement of insects between Bt crops and refuges of non-Bt crops is essential for the refuge strategy because it increases chances that resistant adults mate with susceptible adults from refuges. Conclusions about optimal levels of movement for delaying resistance are not consistent among previous modeling studies. To clarify the effects of movement on resistance evolution, we analyzed simulations of a spatially explicit model based partly on the interaction of pink bollworm, Pectinophora gossypiella (Saunders), with Bt cotton. We examined resistance evolution as a function of insect movement under 12 sets of assumptions about the relative abundance of Bt cotton (50 and 75%), temporal distribution of Bt cotton and refuge fields (fixed, partial rotation, and full rotation), and spatial distribution of fields (random and uniform). The results show that interactions among the relative abundance and distribution of refuges and Bt cotton fields can alter the effects of movement on resistance evolution. The results also suggest that differences in conclusions among previous studies can be explained by differences in assumptions about the relative abundance and distribution of refuges and Bt crop fields. With fixed field locations and all Bt cotton fields adjacent to at least one refuge, resistance evolved slowest with low movement. However, low movement and fixed field locations favored rapid resistance evolution when some Bt crop fields were isolated from refuges. When refuges and Bt cotton fields were rotated to the opposite crop type each year, resistance evolved fastest with low movement. Nonrecessive inheritance of resistance caused rapid resistance evolution regardless of movement rate. Confirming previous reports, results described here show that resistance can be delayed effectively by fixing field locations and distributing refuges uniformly to ensure that Bt crop fields are not isolated from refuges. However, rotating fields provided better insect control and reduced the need for insecticide sprays. © 2005 Entomological Society of America.
• Tabashnik, B. (2005). Refuges in India and delayed resistance to Bt crops [2] (multiple letters). Nature Biotechnology, 23(4), 414-.
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  PMID: 15815660;
• Tabashnik, B. E., Biggs, R. W., Higginson, D. M., Henderson, S., Unnithan, D. C., Unnithan, G. C., Ellers-Kirk, C., Sisterson, M. S., Dennehy, T. J., Carrière, Y., & Morin, S. (2005). Association between resistance to Bt cotton and cadherin genotype in pink bollworm. Journal of Economic Entomology, 98(3), 635-644.
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  PMID: 16022286;Abstract: Two strains of pink bollworm, Pectinophora gossypiella (Saunders), each derived in 1997 from a different field population, were selected for resistance to Bacillus thuringiensis (Bt) toxin Cry1Ac in the laboratory. One strain (MOV97-R) originated from Mohave Valley in western Arizona; the other strain (SAF97-R) was from Safford in eastern Arizona. Relative to a susceptible laboratory strain, Cry1Ac resistance ratios were 1700 for MOV97-R and 520 for SAF97-R. For the two resistant strains, larval survival did not differ between non-Bt cotton and transgenic cotton producing Cry1Ac. In contrast, larval survival on Bt cotton was 0% for the two unselected parent strains from which the resistant strains were derived. Previously identified resistance (r) alleles of a cadherin gene (BtR) occurred in both resistant strains: r1 and r3 in MOV97-R, and r1 and r2 in SAF97-R. The frequency of individuals carrying two r alleles (rr) was 1.0 in the two resistant strains and 0.02 in each of the two unselected parent strains. Furthermore, in two hybrid strains with a mixture of susceptible (s) and r alleles at the BtR locus, all survivors on Bt cotton had two r alleles. The results show that resistance to Cry1Ac-producing Bt cotton is associated with recessive r alleles at the BtR locus in the strains of pink bollworm tested here. In conjunction with previous results from two other Bt-resistant strains of pink bollworm (APHIS-98R and AZP-R), results reported here identify the cadherin locus as the leading candidate for molecular monitoring of pink bollworm resistance to Bt cotton. © 2005 Entomological Society of America.
• Tabashnik, B. E., Dennehy, T. J., & Carrière, Y. (2005). Delayed resistance to transgenic cotton in pink bollworm. Proceedings of the National Academy of Sciences of the United States of America, 102(43), 15389-15393.
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  PMID: 16227430;PMCID: PMC1255739;Abstract: Transgenic crops producing Bacillus thuringiensis (Bt) toxins kill some key insect pests and thus can reduce reliance on insecticides. Widespread planting of such Bt crops increased concerns that their usefulness would be cut short by rapid evolution of resistance to Bt toxins by pests. Pink bollworm (Pectinophora gossypiella) is a major pest that has experienced intense selection for resistance to Bt cotton in Arizona since 1997. We monitored pink bollworm resistance to Bt toxin for 8 years with laboratory bioassays of strains derived annually from 10-17 cotton fields statewide. Bioassay results show no net increase from 1997 to 2004 in the mean frequency of pink bollworm resistance to Bt toxin. A synthesis of experimental and modeling results suggests that this delay in resistance can be explained by refuges of cotton without Bt toxin, recessive inheritance of resistance, incomplete resistance, and fitness costs associated with resistance. © 2005 by The National Academy of Sciences of the USA.
• Tabashnik, B., Carrière, Y., Ellers-Kirk, C., Kumar, K., Heuberger, S., Whitlow, M., Antilla, L., Dennehy, T. J., & Tabashnik, B. E. (2005). Long-term evaluation of compliance with refuge requirements for Bt cotton. Pest management science, 61(4).
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  The success of the refuge strategy for delaying pest resistance to Bt crops depends on compliance by farmers. However, the accuracy of previous estimates of compliance has been questioned. We have applied a novel approach based on the use of Geographical Information System (GIS) technology to measure compliance with refuge requirements for Bt cotton in six Arizona regions from 1998 to 2003. Although compliance varied among regions, overall compliance was above 88% in five of six years. With the cooperation of farmers, our approach allows precise and economical assessment of compliance with the refuge strategy.
• Tabashnik, B., Sisterson, M. S., Carrière, Y., Dennehy, T. J., & Tabashnik, B. E. (2005). Evolution of resistance to transgenic crops: interactions between insect movement and field distribution. Journal of economic entomology, 98(6).
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  The refuge strategy is designed to delay evolution of pest resistance to transgenic crops producing Bacillus thuringiensis Berliner (Bt) toxins. Movement of insects between Bt crops and refuges of non-Bt crops is essential for the refuge strategy because it increases chances that resistant adults mate with susceptible adults from refuges. Conclusions about optimal levels of movement for delaying resistance are not consistent among previous modeling studies. To clarify the effects of movement on resistance evolution, we analyzed simulations of a spatially explicit model based partly on the interaction of pink bollworm, Pectinophora gossypiella (Saunders), with Bt cotton. We examined resistance evolution as a function of insect movement under 12 sets of assumptions about the relative abundance of Bt cotton (50 and 75%), temporal distribution of Bt cotton and refuge fields (fixed, partial rotation, and full rotation), and spatial distribution of fields (random and uniform). The results show that interactions among the relative abundance and distribution of refuges and Bt cotton fields can alter the effects of movement on resistance evolution. The results also suggest that differences in conclusions among previous studies can be explained by differences in assumptions about the relative abundance and distribution of refuges and Bt crop fields. With fixed field locations and all Bt cotton fields adjacent to at least one refuge, resistance evolved slowest with low movement. However, low movement and fixed field locations favored rapid resistance evolution when some Bt crop fields were isolated from refuges. When refuges and Bt cotton fields were rotated to the opposite crop type each year, resistance evolved fastest with low movement. Nonrecessive inheritance of resistance caused rapid resistanceevolution regardless of movement rate. Confirming previous reports, results described here show that resistance can be delayed effectively by fixing field locations and distributing refuges uniformly to ensure that Bt crop fields are not isolated from refuges. However, rotating fields provided better insect control and reduced the need for insecticide sprays.
• Carrière, Y., Dutilleul, P., Ellers-Kirk, C., Pedersen, B., Haller, S., Antilla, L., Dennehy, T. J., & Tabashnik, B. E. (2004). Sources, sinks, and the zone of influence of refuges for managing insect resistance to Bt crops. Ecological Applications, 14(6), 1615-1623.
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  Abstract: The refuge strategy is central for delaying insect resistance to transgenic crops that produce Bacillus thuringiensis (Bt) toxins, but determining an effective spatial configuration of refuges has been problematic. We developed a spatially explicit, demographically based method for estimating the zone of influence of refuges, the area over which refuges increase an insect's population density. The method relied on global positioning system (GPS) and geographic information system (GIS) technologies combined with spatial statistics. We applied it in two successive years to Arizona populations of pink bollworm, a major cotton pest. Refuges at 0.75 km or less from Bt cotton had the greatest potential for delaying resistance in both years studied. However, the zone of influence of refuges varied between years and among sites. The density of susceptible moths was significantly lower at sites with high compared to low abundance of Bt cotton relative to non-Bt cotton. Thus, abundance of a Bt crop relative to a non-Bt crop may influence the effectiveness of refuges. Our spatially explicit approach takes such source-sink dynamics into account and hence is well suited to help determine how refuges should be deployed.
• Carrière, Y., Ellers-Kirk, C., Biggs, R., Higginson, D. M., Dennehy, T. J., & Tabashnik, B. E. (2004). Effects of gossypol on fitness costs associated with resistance to Bt cotton in pink bollworm. Journal of Economic Entomology, 97(5), 1710-1718.
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  PMID: 15568363;Abstract: Fitness costs associated with insect resistance to Bacillus thuringiensis (Bt) crops may help to delay or prevent the spread of resistance alleles, especially when refuges of non-Bt host plants are present. The potential for such delays increases as the magnitude and dominance of fitness costs increase. Here, we examined the idea that plant secondary chemicals affect expression of fitness costs associated with resistance to Bt cotton in Pectinophora gossypiella (Saunders). Specifically, we tested the hypotheses that gossypol affects the magnitude or dominance of fitness costs, by measuring performance of three independent sets of pink bollworm populations fed artificial diet with and without gossypol. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. No individuals completed development on diets with gossypol in one set, suggesting that these individuals partially lost the ability to detoxify this chemical. In the other two sets, costs affecting survival did not support the hypotheses, but costs affecting pupal weight did. Adding gossypol to diet increased the magnitude and dominance of costs affecting pupal weight. In one of the two sets with survivors on diet with gossypol, costs affecting development time were less recessive when gossypol was present in diet. These results indicate that gossypol increased the magnitude and dominance of some fitness costs. Better understanding of the effects of natural plant defenses on fitness costs could improve our ability to design refuges for managing insect resistance to Bt crops.
• Chilcutt, C. F., & Tabashnik, B. E. (2004). Contamination of refuges by Bacillus thuringiensis toxin genes from transgenic maize. Proceedings of the National Academy of Sciences of the United States of America, 101(20), 7526-7529.
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  PMID: 15136739;PMCID: PMC419639;Abstract: Transgenic crops producing insecticidal toxins from Bacillus thuringiensis (Bt) are widely used to control pests, but their benefits will be lost if pests evolve resistance. The mandated high-dose/refuge strategy for delaying pest resistance requires planting refuges of toxin-free crops near Bt crops to promote survival of susceptible pests. We report that pollen-mediated gene flow up to 31 m from Bt maize caused low to moderate Bt toxin levels in kernels of non-Bt maize refuge plants. Immunoassays of non-Bt maize sampled from the field showed that the mean concentration of Bt toxin Cry1Ab in kernels and the percentage of kernels with Cry1Ab decreased with distance from Bt maize. The highest Bt toxin concentration in pooled kernels of non-Bt maize plants was 45% of the mean concentration in kernels from adjacent Bt maize plants. Most previous work on gene flow from transgenic crops has emphasized potential effects of transgene movement on wild relatives of crops, landraces, and organic plantings, whereas implications for pest resistance have been largely ignored. Variable Bt toxin production in seeds of refuge plants undermines the high-dose/refuge strategy and could accelerate pest resistance to Bt crops. Thus, guidelines should be revised to reduce gene flow between Bt crops and refuge plants.
• Sisterson, M. S., Antilla, L., Carrière, Y., Ellers-Kirk, C., & Tabashnik, B. E. (2004). Effects of insect population size on evolution of resistance to transgenic crops. Journal of Economic Entomology, 97(4), 1413-1424.
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  PMID: 15384355;Abstract: Models of the evolution of insect resistance to transgenic crops have often assumed that population size is infinite or that carrying capacity is fixed. To evaluate potential effects of population size on resistance evolution, we conducted sensitivity analyses by using a stochastic, spatially explicit model based partly on the interaction between pink bollworm and Bacillus thuringiensis (Bt) cotton. We examined interactions of carrying capacity, region size, dispersal, and percentage of fields planted with Bt cotton. The median and variance in the time to resistance decreased as region size increased, regardless of carrying capacity. This occurred because larger regions were more likely to have at least one field in which resistance evolved rapidly and served as a source from which resistance spread throughout the region. Carrying capacity significantly affected the median time to resistance with 75% of fields planted with Bt cotton, but not with 50% Bt cotton. In contrast, carrying capacity significantly influenced the variance in the time to resistance with 50% Bt cotton, but not with 75% Bt cotton. We also found resistance evolution was affected by interactions between carrying capacity, dispersal, and the percentage of fields planted with Bt cotton. The high variability observed in our simulations indicates that factors affecting stochastic events can play an important role in the evolution of resistance. Because population size determines the extent to which stochastic events are important, reasonable estimates of population size are essential for devising robust models of resistance evolution.
• Sisterson, M. S., Biggs, R. W., Olson, C., Carrière, Y., Dennehy, T. J., & Tabashnik, B. E. (2004). Arthropod abundance and diversity in Bt and non-Bt cotton fields. Environmental Entomology, 33(4), 921-929.
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  Abstract: The widespread planting of crops genetically modified to produce Bacillus thuringeinsis Berliner (Bt) toxins for pest control may affect nontarget arthropods. To address this issue, we compared the abundance and diversity of arthropods on plants in field plots of Bt cotton, non-Bt cotton, and a row mixture of 75% Bt cotton and 25% non-Bt cotton at two sites in Arizona. Over three sampling dates during 2 yr, we recorded all of the arthropods found on 120 cotton plants per treatment. This yielded 3,309 individual arthropods from 69 families. Excluding pink bollworm, Pectinophora gossypiella Saunders, the pest targeted by Bt cotton, we compared the abundance and diversity of all arthropods, chewing herbivores, sucking herbivores, rasping-sucking arthropods, and natural enemies. Arthropod abundance was significantly affected by site, plant height, and cotton type. More arthropods were collected from row mixture plots than Bt plots, but arthropod abundance did not differ significantly between Bt plots and non-Bt plots. The number of families collected was 57 for row mixture plots, 55 for non-Bt plots, and 47 for Bt plots. The number of families increased as arthropod abundance increased, suggesting that the differences in diversity among treatments were caused by differences in abundance. Within row mixture plots, arthropod abundance and diversity did not differ significantly between Bt plants and non-Bt plants. We conclude that the differences between Bt and non-Bt cultivars had relatively minor effects on the arthropod community on cotton plants.
• Tabashnik, B. E., & Carrière, Y. (2004). Bt transgenic crops do not have favorable effects on resistant insects. Journal of Insect Science, 4.
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  Abstract: Sayyed et al. (Ecology Letters (2003) 6: 167-169) hypothesized that insecticidal Bacillus thuringiensis (Bt) toxins produced by transgenic crops could have nutritionally favorable effects that increase the fitness of resistant insects eating such crops. This idea was based on increased pupal weight of resistant larvae of diamondback moth, Plutella xylostella (L.), fed leaf discs treated externally with a Bt toxin. We summarize evidence from diamondback moth and other pests showing that the Bt toxins in transgenic crops do not enhance performance of resistant insects. Aside from a few notable exceptions in which performance of resistant insects did not differ between Bt and non-Bt crops, Bt crops had adverse affects on resistant insects.
• Tabashnik, B. E., & Carrière, Y. (2004). Bt transgenic crops do not have favorable effects on resistant insects.. Journal of insect science (Online), 4, 4-.
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  PMID: 15861220;PMCID: PMC455678;Abstract: Sayyed et al. (Ecology Letters (2003) 6: 167-169) hypothesized that insecticidal Bacillus thuringiensis (Bt) toxins produced by transgenic crops could have nutritionally favorable effects that increase the fitness of resistant insects eating such crops. This idea was based on increased pupal weight of resistant larvae of diamondback moth, Plutella xylostella (L.), fed leaf discs treated externally with a Bt toxin. We summarize evidence from diamondback moth and other pests showing that the Bt toxins in transgenic crops do not enhance performance of resistant insects. Aside from a few notable exceptions in which performance of resistant insects did not differ between Bt and non-Bt crops, Bt crops had adverse affects on resistant insects.
• Tabashnik, B. E., Gould, F., & Carrière, Y. (2004). Delaying evolution of insect resistance to transgenic crops by decreasing dominance and heritability. Journal of Evolutionary Biology, 17(4), 904-912.
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  PMID: 15271091;Abstract: The refuge strategy is used widely for delaying evolution of insect resistance to transgenic crops that produce Bacillus thuringiensis (Bt) toxins. Farmers grow refuges of host plants that do not produce Bt toxins to promote survival of susceptible pests. Many modelling studies predict that refuges will delay resistance longest if alleles conferring resistance are rare, most resistant adults mate with susceptible adults, and Bt plants have sufficiently high toxin concentration to kill heterozygous progeny from such matings. In contrast, based on their model of the cotton pest Heliothis virescens, Vacher et al. (Journal of Evolutionary Biology, 16, 2003, 378) concluded that low rather than high toxin doses would delay resistance most effectively. We demonstrate here that their conclusion arises from invalid assumptions about larval concentration-mortality responses and dominance of resistance. Incorporation of bioassay data from H. virescens and another key cotton pest (Pectinophora gossypiella) into a population genetic model shows that toxin concentrations high enough to kill all or nearly all heterozygotes should delay resistance longer than lower concentrations.
• Tabashnik, B. E., Liu, Y., Unnithan, D. C., Carrière, Y., Dennehy, T. J., & Morin, S. (2004). Shared genetic basis of resistance to Bt toxin Cry1Ac in independent strains of pink bollworm. Journal of Economic Entomology, 97(3), 721-726.
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  PMID: 15279243;Abstract: Classical and molecular genetic analyses show that two independently derived resistant strains of pink bollworm, Pectinophora gossypiella (Saunders), share a genetic locus at which three mutant alleles confer resistance to Bacillus thuringiensis (Bt) toxin Cry1Ac. One laboratory-selected resistant strain (AZP-R) was derived from individuals collected in 1997 from 10 Arizona cotton fields, whereas the other (APHIS-98R) was derived from a long-term susceptible laboratory strain. Both strains were previously reported to show traits of "mode 1" resistance, the most common type of lepidopteran resistance to Cry1A toxins. Inheritance of resistance to a diagnostic concentration of Cry1Ac (10 μg per gram of diet) was recessive in both strains. In interstrain complementation tests for allelism, F1 progeny from crosses between the two strains were resistant to the diagnostic concentration of Cry1Ac. These results indicate that a major resistance locus is shared by the two strains. Analysis of DNA from the pink bollworm cadherin gene (BtR) using allele-specific polymerase chain reaction (PCR) tests showed that the previously identified resistance alleles (r1, r2, and r3) occurred in both strains, but their frequencies differed between strains. In conjunction with previous findings, the results reported here suggest that PCR-based detection of the three known cadherin resistance alleles might be useful for monitoring resistance to Cry1Ac-producing Bt cotton in field populations of pink bollworm.
• Tabashnik, B., Carrière, Y., Ellers-Kirk, C., Biggs, R., Higginson, D. M., Dennehy, T. J., & Tabashnik, B. E. (2004). Effects of gossypol on fitness costs associated with resistance to Bt cotton in pink bollworm. Journal of economic entomology, 97(5).
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  Fitness costs associated with insect resistance to Bacillus thuringiensis (Bt) crops may help to delay or prevent the spread of resistance alleles, especially when refuges of non-Bt host plants are present. The potential for such delays increases as the magnitude and dominance of fitness costs increase. Here, we examined the idea that plant secondary chemicals affect expression of fitness costs associated with resistance to Bt cotton in Pectinophora gossypiella (Saunders). Specifically, we tested the hypotheses that gossypol affects the magnitude or dominance of fitness costs, by measuring performance of three independent sets of pink bollworm populations fed artificial diet with and without gossypol. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. No individuals completed development on diets with gossypol in one set, suggesting that these individuals partially lost the ability to detoxify this chemical. In the other two sets, costs affecting survival did not support the hypotheses, but costs affecting pupal weight did. Adding gossypol to diet increased the magnitude and dominance of costs affecting pupal weight. In one of the two sets with survivors on diet with gossypol, costs affecting development time were less recessive when gossypol was present in diet. These results indicate that gossypol increased the magnitude and dominance of some fitness costs. Better understanding of the effects of natural plant defenses on fitness costs could improve our ability to design refuges for managing insect resistance to Bt crops.
• Tabashnik, B., Sisterson, M. S., Antilla, L., Carrière, Y., Ellers-Kirk, C., & Tabashnik, B. E. (2004). Effects of insect population size on evolution of resistance to transgenic crops. Journal of economic entomology, 97(4).
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  Models of the evolution of insect resistance to transgenic crops have often assumed that population size is infinite or that carrying capacity is fixed. To evaluate potential effects of population size on resistance evolution, we conducted sensitivity analyses by using a stochastic, spatially explicit model based partly on the interaction between pink bollworm and Bacillus thuringiensis (Bt) cotton. We examined interactions of carrying capacity, region size, dispersal, and percentage of fields planted with Bt cotton. The median and variance in the time to resistance decreased as region size increased, regardless of carrying capacity. This occurred because larger regions were more likely to have at least one field in which resistance evolved rapidly and served as a source from which resistance spread throughout the region. Carrying capacity significantly affected the median time to resistance with 75% of fields planted with Bt cotton, but not with 50% Bt cotton. In contrast, carrying capacity significantly influenced the variance in the time to resistance with 50% Bt cotton, but not with 75% Bt cotton. We also found resistance evolution was affected by interactions between carrying capacity, dispersal, and the percentage of fields planted with Bt cotton. The high variability observed in our simulations indicates that factors affecting stochastic events can play an important role in the evolution of resistance. Because population size determines the extent to which stochastic events are important, reasonable estimates of population size are essential for devising robust models of resistance evolution.
• Carrière, Y., Ellers-Kirk, C., Sisterson, M., Antilla, L., Whitlow, M., Dennehy, T. J., & Tabashnik, B. E. (2003). Long-term regional suppression of pink bollworm by Bacillus thuringiensis cotton. Proceedings of the National Academy of Sciences of the United States of America, 100(4), 1519-1523.
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  PMID: 12571355;PMCID: PMC149864;Abstract: Despite the potentially profound impact of genetically modified crops on agriculture and the environment, we know little about their long-term effects. Transgenic crops that produce toxins from Bacillus thuringiensis (Bt) to control insects are grown widely, but rapid evolution of resistance by pests could nullify their benefits. Here, we present theoretical analyses showing that long-term suppression of pest populations is governed by interactions among reproductive rate, dispersal propensity, and regional abundance of a Bt crop. Supporting this theory, a 10-year study in 15 regions across Arizona shows that Bt cotton suppressed a major pest, pink bollworm (Pectinophora gossypiella), independent of demographic effects of weather and variation among regions. Pink bollworm population density declined only in regions where Bt cotton was abundant. Such long-term suppression has not been observed with insecticide sprays, showing that transgenic crops open new avenues for pest control. The debate about putative benefits of Bt crops has focused primarily on short-term decreases in insecticide use. The present findings suggest that long-term regional pest suppression after deployment of Bt crops may also contribute to reducing the need for insecticide sprays.
• Gonzátlez-Cabrera, J., Escriche, B., Tabashnik, B. E., & Ferré, J. (2003). Binding of Bacillus thuringiensis toxins in resistant and susceptible strains of pink bollworm (Pectinophora gossypiella). Insect Biochemistry and Molecular Biology, 33(9), 929-935.
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  PMID: 12915184;Abstract: Evolution of resistance by pests could cut short the success of transgenic plants producing toxins from Bacillus thuringiensis, such as Bt cotton. The most common mechanism of insect resistance to B. thuringiensis is reduced binding of toxins to target sites in the brush border membrane of the larval midgut. We compared toxin binding in resistant and susceptible strains of Pectinophora gossypiella, a major pest of cotton worldwide. Using Cry1Ab and Cry1Ac labeled with 125I and brush border membrane vesicles (BBMV), competition experiments were performed with unlabeled Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ba, Cry1Ca, Cry1Ja, Cry2Aa, and Cry9Ca. In the susceptible strain, Cry1Aa, Cry1Ab, Cry1Ac, and Cry1Ja bound to a common binding site that was not shared by the other toxins tested. Reciprocal competition experiments with Cry1Ab, Cry1Ac, and Cry1Ja showed that these toxins do not bind to any additional binding sites. In the resistant strain, binding of 125I-Cry1Ac was not significantly affected; however, 125I-Cry1Ab did not bind to the BBMV. This result, along with previous data from this strain, shows that the resistance fits the "mode 1" pattern of resistance described previously in Plutella xylostella, Plodia interpunctella, and Heliothis virescens. © 2003 Elsevier Ltd. All rights reserved.
• Sisterson, M. S., Liu, Y. B., Kerns, D. L., & Tabashnik, B. E. (2003). Effects of Kaolin Particle Film on Oviposition, Larval Mining, and Infestation of Cotton by Pink Bollworm (Lepidoptera: Gelechiidae). Journal of Economic Entomology, 96(3), 805-810.
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  PMID: 12852620;Abstract: We tested effects of kaolin particle film on oviposition, larval mining, and infestation of cotton by pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), in the laboratory, greenhouse, and field. In laboratory choice tests, females laid seven times more eggs on untreated bolls than on bolls treated with kaolin. When neonates were put on bolls in the laboratory, each boll with a treated and untreated half, larvae and mines were found 24 h later on the untreated half but not on the treated half. In oviposition choice tests with whole plants in the greenhouse, females laid four times more eggs on untreated plants than on treated plants and the number of eggs on bolls was five times higher for untreated plants than for treated plants. Kaolin treatments altered the distribution of eggs among plant parts, with untreated bolls receiving a higher percentage than treated bolls, whereas the opposite occurred for petioles. In field tests, treatment with kaolin alone reduced the proportion of bolls infested with pink bollworm, but a mixture of kaolin and the pyrethroid lambda-cyhalothrin was most effective. The results suggest that kaolin particle film may be useful against pink bollworm, particularly in conjunction with other control tactics.
• Tabashnik, B. E., Carrière, Y., Dennehy, T. J., Morin, S., Sisterson, M. S., Roush, R. T., Shelton, A. M., & Zhao, J. (2003). Insect resistance to transgenic Bt crops: Lessons from the laboratory and field. Journal of Economic Entomology, 96(4), 1031-1038.
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  PMID: 14503572;Abstract: Transgenic crops that produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) grew on >62 million ha worldwide from 1996 to 2002. Despite expectations that pests would rapidly evolve resistance to such Bt crops, increases in the frequency of resistance caused by exposure to Bt crops in the field have not yet been documented. In laboratory and greenhouse tests, however, at least seven resistant laboratory strains of three pests (Plutella xylostella [L.], Pectinophora gossypiclla [Saunders], and Helicocerpa annigera [Hübner]) have completed development on Bt crops. In contrast, several other laboratory strains with 70- to 10,100-fold resistance to Bt toxins in diet did not survive on Bt crops. Monitoring of field populations in regions with high adoption of Bt crops has not yet detected increases in resistance frequency. Resistance monitoring examples include Ostrinia nubilalis (Hübner) in the United States (6 yr), P. gossypiella in Arizona (5 yr), H. arinigera in northern China (3 yr), and Hclicocerpa zea (Boddie) in North Carolina (2 yr). Key factors delaying resistance to Bt crops are probably refuges of non-Bt host plants that enable survival of susceptible pests, low initial resistance allele frequencies, recessive inheritance of resistance to Bt crops, costs associated with resistance that reduce fitness of resistant individuals relative to susceptible individuals on non-Bt hosts ("fitness costs"), and disadvantages suffered by resistant strains on Bt hosts relative to their performance on non-Bt hosts ("incomplete resistance"). The relative importance of these factors varies among pest-Bt crop systems, and violations of key assumptions of the refuge strategy (low resistance allele frequency and recessive inheritance) may occur in some cases. The success of Bt crops exceeds expectations of many, but does not preclude resistance problems in the future.
• Tabashnik, B. E., Dennehy, T. J., Carrière, Y., Liu, Y., Meyer, S. K., Patin, A., Sims, M., & Ellers-Kirk, C. (2003). Resistance management: Slowing pest adaptation to transgenic crops. Acta Agriculturae Scandinavica - Section B: Soil and Plant Science, 53(SUPPL. 1), 51-56.
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  Abstract: Transgenic crops that are genetically modified to produce insecticidal proteins from the common bacterium Bacillus thuringiensis (Bt) can help to control pests while reducing reliance on insecticide sprays. So far, no insects have evolved resistance in the field to Bt transgenic crops. However, diamondback moth populations have evolved resistance to Bt sprays in the field and many pests have evolved resistance to Bt toxins in the laboratory. To delay resistance, the refuge strategy provides host plants that do not produce Bt toxins, thereby promoting survival of susceptible pests. In Arizona, Bt cotton has been extremely effective in controlling the pink bollworm (Pectinophora gossypiella), a major pest. Despite a surprisingly high frequency of resistance in 1997, resistance did not increase in Arizona field populations of pink bollworm from 1997 to 1999. Nonetheless, pink bollworm and other insects will eventually evolve resistance, so any particular transgenic crop variety is not a permanent solution to pest problems. Instead, transgenic crops can be used in harmony with other tactics as part of integrated pest management. Evaluations of transgenic crops should consider their advantages and disadvantages compared with alternatives. If transgenic crops can greatly reduce use of hazardous insecticides, as achieved in Arizona cotton, great benefits may occur. © 2003 Taylor & Francis.
• Tabashnik, B., Carrière, Y., Ellers-Kirk, C., Sisterson, M., Antilla, L., Whitlow, M., Dennehy, T. J., & Tabashnik, B. E. (2003). Long-term regional suppression of pink bollworm by Bacillus thuringiensis cotton. Proceedings of the National Academy of Sciences of the United States of America, 100(4).
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  Despite the potentially profound impact of genetically modified crops on agriculture and the environment, we know little about their long-term effects. Transgenic crops that produce toxins from Bacillus thuringiensis (Bt) to control insects are grown widely, but rapid evolution of resistance by pests could nullify their benefits. Here, we present theoretical analyses showing that long-term suppression of pest populations is governed by interactions among reproductive rate, dispersal propensity, and regional abundance of a Bt crop. Supporting this theory, a 10-year study in 15 regions across Arizona shows that Bt cotton suppressed a major pest, pink bollworm (Pectinophora gossypiella), independent of demographic effects of weather and variation among regions. Pink bollworm population density declined only in regions where Bt cotton was abundant. Such long-term suppression has not been observed with insecticide sprays, showing that transgenic crops open new avenues for pest control. The debate about putative benefits of Bt crops has focused primarily on short-term decreases in insecticide use. The present findings suggest that long-term regional pest suppression after deployment of Bt crops may also contribute to reducing the need for insecticide sprays.
• Tabashnik, B., Morin, S., Biggs, R. W., Sisterson, M. S., Shriver, L., Ellers-Kirk, C., Higginson, D., Holley, D., Gahan, L. J., Heckel, D. G., Carrière, Y., Dennehy, T. J., Brown, J. K., & Tabashnik, B. E. (2003). Three cadherin alleles associated with resistance to Bacillus thuringiensis in pink bollworm. Proceedings of the National Academy of Sciences of the United States of America, 100(9).
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  Evolution of resistance by pests is the main threat to long-term insect control by transgenic crops that produce Bacillus thuringiensis (Bt) toxins. Because inheritance of resistance to the Bt toxins in transgenic crops is typically recessive, DNA-based screening for resistance alleles in heterozygotes is potentially much more efficient than detection of resistant homozygotes with bioassays. Such screening, however, requires knowledge of the resistance alleles in field populations of pests that are associated with survival on Bt crops. Here we report that field populations of pink bollworm (Pectinophora gossypiella), a major cotton pest, harbored three mutant alleles of a cadherin-encoding gene linked with resistance to Bt toxin Cry1Ac and survival on transgenic Bt cotton. Each of the three resistance alleles has a deletion expected to eliminate at least eight amino acids upstream of the putative toxin-binding region of the cadherin protein. Larvae with two resistance alleles in any combination were resistant, whereas those with one or none were susceptible to Cry1Ac. Together with previous evidence, the results reported here identify the cadherin gene as a leading target for DNA-based screening of resistance to Bt crops in lepidopteran pests.
• Tabashnik, B., Sisterson, M. S., Liu, Y. B., Kerns, D. L., & Tabashnik, B. E. (2003). Effects of kaolin particle film on oviposition, larval mining, and infestation of cotton by pink bollworm (Lepidoptera: Gelechiidae). Journal of economic entomology, 96(3).
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  We tested effects of kaolin particle film on oviposition, larval mining, and infestation of cotton by pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), in the laboratory, greenhouse, and field. In laboratory choice tests, females laid seven times more eggs on untreated bolls than on bolls treated with kaolin. When neonates were put on bolls in the laboratory, each boll with a treated and untreated half, larvae and mines were found 24 h later on the untreated half but not on the treated half. In oviposition choice tests with whole plants in the greenhouse, females laid four times more eggs on untreated plants than on treated plants and the number of eggs on bolls was five times higher for untreated plants than for treated plants. Kaolin treatments altered the distribution of eggs among plant parts, with untreated bolls receiving a higher percentage than treated bolls, whereas the opposite occurred for petioles. In field tests, treatment with kaolin alone reduced the proportion of bolls infested with pink bollworm, but a mixture of kaolin and the pyrethroid lambda-cyhalothrin was most effective. The results suggest that kaolin particle film may be useful against pink bollworm, particularly in conjunction with other control tactics.
• Liu, Y., Tabashnik, B. E., Dennehy, T. J., Carrière, Y., Sims, M. A., & Meyer, S. K. (2002). Oviposition on and mining in bolls of Bt and non-Bt cotton by resistant and susceptible pink bollworm (Lepidoptera: Gelechiidae).. Journal of Economic Entomology, 95(1), 143-148.
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  PMID: 11942750;Abstract: Transgenic cotton that produces insecticidal crystal protein Cry1Ac of Bacillus thuringiensis (Bt) has been effective in controlling pink bollworm, Pectinophora gossypiella (Saunders). We compared responses to bolls of Bt cotton and non-Bt cotton by adult females and neonates from susceptible and Cry1Ac-resistant strains of pink bollworm. In choice tests on caged cotton plants in the greenhouse, neither susceptible nor resistant females laid fewer eggs on Bt cotton bolls than on non-Bt cotton bolls, indicating that the Bt toxin did not deter oviposition. Multiple regression revealed that the number of eggs laid per boll was negatively associated with boll age and positively associated with boll diameter. Females also laid more eggs per boll on plants with more bolls. The distribution of eggs among bolls of Bt cotton and non-Bt cotton was clumped, indicating that boll quality rather than avoidance of previously laid eggs was a primary factor in oviposition preference. Parallel to the results from oviposition experiments, in laboratory no-choice tests with 10 neonates per boll, the number of entrance holes per boll did not differ between Bt cotton and non-Bt cotton for susceptible and resistant neonates. Also, like females, neonates preferred younger bolls and larger bolls. Thus, acceptance of bolls by females for oviposition and by neonates for mining was affected by boll age and diameter, but not by Bt toxin in bolls. The lack of discrimination between Bt and non-Bt cotton bolls by pink bollworm from susceptible and resistant strains indicates that oviposition and mining initiation are independent of susceptibility to Cry1Ac.
• Masson, L., Tabashnik, B. E., Mazza, A., Préfontaine, G., Potvin, L., Brousseau, R., & Schwartz, J. -. (2002). Mutagenic analysis of a conserved region of domain III in the Cry1ac toxin of Bacillus thuringiensis. Applied and Environmental Microbiology, 68(1), 194-200.
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  PMID: 11772627;PMCID: PMC126535;Abstract: We used site-directed mutagenesis to probe the function of four alternating arginines located at amino acid positions 525, 527, 529, and 531 in a highly conserved region of domain III in the Cry1Ac toxin of Bacillus thuringiensis. We created 10 mutants: eight single mutants, with each arginine replaced by either glycine (G) or aspartic acid (D), and two double mutants (R525G/R527G and R529G/R531G). In lawn assays of the 10 mutants with a cultured Choristoneura fumiferana insect cell line (Cf1), replacement of a single arginine by either glycine or aspartic acid at position 525 or 529 decreased toxicity 4- to 12-fold relative to native Cry1Ac toxin, whereas replacement at position 527 or 531 decreased toxicity only 3-fold. The reduction in toxicity seen with double mutants was 8-fold for R525G/R527G and 25-fold for R529G/R531G. Five of the mutants (R525G, R525D, R527G, R529D, and R525G/R527G) were tested in bioassays with Plutella xylostella larvae and ion channel formation in planar lipid bilayers. In the bioassays, R525D, R529D, and R525G/R527G showed reduced toxicity. In planar lipid bilayers, the conductance and the selectivity of the mutants were similar to those of native Cry1Ac. Toxins with alteration at position 527 or 529 tended to remain in their subconducting states rather than the maximally conducting state. Our results suggest that the primary role of this conserved region is to maintain both the structural integrity of the native toxin and the full functionality of the formed membrane pore.
• Tabashnik, B. E., Dennehy, T. J., Sims, M. A., Larkin, K., Head, G. P., Moar, W. J., & Carrièree, Y. (2002). Control of resistant pink bollworm (Pectinophora gossypiella) by transgenic cotton that produces Bacillus thuringiensis toxin Cry2Ab. Applied and Environmental Microbiology, 68(8), 3790-3794.
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  PMID: 12147473;PMCID: PMC124036;Abstract: Crops genetically engineered to produce Bacillus thuringiensis toxins for insect control can reduce use of conventional insecticides, but insect resistance could limit the success of this technology. The first generation of transgenic cotton with B. thuringiensis produces a single toxin, Cry1Ac, that is highly effective against susceptible larvae of pink bollworm (Pectinophora gossypiella), a major cotton pest. To counter potential problems with resistance, second-generation transgenic cotton that produces B. thuringiensis toxin Cry2Ab alone or in combination with Cry1Ac has been developed. In greenhouse bioassays, a pink bollworm strain selected in the laboratory for resistance to Cry1Ac survived equally well on transgenic cotton with Cry1Ac and on cotton without Cry1Ac. In contrast, Cry1Ac-resistant pink bollworm had little or no survival on second-generation transgenic cotton with Cry2Ab alone or with Cry1Ac plus Cry2Ab. Artificial diet bioassays showed that resistance to Cry1Ac did not confer strong cross-resistance to Cry2Aa. Strains with >90% larval survival on diet with 10 μg of Cry1Ac per ml showed 0% survival on diet with 3.2 or 10 μg of Cry2Aa per ml. However, the average survival of larvae fed a diet with 1 μg of Cry2Aa per ml was higher for Cry1Ac-resistant strains (2 to 10%) than for susceptible strains (0%). If plants with Cry1Ac plus Cry2Ab are deployed while genes that confer resistance to each of these toxins are rare, and if the inheritance of resistance to both toxins is recessive, the efficacy of transgenic cotton might be greatly extended.
• Tabashnik, B. E., Liu, Y., Dennehy, T. J., Sims, M. A., Sisterson, M. S., Biggs, R. W., & Carrière, Y. (2002). Inheritance of resistance to Bt toxin crylac in a field-derived strain of pink bollworm (Lepidoptera: Gelechiidae).. Journal of Economic Entomology, 95(5), 1018-1026.
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  PMID: 12403429;Abstract: Laboratory selection with Cry1Ac, the Bacillus thuringiensis (Bt) toxin in transgenic cotton, initially produced 300-fold resistance in a field-derived strain of pink bollworm, Pectinophora gossypiella (Saunders), a major cotton pest. After additional selection increased resistance to 3,100-fold, we tested the offspring of various crosses to determine the mode of inheritance of resistance to Cry1Ac. The progeny of reciprocal F1 crosses (resistant male x susceptible female and vice versa) responded alike in bioassays, indicating autosomal inheritance. Consistent with earlier findings, resistance was recessive at a high concentration of Cry1Ac. However, the dominance of resistance increased as the concentration of Cry1Ac decreased. Analysis of survival and growth of progeny from backcrosses (F1 x resistant strain) suggest that resistance was controlled primarily by one or a few major loci. The progression of resistance from 300- to 3,100-fold rules out the simplest model with one locus and two alleles. Overall the patterns observed can be explained by either a single resistance gene with three or more alleles or by more than one resistance gene. The pink bollworm resistance to Cry1Ac described here fits "mode 1" resistance, the most common type of resistance to Cry1A toxins in Lepidoptera.
• Carrière, Y., & Tabashnik, B. E. (2001). Reversing insect adaptation to transgenic insecticidal plants. Proceedings of the Royal Society B: Biological Sciences, 268(1475), 1475-1480.
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  PMID: 11454291;PMCID: PMC1088766;Abstract: The refuge-high-dose strategy for delaying insect adaptation to transgenic plants produces non-transgenic plants that enable survival of susceptible individuals. Previous theoretical work has suggested three requirements for success of the refuge-high-dose strategy: a low initial frequency of the resistance allele, extensive mating between resistant and susceptible adults and recessive inheritance of resistance. In order to understand an observed decrease in resistance frequency and improve the potential for managing resistance better, we used analytical and simulation models for exploring the conditions that prevent or reverse the evolution of resistance, even when resistance is not rare initially. Assuming random mating and recessive or nearly recessive inheritance of resistance, the factors favouring reversal of resistance are non-recessive costs of resistance, low initial resistance allele frequency, large refuges, incomplete resistance and density-independent population growth in refuges.
• Carrière, Y., Dennehy, T. J., Pedersen, B., Haller, S., Ellers-Kirk, C., Antilla, L., Liu, Y., Willott, E., & Tabashnik, B. E. (2001). Large-scale management of insect resistance to transgenic cotton in Arizona: Can transgenic insecticidal crops be sustained?. Journal of Economic Entomology, 94(2), 315-325.
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  PMID: 11332820;Abstract: A major challenge for agriculture is management of insect resistance to toxins from Bacillus thuringiensis (Bt) produced by transgenic crops. Here we describe how a large-scale program is being developed in Arizona for management of resistance to Bt cotton in the pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), and other insect pests of cotton. Financial support from growers makes this program possible. Collaboration between the Arizona Cotton Research and Protection Council, the University of Arizona, and government agencies has led to development of resistance management guidelines, a remedial action plan, and tools for monitoring compliance with the proposed guidelines. Direct participation in development of resistance management policies is a strong incentive for growers to invest in resistance management research. However, more research, regularly updated regulations, and increased collaboration between stakeholders are urgently needed to maintain efficacy of Bt toxins in transgenic crops. © 2001 Entomological Society of America.
• Carrière, Y., Ellers-Kirk, C., Liu, Y., Sims, M. A., Patin, A. L., Dennehy, T. J., & Tabashnik, B. E. (2001). Fitness costs and maternal effects associated with resistance to transgenic cotton in the pink bollworm (Lepidoptera: Gelechiidae). Journal of Economic Entomology, 94(6), 1571-1576.
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  PMID: 11777066;Abstract: Transgenic cotton producing a Bacillus thuringiensis (Bt) toxin is widely used for controlling the pink bollworm, Perctinophora gossypiella (Saunders). We compared performance of pink bollworm strains resistant to Bt cotton with performance of their susceptible counterparts on non-Bt cotton. We found fitness costs that reduced survival on non-Bt cotton by an average of 51.5% in two resistant strains relative to the susceptible strains. The survival cost was recessive in one set of crosses between a resistant strain and the susceptible strain from which it was derived. However, crosses involving an unrelated resistant and susceptible strain indicated that the survival cost could be dominant. Development time on non-Bt cotton did not differ between the two related resistant and susceptible strains. A slight recessive cost affecting development time was suggested by comparison of the unrelated resistant and susceptible strains. Maternal effects transmitted by parents that had eaten Bt-treated artificial diet as larvae had negative effects on embryogenesis, adult fertility, or both, and reduced the ability of neonates to enter cotton bolls. These results provide further evidence that fitness costs associated with the evolution of resistance to Bt cotton are substantial in the pink bollworm. © 2001 Entomological Society of America.
• Carrière, Y., Ellers-Kirk, C., Patin, A. L., Sims, M. A., Meyer, S., Liu, Y., Dennehy, T. J., & Tabashnik, B. E. (2001). Overwintering cost associated with resistance to transgenic cotton in the pink bollworm (Lepidoptera: Gelechiidae). Journal of Economic Entomology, 94(4), 935-941.
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  PMID: 11561855;Abstract: Fitness costs associated with resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) may have important effects on the evolution of resistance. We investigated overwintering costs in pink bollworm, Pectinophora gosypiella (Saunders), strains with different degrees of resistance to Bt cotton. Frequency of resistant individuals in a strain was not associated with induction of diapause or emergence from diapause in early winter. Emergence from diapause in the spring was 71% lower in three highly resistant strains than in two heterogeneous strains from which the resistant strains were derived. This underestimates the overwintering cost because the frequency of the resistance allele was relatively high in the heterogeneous strains. Emergence in the spring in hybrid progeny from crosses between the resistant and heterogeneous strains was greater than in resistant strains but did not differ from susceptible strains, showing that the overwintering cost was recessive to some extent. © 2001 Entomological Society of America.
• González-Cabrera, J., Herrero, S., Sayyed, A. H., Escriche, B., Liu, Y. B., Meyer, S. K., Wright, D. J., Tabashnik, B. E., & Ferré, J. (2001). Variation in Susceptibility to Bacillus thuringiensis Toxins among Unselected Strains of Plutella xylostella. Applied and Environmental Microbiology, 67(10), 4610-4613.
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  PMID: 11571163;PMCID: PMC93210;Abstract: So far, the only insect that has evolved resistance in the field to Bacillus thuringiensis toxins is the diamondback moth (Plutella xylostella). Documentation and analysis of resistant strains rely on comparisons with laboratory strains that have not been exposed to B. thuringiensis toxins. Previously published reports show considerable variation among laboratories in responses of unselected laboratory strains to B. thuringiensis toxins. Because different laboratories have used different unselected strains, such variation could be caused by differences in bioassay methods among laboratories, genetic differences among unselected strains, or both. Here we tested three unselected strains against five B. thuringiensis toxins (Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ca, and Cry1Da) using two bioassay methods. Tests of the LAB-V strain from The Netherlands in different laboratories using different bioassay methods yielded only minor differences in results. In contrast, side-by-side comparisons revealed major genetic differences in susceptibility between strains. Compared with the LAB-V strain, the ROTH strain from England was 17- to 170-fold more susceptible to Cry1Aa and Cry1Ac, respectively, whereas the LAB-PS strain from Hawaii was 8-fold more susceptible to Cry1Ab and 13-fold more susceptible to Cry1Da and did not differ significantly from the LAB-V strain in response to Cry1Aa, Cry1Ac, or Cry1Ca. The relative potencies of toxins were similar among LAB-V, ROTH, and LAB-PS, with Cry1Ab and Cry1Ac being most toxic and Cry1Da being least toxic. Therefore, before choosing a standard reference strain upon which to base comparisons, it is highly advisable to perform an analysis of variation in susceptibility among field and laboratory populations.
• Herrero, S., González-Cabrera, J., Tabashnik, B. E., & Ferré, J. (2001). Shared Binding Sites in Lepidoptera for Bacillus thuringiensis Cry1Ja and Cry1A Toxins. Applied and Environmental Microbiology, 67(12), 5729-5734.
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  PMID: 11722929;PMCID: PMC93366;Abstract: Bacillus thuringiensis toxins act by binding to specific target sites in the insect midgut epithelial membrane. The best-known mechanism of resistance to B. thuringiensis toxins is reduced binding to target sites. Because alteration of a binding site shared by several toxins may cause resistance to all of them, knowledge of which toxins share binding sites is useful for predicting cross-resistance. Conversely, cross-resistance among toxins suggests that the toxins share a binding site. At least two strains of diamondback moth (Plutella xylostella) with resistance to Cry1A toxins and reduced binding of CrylA toxins have strong cross-resistance to Cry1Ja. Thus, we hypothesized that Cry1Ja shares binding sites with Cry1A toxins. We tested this hypothesis in six moth and butterfly species, each from a different family: Cacyreus marshalli (Lycaenidae), Lobesia botrana (Tortricidae), Manduca sexta (Sphingidae), Pectinophora gossypiella (Gelechiidae), P. xylostella (Plutellidae), and Spodoptera exigua (Noctuidae). Although the extent of competition varied among species, experiments with biotinylated Cry1Ja and radiolabeled Cry1Ac showed that Cry1Ja and Cry1Ac competed for binding sites in all six species. A recent report also indicates shared binding sites for Cry1Ja and CrylA toxins in Heliothis virescens (Noctuidae). Thus, shared binding sites for Cry1Ja and Cry1A occur in all lepidopteran species tested so far.
• Liu, Y., Tabashnik, B. E., Dennehy, T. J., Patin, A. L., Sims, M. A., Meyer, S. K., & Carrière, Y. (2001). Effects of Bt cotton and Cry1Ac toxin on survival and development of pink bollworm (Lepidoptera: Gelechiidae). Journal of Economic Entomology, 94(5), 1237-1242.
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  PMID: 11681689;Abstract: We evaluated the effects of Bacillus thuringiensis (Bt) toxin Cry1Ac on survival and development of a susceptible strain and laboratory-selected resistant strains of pink bollworm, Pectinophora gossypiella (Saunders). For susceptible and resistant strains tested on artificial diet, increases in Cry1Ac concentration reduced developmental rate and pupal weight. In greenhouse tests, survival of resistant larvae on transgenic cotton that produces Cry1Ac (Bt cotton) was 46% relative to their survival on non-Bt cotton. In contrast, Bt cotton killed all susceptible larvae tested. F1 hybrid progeny of resistant and susceptible adults did not survive on Bt cotton, which indicates recessive inheritance of resistance. Compared with resistant or susceptible larvae reared on non-Bt cotton, resistant larvae reared on Bt cotton had lower survival and slower development, and achieved lower pupal weight and fecundity. Recessive resistance to Bt cotton is consistent with one of the basic assumptions of the refuge strategy for delaying resistance to Bt cotton. Whereas slower development of resistant insects on Bt cotton could increase the probability of mating between resistant adults and accelerate resistance, negative effects of Bt cotton on the survival and development of resistant larvae could delay evolution of resistance.
• Liu, Y., Tabashnik, B. E., Meyer, S. K., & Crickmore, N. (2001). Cross-Resistance and Stability of Resistance to Bacillus thuringiensis Toxin Cry1C in Diamondback Moth. Applied and Environmental Microbiology, 67(7), 3216-3219.
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  PMID: 11425744;PMCID: PMC93003;Abstract: We tested toxins of Bacillus thuringiensis against larvae from susceptible, Cry1C-resistant, and Cry1A-resistant strains of diamondback moth (Plutella xylostella). The Cry1C-resistant strain, which was derived from a field population that had evolved resistance to B. thuringiensis subsp. kurstaki and B. thuringiensis subsp. aizawai, was selected repeatedly with Cry1C in the laboratory. The Cry1C-resistant strain had strong cross-resistance to Cry1Ab, Cry1Ac, and Cry1F, low to moderate cross-resistance to Cry1Aa and Cry9Ca, and no cross-resistance to Cry1Bb, Cry1Ja, and Cry2A. Resistance to Cry1C declined when selection was relaxed. Together with previously reported data, the new data on the cross-resistance of a Cry1C-resistant strain reported here suggest that resistance to Cry1A and Cry1C toxins confers little or no cross-resistance to Cry1Bb, Cry2Aa, or Cry9Ca. Therefore, these toxins might be useful in rotations or combinations with Cry1A and Cry1C toxins. Cry9Ca was much more potent than Cry1Bb or Cry2Aa and thus might be especially useful against diamondback moth.
• Liu, Y., Tabashnik, B. E., Meyer, S. K., Carrière, Y., & Bartlett, A. C. (2001). Genetics of pink bollworm Resistance to Bacillus thuringiensis toxin CrylAc. Journal of Economic Entomology, 94(1), 248-252.
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  PMID: 11233121;Abstract: Laboratory selection increased resistance of pink bollworm (Pectinophora gossypiella) to the Bacillus thuringiensis toxin CrylAc. Three selections with CrylAc in artificial diet increased resistance from a low level to >100-fold relative to a susceptible strain. We used artificial diet bioassays to test F1 hybrid progeny from reciprocal crosses between resistant and susceptible strains. The similarity between F1 progeny from the two reciprocal crosses indicates autosomal inheritance of resistance. The dominance of resistance to CrylAc depended on the concentration. Resistance was codominant at a low concentration of CrylAc, partially recessive at an intermediate concentration, and completely recessive at a high concentration. Comparison of the artificial diet results with previously reported results from greenhouse bioassays shows that the high concentration of CrylAc in bolls of transgenic cotton is essential for achieving functionally recessive inheritance of resistance. © 2001 Entomological Society of America.
• Meyer, S. K., Tabashnik, B. E., Liu, Y. -., Wirth, M. C., & Federici, B. A. (2001). Cyt1A from Bacillus thuringiensis lacks toxicity to susceptible and resistant larvae of diamondback moth (Plutella xylostella) and pink bollworm (Pectinophora gossypiella). Applied and Environmental Microbiology, 67(1), 462-463.
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  PMID: 11133481;PMCID: PMC92601;Abstract: We tested Cyt1Aa, a cytolylic endotoxin of Bacillus thuringiensis, against susceptible and Cry1A-resistant larvae of two lepidopteran pests, diamondback moth (Plutella xylostella) and pink bollworm (Pectinophora gossypiella). Unlike previous results obtained with mosquito and beetle larvae, Cyt1Aa alone or in combination with Cry toxins was not highly toxic to the lepidopteran larvae that we examined.
• Tabashnik, B. E. (2001). Breaking the code of resistance.. Nature biotechnology, 19(10), 922-924.
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  PMID: 11581654;
• Tabashnik, B. E., Dennehy, T. J., Carrière, Y., & Obrycki, J. J. (2001). Supporting a cautious approach to agricultural biotechnology [2] (multiple letters). BioScience, 51(11), 905-906.
• Tabashnik, B., Carrière, Y., & Tabashnik, B. E. (2001). Reversing insect adaptation to transgenic insecticidal plants. Proceedings. Biological sciences / The Royal Society, 268(1475).
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  The refuge-high-dose strategy for delaying insect adaptation to transgenic plants produces non-transgenic plants that enable survival of susceptible individuals. Previous theoretical work has suggested three requirements for success of the refuge-high-dose strategy: a low initial frequency of the resistance allele, extensive mating between resistant and susceptible adults and recessive inheritance of resistance. In order to understand an observed decrease in resistance frequency and improve the potential for managing resistance better, we used analytical and simulation models for exploring the conditions that prevent or reverse the evolution of resistance, even when resistance is not rare initially. Assuming random mating and recessive or nearly recessive inheritance of resistance, the factors favouring reversal of resistance are non-recessive costs of resistance, low initial resistance allele frequency, large refuges, incomplete resistance and density-independent population growth in refuges.
• Tabashnik, B., Carrière, Y., Dennehy, T. J., Pedersen, B., Haller, S., Ellers-Kirk, C., Antilla, L., Liu, Y. B., Willott, E., & Tabashnik, B. E. (2001). Large-scale management of insect resistance to transgenic cotton in Arizona: can transgenic insecticidal crops be sustained?. Journal of economic entomology, 94(2).
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  A major challenge for agriculture is management of insect resistance to toxins from Bacillus thuringiensis (Bt) produced by transgenic crops. Here we describe how a large-scale program is being developed in Arizona for management of resistance to Bt cotton in the pink bollworm, Pectinophora gossypiella (Saunders) (Lepidoptera: Gelechiidae), and other insect pests of cotton. Financial support from growers makes this program possible. Collaboration between the Arizona Cotton Research and Protection Council, the University of Arizona, and government agencies has led to development of resistance management guidelines, a remedial action plan, and tools for monitoring compliance with the proposed guidelines. Direct participation in development of resistance management policies is a strong incentive for growers to invest in resistance management research. However, more research, regularly updated regulations, and increased collaboration between stakeholders are urgently needed to maintain efficacy of Bt toxins in transgenic crops.
• Tabashnik, B., Carrière, Y., Ellers-Kirk, C., Liu, Y. B., Sims, M. A., Patin, A. L., Dennehy, T. J., & Tabashnik, B. E. (2001). Fitness costs and maternal effects associated with resistance to transgenic cotton in the pink bollworm (Lepidoptera: Gelechiidae). Journal of economic entomology, 94(6).
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  Transgenic cotton producing a Bacillus thuringiensis (Bt) toxin is widely used for controlling the pink bollworm, Perctinophora gossypiella (Saunders). We compared performance of pink bollworm strains resistant to Bt cotton with performance of their susceptible counterparts on non-Bt cotton. We found fitness costs that reduced survival on non-Bt cotton by an average of 51.5% in two resistant strains relative to the susceptible strains. The survival cost was recessive in one set of crosses between a resistant strain and the susceptible strain from which it was derived. However, crosses involving an unrelated resistant and susceptible strain indicated that the survival cost could be dominant. Development time on non-Bt cotton did not differ between the two related resistant and susceptible strains. A slight recessive cost affecting development time was suggested by comparison of the unrelated resistant and susceptible strains. Maternal effects transmitted by parents that had eaten Bt-treated artificial diet as larvae had negative effects on embryogenesis, adult fertility, or both, and reduced the ability of neonates to enter cotton bolls. These results provide further evidence that fitness costs associated with the evolution of resistance to Bt cotton are substantial in the pink bollworm.
• Tabashnik, B., Carrière, Y., Ellers-Kirk, C., Patin, A. L., Sims, M. A., Meyer, S., Liu, Y. B., Dennehy, T. J., & Tabashnik, B. E. (2001). Overwintering cost associated with resistance to transgenic cotton in the pink bollworm (Lepidoptera: Gelechiidae). Journal of economic entomology, 94(4).
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  Fitness costs associated with resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) may have important effects on the evolution of resistance. We investigated overwintering costs in pink bollworm, Pectinophora gosypiella (Saunders), strains with different degrees of resistance to Bt cotton. Frequency of resistant individuals in a strain was not associated with induction of diapause or emergence from diapause in early winter. Emergence from diapause in the spring was 71% lower in three highly resistant strains than in two heterogeneous strains from which the resistant strains were derived. This underestimates the overwintering cost because the frequency of the resistance allele was relatively high in the heterogeneous strains. Emergence in the spring in hybrid progeny from crosses between the resistant and heterogeneous strains was greater than in resistant strains but did not differ from susceptible strains, showing that the overwintering cost was recessive to some extent.
• Groeters, F. R., & Tabashnik, B. E. (2000). Roles of selection intensity, major genes, and minor genes in evolution of insecticide resistance. Journal of Economic Entomology, 93(6), 1580-1587.
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  PMID: 11142284;Abstract: A prominent hypothesis about insecticide resistance is that genes of major effect play a key role in field-evolved resistance because the intensity of selection is extremely high in the field. A corollary hypothesis is that the lower intensity of selection in laboratory selection experiments favors polygenic control of insecticide resistance. Contrary to these hypotheses, a literature review revealed that the intensity of selection for insecticide resistance in the field varies widely and overlaps broadly with selection intensities in the laboratory. Also contrary to these hypotheses, results from simulations of population genetic models suggest that selection intensities typical of laboratory selection experiments favor resistance that is conferred by major genes. Major genes dominated responses to selection for resistance across a wide range of simulated selection intensities, with and without fitness costs and refuges. The simulation results also suggest that the intensity of selection, rather than the number of loci conferring resistance, is central in determining rates of resistance evolution and effectiveness of refuges. © 2000 Entomological Society of America.
• Jin, R. -., Liu, Y. -., Tabashnik, B. E., & Borthakur, D. (2000). Development of transgenic cabbage (Brassica oleracea var. capitata) for insect resistance by agrobacterium tumefaciens-mediated transformation. In Vitro Cellular and Developmental Biology - Plant, 36(4), 231-237.
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  Abstract: Transgenic head cabbage (Brassica oleracea var. capitata), resistant to diamondback moth (Plutella xylostella) larvae, was developed through Agrobacterium tumefaciens-mediated transformation with Bacillus thuringiensis (Bt) cry genes using a modified procedure. Factors important for transformation included cabbage cultivar; preculture and coculture of explants on a callus initiation medium; use of appropriate amount; and delay in initial application of selective agents. A total of 15 independent transformed lines with over 100 plants were obtained from several transformation experiments, representing an overall transformation efficiency of ~ 1%. Cabbage plants transformed with a synthetic Bt gene, cry1Ab3, were all resistant to larvae of the diamondback moth, whereas all plants transgenic for cry1Ia3, a wild-type Bt gene, were susceptible. As a first step towards testing the hypothesis that reduced exposure of Bt to target insects would delay the evolution of insect resistance to Bt, cry1Ab3 expression was put under the transcriptional control of the soybean wound-inducible vspB promoter and transgenic cabbage was obtained. Insect bioassay showed that such plants were all resistant to diamondback moth even without induction for the expression of Bt.
• Liu, Y., Tabashnik, B. E., Masson, L., Escriche, B., & Ferré, J. (2000). Binding and toxicity of Bacillus thuringiensis Protein Cry1C to susceptible and resistant diamondback moth (lepidoptera: plutellidae). Journal of Economic Entomology, 93(1), 1-6.
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  PMID: 14658503;Abstract: We studied mechanisms of resistance to Bacillus thuringiensis insecticidal crystal protein Cry1C in the diamondback moth, Plutella xylostella (L.). Binding assays with midgut brush border membrane vesicles prepared from whole larvae showed no significant difference between resistant and susceptible strains in binding of radioactively-labeled Cry1C. These results indicate that reduced binding of Cry1C to midgut membrane target sites did not cause resistance to Cry1C. Thus, the mechanism of resistance to Cry1C differs from that observed in several previously reported cases of resistance to Cry1A toxins in diamondback moth. We tested Cry1C toxin and Cry1C crystalline protoxin against resistant and susceptible larvae using leaf disk bioassays. After adjusting for the size difference between Cry1C toxin and protoxin, we found that with resistant larvae, toxin was significantly more toxic than protoxin. In contrast, with susceptible larvae, no significant difference in toxicity occurred between Cry1C toxin and protoxin. The resistance ratios for Cry1C were 19 for toxin and 48 for protoxin. These results suggest that reduced conversion of Cry1C protoxin to toxin is a minor mechanism of resistance to Cry1C. Because neither reduced binding nor reduced conversion of protoxin to toxin appear to be major mechanisms, one or more other mechanisms are important in diamondback moth resistance to Cry1C. © 2000 Entomological Society of America.
• Tabashnik, B. E., Liu, Y. -., Maagd, R. D., & Dennehy, T. J. (2000). Cross-resistance of pink bollworm (Pectinophora gossypiella) to Bacillus thuringiensis toxins. Applied and Environmental Microbiology, 66(10), 4582-4584.
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  PMID: 11010923;PMCID: PMC92349;Abstract: Two strains of pink bollworm (Pectinophora gossypiella) selected in the laboratory for resistance to Bacillus thuringiensis toxin Cry1Ac had substantial cross-resistance to Cry1Aa and Cry1Ab but not to Cry1Bb, Cry1Ca, Cry1Da, Cry1Ea, Cry1Ja, Cry2Aa, Cry9Ca, H04, or H205. The narrow spectrum of resistance and the cross-resistance to activated toxin Cry1Ab suggest that reduced binding of toxin to midgut target sites could be an important mechanism of resistance.
• Tabashnik, B. E., Patin, A. L., Dennehy, T. J., Liu, Y., Carrière, Y., Sims, M. A., & Antilla, L. (2000). Frequency of resistance to Bacillus thuringiensis in field populations of pink bollworm. Proceedings of the National Academy of Sciences of the United States of America, 97(24), 12980-12984.
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  PMID: 11087854;PMCID: PMC27164;Abstract: Strategies for delaying pest resistance to genetically modified crops that produce Bacillus thuringiensis (Bt) toxins are based primarily on theoretical models. One key assumption of such models is that genes conferring resistance are rare. Previous estimates for lepidopteran pests targeted by St crops seem to meet this assumption. We report here that the estimated frequency of a recessive allele conferring resistance to Bt toxin Cry1Ac was 0.16 (95% confidence interval = 0.05-0.26) in strains of pink bollworm (Pectinophora gossypiella) derived from 10 Arizona cotton fields during 1997. Unexpectedly, the estimated resistance allele frequency did not increase from 1997 to 1999 and St cotton remained extremely effective against pink bollworm. These results demonstrate that the assumptions and predictions of resistance management models must be reexamined.
• Tabashnik, B. E., Roush, R. T., Earle, E. D., Shelton, A. M., Huang, F., Buschman, L., Higgins, R., & McGaughey, W. (2000). Resistance to Bt toxins [3] (multiple letters). Science, 287(5450), 42-.
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  PMID: 10644220;
• Tabashnik, B. E., Tabashnik, B. E., Johnson, K. W., Johnson, K. W., Engleman, J. T., Engleman, J. T., Baum, J. A., & Baum, J. A. (2000). Cross-resistance to Bacillus thuringiensis toxin Cry1Ja in a strain of diamondback moth adapted to artificial diet. Journal of Invertebrate Pathology, 76(1), 81-83.
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  PMID: 10963409;
• Tabashnik, B., Groeters, F. R., & Tabashnik, B. E. (2000). Roles of selection intensity, major genes, and minor genes in evolution of insecticide resistance. Journal of economic entomology, 93(6).
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  A prominent hypothesis about insecticide resistance is that genes of major effect play a key role in field-evolved resistance because the intensity of selection is extremely high in the field. A corollary hypothesis is that the lower intensity of selection in laboratory selection experiments favors polygenic control of insecticide resistance. Contrary to these hypotheses, a literature review revealed that the intensity of selection for insecticide resistance in the field varies widely and overlaps broadly with selection intensities in the laboratory. Also contrary to these hypotheses, results from simulations of population genetic models suggest that selection intensities typical of laboratory selection experiments favor resistance that is conferred by major genes. Major genes dominated responses to selection for resistance across a wide range of simulated selection intensities, with and without fitness costs and refuges. The simulation results also suggest that the intensity of selection, rather than the number of loci conferring resistance, is central in determining rates of resistance evolution and effectiveness of refuges.
• Ballester, V., Granero, F., Tabashnik, B. E., Malvar, T., & Ferré, J. (1999). Integrative model for binding of Bacillus thuringiensis toxins in susceptible and resistant larvae of the diamondback moth (Plutella xylostella). Applied and Environmental Microbiology, 65(4), 1413-1419.
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  PMID: 10103230;PMCID: PMC91200;Abstract: Insecticidal crystal proteins from Bacillus thuringiensis in sprays and transgenic crops are extremely useful for environmentally sound pest management, but their long-term efficacy is threatened by evolution of resistance by target pests. The diamondback moth (Plutella xylostella) is the first insect to evolve resistance to B. thuringiensis in open-field populations. The only known mechanism of resistance to B. thuringiensis in the diamondback moth is reduced binding of toxin to midgut binding sites. In the present work we analyzed competitive binding of B. thuringiensis toxins Cry1Aa, Cry1Ab, Cry1Ac, and Cry1F to brush border membrane vesicles from larval midguts in a susceptible strain and in resistant strains from the Philippines, Hawaii, and Pennsylvania. Based on the results, we propose a model for binding of B. thuringiensis crystal proteins in susceptible larvae with two binding sites for Cry1Aa, one of which is shared with Cry1Ab, Cry1AC, and Cry1F. Our results show that the common binding site is altered in each of the three resistant strains. In the strain from the Philippines, the alteration reduced binding of Cry1Ab but did not affect binding of the other crystal proteins. In the resistant strains from Hawaii and Pennsylvania, the alteration affected binding of Cry1Aa, Cry1Ab, Cry1Ac, and Cry1F. Previously reported evidence that a single mutation can confer resistance to Cry1Ab, Cry1Ac, and Cry1F corresponds to expectations based on the binding model. However, the following two other observations do not: the mutation in the Philippines strain affected binding of only Cry1Ab, and one mutation was sufficient for resistance to Cry1Aa. The imperfect correspondence between the model and observations suggests that reduced binding is not the only mechanism of resistance in the diamondback moth and that some, but not all, patterns of resistance and cross-resistance can be predicted correctly from the results of competitive binding analyses of susceptible strains.
• Chang, W. X., Gahan, L. J., Tabashnik, B. E., & Heckel, D. G. (1999). A new aminopeptidase from diamondback moth provides evidence for a gene duplication event in Lepidoptera. Insect Molecular Biology, 8(2), 171-177.
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  PMID: 10380100;Abstract: We screened a midgut cDNA library from diamondback moth, Plutella xylostella, with a probe generated using sequence information from an aminopeptidase N gene from Manduca sexta (MsAPN-1). The sequence recovered (PxAPN-A) encodes a protein of 988 resides with a 60% sequence identity to MsAPN-1. The two proteins share a signal peptide which directs processing by the endoplasmic reticulum, a C-terminal hydrophobic region satisfying the criterion for a GPI anchor and cleavage, and the possibility of an O-glycosylated rigid stalk attached to the GPI anchor. PxAPN-A is more closely related to MsAPN-1 than it is to another aminopeptidase recently reported from P. xylostella. Sequence comparisons with other species suggests that at least one aminopeptidase gene duplication occurred in an ancestral lepidopteran.
• Chilcutt, C. F., & Tabashnik, B. E. (1999). Effects of Bacillus thuringiensis on adults of Cotesia plutellae (Hymenoptera: Braconidae), a parasitoid of the Diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae). Biocontrol Science and Technology, 9(3), 435-440.
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  Abstract: To complement existing information on the mortality of larvae of the wasp Cotesia plutellae attacking moth caterpillars infected with Bacillus thuringiensis (Bt) we tested the direct and indirect effects of the bacterium on adult wasp longevity and oviposition behaviour. In one experiment with female parasitoids, mean longevity (± SEM) was not significantly different between females exposed to Bt (1.98±0.08 days) and those not exposed (2.18±0.13 days). In a second experiment with both males and females, Bt treatment did not significantly effect either male or female parasitoids exposed to Bt. To observe the possible effects of Bt on oviposition behaviour of C. plutellae each of 10 females were given five larvae that had been treated with Bt and five untreated larvae at the same time. All parasitoids were observed to make oviposition attempts in both untreated and treated larvae. Upon dissection of the host larvae, one or more C. plutellae eggs were found in each of the larvae in which a parasitoid attempted oviposition. There was no effect of Bt treatment on parasitoid oviposition. The mean number of ovipositions in treated larvae (4.2±0.3) was not significantly different from untreated larvae (4.7±0.2).
• Chilcutt, C. F., & Tabashnik, B. E. (1999). Simulation of Integration of Bacillus thuringiensis and the Parasitoid Cotesia plutellae (Hymenoptera: Braconidae) for Control of Susceptible and Resistant Diamondback Moth (Lepidoptera: Plutellidae). Environmental Entomology, 28(3), 505-512.
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  Abstract: We used computer simulations to better understand how to combine the microbial insecticide Bacillus thuringiensis Berliner and the parasitoid Cotesia plutellae Kurdjurnov to control diamondback moth, Plutella xylostella (L.). The simulations incorporated potential for the evolution of moth resistance to Bacillus thuringiensis with Bacillus thuringiensis-parasitoid-pest interactions. We considered the effects of pest immigration, pesticide refuge, and pesticide dose on diamondback moth control and resistance development by diamondback moth. Four major conclusions were drawn. First, C. plutellae and B thuringiensis could be used effectively to control diamondback moth. Second, the use of parasitoids such as C. plutellae could slow the evolution of pesticide resistance by decreasing the number of generations in which pesticide treatment is required. Third, parasitoids could be complementary to refuges from pesticides, with parasitoids maintaining control of a pest while refuges slow the development of resistance to the pesticide by the pest. Fourth, in conjunction with the use of parasitoids, high dose or low dose strategies have similar consequences on pest density over long periods. However, unless there is substantial immigration of susceptible individuals into the pest population or some type of pesticide refuge, a low dose strategy is better for controlling pests.
• Heckel, D. G., Gahan, L. J., Liu, Y., & Tabashnik, B. E. (1999). Genetic mapping of resistance to Bacillus thuringiensis toxins in diamondback moth using biphasic linkage analysis. Proceedings of the National Academy of Sciences of the United States of America, 96(15), 8373-8377.
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  PMID: 10411882;PMCID: PMC17523;Abstract: Transgenic plants producing environmentally benign Bacillus thuringiensis (Bt) toxins are deployed increasingly for insect control, but their efficacy will be short-lived if pests adapt quickly. The diamondback moth (Plutella xylostella), a worldwide pest of vegetables, is the first insect to evolve resistance to Bt toxins in open-field populations. A recessive autosomal gene confers resistance to at least four Bt toxins and enables survival without adverse effects on transgenic plants. Allelic variants of this gene confer resistance in strains from Hawaii, Pennsylvania, and the Philippines. Here we exploited the biphasic nature of Lepidopteran genetic linkage to map this gene in diamondback moth with 207 amplified fragment length polymorphisms as DNA markers. We also cloned and sequenced an amplified fragment length polymorphism marker for the chromosome containing the Bt resistance gene. The results provide a powerful tool for facilitating progress in understanding, monitoring, and managing resistance to Bt.
• Jin, R., Liu, Y., Tabashnik, B. E., & Borthakur, D. (1999). Tissue culture and Agrobacterium-mediated transformation of watercress. Plant Cell, Tissue and Organ Culture, 58(3), 171-176.
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  Abstract: Adventitious shoot regeneration could be obtained from more than 80% of the calluses initiated from stem explants of watercress (Rorippa nasturtium-aquaticum) by using an induction medium and a shoot regeneration medium. The induction medium contained 1.15 μM 2,4-dichlorophenoxyacetic acid and 5 μM thidiazuron; the shoot regeneration medium was composed of 0.5 μM thidiazuron and 2.25 μM 6-benzylaminopurine. This regeneration procedure was incorporated into an Agrobacterium-mediated transformation procedure for gene transfer into watercress. Factors affecting transformation included preculture, selection agents, use of tobacco nurse cells, and the length of coculture. A transgenic line of watercress transformed with a wild-type Bacillus thuringiensis insecticidal gene, cryIIa3, was not toxic to larvae of the diamondback moth (Plutella xylostella), presumably due to premature polyadenylation of the transcript encoded by this gene in the plant.
• Liu, Y. -., Tabashnik, B. E., Dennehy, T. J., Patin, A. L., & Bartlett, A. C. (1999). Development time and resistance to Bt crops [5]. Nature, 400(6744), 519-.
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  PMID: 10448853;
• Masson, L., Tabashnik, B. E., Yong-Biao, L., Brousseau, R., & Schwartz, J. (1999). Helix 4 of the Bacillus thuringiensis Cry1Aa toxin lines the lumen of the ion channel. Journal of Biological Chemistry, 274(45), 31996-32000.
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  PMID: 10542230;Abstract: The mode of action of Bacillus thuringiensis insecticidal proteins is not well understood. Based on analogies with other bacterial toxins and ion channels, we hypothesized that charged amino acids in helix 4 of the Cry1Aa toxin are critical for toxicity and ion channel function. Using Plutella xylostella as a model target, we analyzed responses to Cry1Aa and eight proteins with altered helix 4 residues. Toxicity was abolished in five charged residue mutants (E129K, R131Q, R131D, D136N, D136C), however, two charged (R127E and R127N) and one polar (N138C) residue mutant retained wild- type toxicity. Compared with Cry1Aa and toxic mutants, nontoxic mutants did not show greatly reduced binding to brush border membrane vesicles, but their ion channel conductance was greatly reduced in planar lipid bilayers. Substituted cysteine accessibility tests showed that in situ restoration of the negative charge of D136C restored conductance to wild-type levels. The results imply that charged amino acids on the Asp-136 side of helix 4 are essential for toxicity and passage of ions through the channel. These results also support a refined version of the umbrella model of membrane integration in which the side of helix 4 containing Asp-136 faces the aqueous lumen of the ion channel.
• Patin, A. L., Dennehy, T. J., Sims, M. A., Tabashnik, B. E., Liu, Y. -., Antilla, L., Gouge, D., Henneberry, T. J., & Staten, R. (1999). Status of pink bollworm susceptibility to Bt in Arizona. Proceedings of the 1999 Beltwide Cotton Conference, January 1999, Orlando, Florida, USA, 991-996.
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  Abstract: Statewide monitoring of pink bollworm susceptibility to Bt toxin was conducted in Arizona in 1997 and 1998. In bioassays in which Bt toxin was incorporated into insect diet, a concentration of 10 μg/ml Cry 1 Ac caused >80% mortality of all field populations. LC50s of Arizona populations ranged from 0.35 to 1.7 μg Cry 1 Ac/ml of insect diet. A laboratory strain of pink bollworm had an LC50 of 0.53 μg/ml. A selection experiment was conducted in which survivors of bioassays of 3.2 and 10 μg/ml of Cry 1 Ac from all Arizona populations tested in 1997 were pooled and reared for one generation on diet containing 10 μg Cry 1 Ac/ml. Selection produced a strain of pink bollworm that was 100- to 460-fold less susceptible to Cry 1 Ac, than were the field populations from which it was derived. Preliminary results from greenhouse evaluations concerning survival of this resistant strain on Bt cotton showed that larvae were able to complete development in bolls of Bt cotton, pupate, and successfully reproduce. We conclude from these preliminary results that the type of resistance we have isolated in the laboratory is likely to negatively impact field performance of Bt cotton in Arizona in the future. However, we found no evidence that field performance of Bt cotton has yet been compromised. In large field trials, in-field refuges of non-Bt cotton were contrasted with the standard, external refuges. During both 1997 and 1998, in-field refuge plots yielded comparable to, or better than, plots of 100% Bt cotton, yet they produced substantial numbers of pink bollworm late in the season on non-Bt plants.
• Tabashnik, B. E., Patin, A. L., Dennehy, T. J., Liu, Y. -., Miller, E., & Staten, R. T. (1999). Dispersal of pink bollworm (Lepidoptera: Gelechiidae) males in transgenic cotton that produces a Bacillus thuringiensis toxin. Journal of Economic Entomology, 92(4), 772-780.
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  Abstract: To delay evolution of pest resistance to transgenic cotton that produces Bacillus thuringiensis toxin Cry1Ac (Bt cotton), refuges composed of cotton that does not produce Cry1Ac (non-Bt cotton) are mandated by the United States Environmental Protection Agency. One critical assumption of this strategy is that susceptible adults emerging from non-Bt cotton refuges mate with resistant adults emerging from Bt cotton, which requires movement of adults between Bt cotton and non-Bt cotton. To better understand movements of pink bollworm, the key lepidopteran pest of cotton in Arizona, we analyzed the distribution of wild males and dispersal of sterile males during 1997 on a 259-ha block of a commercial farm containing 76 ha of Bt cotton (69%) and 34 ha of non-Bt cotton (31%). The distribution of 28,397 wild males caught in pheromone traps between 22 May and 20 September suggests that many moved at least 400 m from non-Bt cotton to Bt cotton, yet movement was not sufficient to distribute wild males randomly between Bt cotton and non-Bt cotton. The average number of wild males caught per trap in non-Bt cotton was nearly double that for Bt cotton. Of the estimated 176,000 sterile males released on 8 dates from 18 June through 4 July, we captured 4,447 in pheromone traps between 19 June and 24 July, which represents a recovery rate of 2.5%. The results show that 66-94% of captured sterile males dispersed 400 m or less from the release sites. Along with previously published data, the results reported here suggest that, to promote mating between susceptible and resistant adults, refuges for pink bollworm should be close to Bt cotton.
• Baur, M. E., Kaya, H. K., Tabashnik, B. E., & Chilcutt, C. F. (1998). Suppression of Diamondback Moth (Lepidoptera: Plutellidae) with an Entomopathogenic Nematode (Rhabditida: Steinernematidae) and Bacillus thuringiensis Berliner. Journal of Economic Entomology, 91(5), 1089-1095.
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  PMID: 9805498;Abstract: We tested the efficacy of the All strain of Steinernema carpocapsae (Weiser) against larvae of the diamondback moth, Plutella xylostella (L.). In laboratory bioassays we found that (1) commercially formulated nematodes produced in vitro were as effective as nematodes produced in vivo, (2) resistance of P. xylostella to Bacillus thuringiensis Berliner subsp. kurstaki did not confer cross-resistance to nematodes, (3) mortality caused by nematodes was higher for early than late 3rd-instar P. xylostella larvae, and (4) no interaction occurred when B. thuringieinsis and nematodes were combined against a susceptible strain of P. xylostella, but an antagonistic interaction occurred between the 2 pathogens against a strain of P. xylostella resistant to B. thuringiensis. In field trials conducted on 2 watercress [Rorippa Nasturtium-aquaticum (L.) Hayek] farms in Hawaii, nematodes provided 41% control, B. thuringiensis subsp. aizawai gave 44% control, and the combined treatment (B. thuringiensis plus nematodes both at half rate) resulted in 58% control. Using nemodes to control diamondback moth can theoretically reduce resistance development in diamondback moth populations to B. thuringiensis products, but repeated applications of nematodes will probably be ineffective in attaining control (suggested in simulation model). The results of this study demonstrate that nematodes may be a useful component of integrated pest management programs if efficacy can be increased, especially for populations of P. xylostella that are resistant to B. thuringiensis.
• Liu, Y., & Tabashnik, B. E. (1998). Elimination of a Recessive Allele Conferring Resistance to Bacillus thuringiensis from a Heterogeneous Strain of Diamondback Moth (Lepidoptera: Plutellidae). Journal of Economic Entomology, 91(5), 1032-1037.
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  Abstract: We devised and tested a procedure for eliminating a recessive allele conferring resistance to Bacillus thuringiensis subsp. kurstaki from a laboratory strain of diamondback moth, Plutella xylostella (L.), composed of resistant and susceptible individuals. We established a homozygous susceptible strain (LAB-PS) from a heterogeneous strain (LAB-P) as follows: We obtained F1 progeny from 7 single-pair families from the heterogeneous strain. Hybrid F2 progeny were produced in 7 mass crosses, each of which involved 10 female F1 progeny from a single-pair family and 10 males from a resistant strain (NO-QA). The hybrid F2 progeny were tested in bioassays with a liquid formulation of B. thuringiensis toxin Cry1Ab at a diagnostic concentration that kills susceptible homozygotes and heterozygotes, but not resistant homozygotes. If the resistance allele occurred in either of the 2 parents that produced a particular F1 family, the hybrid F2 progeny derived from that family were expected to contain at least 25% homozygous resistant individuals that would survive exposure to the diagnostic concentration of Cry1Ab. Conversely, 0% survival of a set of hybrid F2 progeny in the diagnostic bioassay would indicate that the single-pair family from which it was derived was homozygous susceptible. We found 0% survival in 1 set of hybrid F2 progeny and used the F1 single-pair family from which this set was derived to establish a homozygous susceptible strain. Subsequent bioassays showed that diagnostic concentrations of Cry1Ab or Cry1Aa killed 100% of larvae tested from this strain. The LC50 of Cry1Ab at 5 d for the susceptible strain was 7-fold lower than that for the heterogeneous strain. The procedure can be adapted for other insects and other traits, such as resistance to other insecticides.
• Liu, Y., Tabashnik, B. E., Moar, W. J., & Smith, R. A. (1998). Synergism between Bacillus thuringiensis spores and toxins against resistant and susceptible diamondback moths (Plutella xylostella). Applied and Environmental Microbiology, 64(4), 1385-1389.
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  PMID: 16349543;PMCID: PMC106159;Abstract: We studied the effects of combinations of Bacillus thuringiensis spores and toxins on the mortality of diamondback moth (Plutella xylostella) larvae in leaf residue bioassays. Spores of B. thurigiensis subsp. kurstaki increased the toxicity of crystals of B. thuringiensis subsp. kurstaki to both resistant and susceptible larvae. For B. thuringiensis subsp. kurstaki, resistance ratios were 1,200 for a spore-crystal mixture and 56,000 for crystals without spores. Treatment of a spore-crystal formulation of B. thuringiensis subsp. kurstaki with the antibiotic streptomycin to inhibit spore germination reduced toxicity to resistant larvae but not to susceptible larvae. In contrast, analogous experiments with B. thuringiensis subsp. aizawai revealed no significant effects of adding spores to crystals or of treating a spore-crystal formulation with streptomycin. Synergism occurred between Cry2A and B. thuringiensis subsp. kurstaki spores against susceptible larvae and between Cry1C and B. thuringiensis subsp. aizawai spores against resistant and susceptible larvae. The results show that B. thuringiensis toxins combined with spores can be toxic even though the toxins and spores have little or no independent toxicity. Results reported here and previously suggest that, for diamondback moth larvae, the extent of synergism between spores and toxins or B. thuringiensis depends on the strain of insect, the type of spore, the set of toxins, the presence of other materials such as formulation ingredients, and the concentrations of spores and toxins.
• Ramachandran, S., Buntin, G. D., All, J. N., Tabashnik, B. E., Raymer, P. L., Adang, M. J., Pulliam, D. A., & Stewart Jr., C. N. (1998). Survival, Development, and Oviposition of Resistant Diamondback Moth (Lepidoptera: Plutellidae) on Transgenic Canola Producing a Bacillus thuringiensis Toxin. Journal of Economic Entomology, 91(6), 1239-1244.
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  Abstract: We measured responses of diamondback moth, Plutella xylostella L., to transgenic and nontransgenic canola, Brassica napus L. Transgenic canola expressed a cry1Ac gene of Bacillus thuringiensis Berliner resulting in 238 ±29 ng of Cry1Ac protein per milligram of total extractable protein in leaves. We tested 2 Hawaiian strains of diamondback moth: NO-QA was resistant to Cry1Ac and LAB-PS was susceptible. Larval and pupal durations, pupal weights, and adult emergence of the 2 strains were similar on nontransgenic canola, but differed significantly on transgenic canola. Transgenic canola killed all larvae tested from the susceptible strain. In contrast, for the resistant strain, no differences occurred between transgenic and nontransgenic canola in larval survival and head capsule width at day 5, percentage pupation, pupal weight, percentage adult emergence, and extent of defoliation. For both the susceptible and resistant strains of diamondback moth, no differences were detected between transgenic and nontransgenic canola in feeding initiation or oviposition preference. The lack of discrimination between transgenic and nontransgenic canola by neonates and ovipositing females indicates that host choice behavior is independent from susceptibility to Cry1Ac. Development of resistant diamondback moth on transgenic canola without any adverse effects provides an example of a pest that has completely overcome high levels of a B. thuringiensis toxin expressed by a genetically engineered plant.
• Simmons, A. L., Dennehy, T. J., Tabashnik, B. E., Antilla, L., Bartlett, A., Gouge, D., & Staten, R. (1998). Evaluation of B.t. cotton deployment strategies and efficacy against pink bollworm in Arizona. 1998 proceedings Beltwide Cotton Conference, January, San Diego, volume 2, 1025-1030.
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  Abstract: A multi-agency team in Arizona in 1997 evaluated B.t. cotton deployment strategies in a large field trial; conducted statewide monitoring of pink bollworm (PBW) susceptibility to the CrylAc endotoxin, and established a Rapid Response Team that investigated claims of unacceptable performance of B.t. cotton. Though needing further evaluation, in-field refuges of one row of non-B.t. cotton for each five rows of B.t. cotton showed promise as an alternative to the current recommendation of external refuges for planting B.t. cotton. Preliminary results of statewide monitoring showed that four field populations were more susceptible to CrylAc than were two reference susceptible laboratory strains. A strain of PBW previously reported to be resistant to CrylAc was confirmed to be significantly less susceptible to this toxin than were the two susceptible laboratory strains or the four field populations tested. The Rapid Response Team, based at the Arizona Cotton Growers Association, investigated nine reports of unusual larval survivorship in B.t. cotton. Only one of these, which has been placed in culture, was confirmed to have resulted in substantial numbers of large larvae surviving in bolls of putatively B.t. cotton. Further investigations of this population and the plants from which it was derived are underway.
• Tabashnik, B. E., Liu, Y., Malvar, T., Heckel, D. G., Masson, L., & Ferré, J. (1998). Insect resistance to Bacillus thuringiensis: Uniform or diverse?. Philosophical Transactions of the Royal Society B: Biological Sciences, 353(1376), 1751-1756.
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  Abstract: Resistance to the insecticidal proteins produced by the soil bacterium Bacillus thuringiensis (Bt) has been documented in more than a dozen species of insect. Nearly all of these cases have been produced primarily by selection in the laboratory, but one pest, the diamondback moth (Plutella xylostella), has evolved resistance in open-field populations. Insect resistance to Bt has immediate and widespread significance because of increasing reliance on Bt toxins in genetically engineered crops and conventional sprays. Furthermore, intense interest in Bt provides an opportunity to examine the extent to which evolutionary pathways to resistance vary among and within species of insect. One mode of resistance to Bt is characterized by more than 500-fold resistance to at least one Cry1A toxin, recessive inheritance, little or no cross-resistance to Cry1C, and reduced binding of at least one Cry1A toxin. Analysis of resistance to Bt in the diamondback moth and two other species of moths suggests that although this particular mode of resistance may be the most common, it is not the only means by which insects can attain resistance to Bt.
• Baur, M. E., Kaya, H. K., & Tabashnik, B. E. (1997). Efficacy of a Dehydrated Steinernematid Nematode Against Black Cutworm (Lepidoptera: Noctuidae) and Diamondback Moth (Lepidoptera: Plutellidae). Journal of Economic Entomology, 90(5), 1200-1206.
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  PMID: 9374596;Abstract: We compared the ability of in vitro-produced, commercially formulated with in vivo-produced, nonformulated Steinernema carpocapsae (Weiser) Poinar. All strain to infect and kill larvae of black cutworm, Agrotis ipsilon (Hufnagel), and diamondback moth, Plutella xylostella (L.). In vitro-produced nematodes formulated in wettable dispersible granules, which were stored dry, were rehydrated in water for 0-72 h before application. Against black cutworms, the efficacy of nematodes was (from most to least effective): in vivo > in vitro rehydrated for 72 h > in vitro rehydrated for 48 h > in vitro rehydrated for 24 h > dehydrated (0 h). Nematodes rehydrated for 72 h in water or moist soil were equally effective against black cutworm larvae, and both were significantly more effective than nematodes without rehydration. These results indicated that nematodes in the wettable dispersible granule formulation required time to rehydrate in the soil before infecting black cutworm larvae. Nematode treatments described above were applied to radish plants held at 100 or 75% RH and tested against diamondback moth larvae. At 100% RH, nematode efficacy was (from most to least effective): in vitro rehydrated for 72 h > in vivo > in vitro rehydrated 48 h > in vitro rehydrated 24 h > dehydrated (0 h). The efficacy of all treatments was lower at 75% than at 100% RH, and the ranking of in vivo and in vitro nematodes rehydrated for 72 h was reversed. The nematodes in the wettable dispersible granule formulation were effective for foliar treatments when humidity was high and nematodes were rehydrated for at least 48 h before application. The data show that nematode infectivity was reduced unless nematodes were rehydrated.
• Baur, M. E., Kaya, H. K., Gaugler, R., & Tabashnik, B. (1997). Effects of adjuvants on entomopathogenic nematode persistence and efficacy against Plutella xylostella. Biocontrol Science and Technology, 7(4), 513-525.
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  Abstract: The effects of spray additives on entomopathogenic nematode persistence and efficacy against Plutella xylostella (L.) were studied. Several adjuvants were toxic to radish seedlings (Raphanus sativus var. capitata L.) but none was toxic to the nematodes or P. xylostella. In the laboratory, the adjuvants that provided the best antidesiccant activity based on a rank score were TX7719, Rodspray oil and Nufilm P. Those providing less protection but better than the remaining adjuvants were 38-F, dextrose and Pluronic F-127. In greenhouse trials, TX7719 and Rodspray oil were more effective than the other adjuvants tested. The stilbene brightener, Blankophor BBH, did not increase nematode efficacy consistently in greenhouse trials probably because the concentration used was too low. In field trials, the combination of TX7719 plus Blankophor BBH increased nematode persistence on watercress leaves (Nasturium officinale R. Br.) and efficacy against P. xylostella significantly. In vitro-produced nematodes benefited more from additives than in vivo-produced nematodes in the laboratory, but that difference was lost in the field. Overall, it was found that additives generally improved nematode persistence and efficacy, but the improvement was probably not sufficient to increase the feasibility of foliar applications of nematodes against P. xylostella. However, further evaluation of adjuvants is warranted for applications of nematodes to watercress for the control of P. xylostella.
• Chilcutt, C. F., & Tabashnik, B. E. (1997). Host-Mediated Competition between the Pathogen Bacillus thuringiensis and the Parasitoid Cotesia plutellae of the Diamondback Moth (Lepidoptera: Plutellidae). Environmental Entomology, 26(1), 38-45.
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  Abstract: We evaluated the effects of variation in phenotype of Plutella xylostella (L.), diamondback moth, on within-host interactions between the bacterial pathogen Bacillus thuringiensis Berliner and larvae of the parasitoid wasp, Cotesia plutellae Kurdjumov. The outcome of interactions between the parasitoid and pathogen depended on host phenotype. Host resistance to the pathogen reduced the success of both the parasitoid and the pathogen. In susceptible hosts, the parasitoid did not affect performance of the pathogen, but the pathogen had a significant negative effect on the parasitoid. In moderately resistant hosts, the interaction between the parasitoid and pathogen was symmetrical and competitive; the parasitoid had a significant negative effect on pathogen performance and the pathogen had a significant negative effect on parasitoid performance. Highly resistant hosts were not susceptible to infection by the pathogen, which created a refugium from competition for the parasitoid. The results show that, similar to host-plant-mediated competition between herbivores, host phenotype can influence competitive interactions between parasitoids and pathogens.
• Chilcutt, C. F., & Tabashnik, B. E. (1997). Independent and Combined Effects of Bacillus thuringiensis and the Parasitoid Cotesia plutellae (Hymenoptera: Braconidae) on Susceptible and Resistant Diamondback Moth (Lepidoptera: Plutellidae). Journal of Economic Entomology, 90(2), 397-403.
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  Abstract: We evaluated mortality to larvae of diamondback moth, Plutella xylostella (L.), caused by the microbial pathogen Bacillus thuringiensis Berliner and the endoparasitic wasp Cotesia plutellae Kurdjumov, separately and in combination. Each of 3 colonies of diamondback moth (susceptible, moderately resistant, and highly resistant to B. thuringiensis) received the following 4 treatments: (1) control, (2) B. thuringiensis only, (3) parasitoids only, and (4) B. thuringiensis plus parasitoids. With increasing colony resistance, the effect of B. thuringiensis decreased but the effect of parasitoids remained the same. The effect of B. thuringiensis plus parasitoids depended on the susceptibility of the host to B. thuringiensis. For the susceptible colony, highest diamondback moth mortality was caused by B. thuringiensis plus parasitoids, but for the 2 resistant colonies, parasitoids alone caused as much mortality as B. thuringiensis and parasitoids combined. For the susceptible and highly resistant colonies, we also evaluated the effect of varying the time interval between parasitism by C. plutellae and exposure to B. thuringiensis on diamondback moth mortality and C. plutellae survival. For the susceptible colony, diamondback moth mortality and C. plutellae survival did not vary as a function of the time interval between parasitism and exposure to B. thuringiensis. For the resistant colony, delaying B. thuringiensis treatment from 0 to 4 d after parasitism occurred did not affect diamondback moth mortality, but it significantly increased parasitoid survival. Efforts to integrate B. thuringiensis and C. plutellae must consider mortality of immature parasitoids inside of susceptible hosts.
• Hollingsworth, R. G., Tabashnik, B. E., Johnson, M. W., Messing, R. H., & Ullman, D. E. (1997). Relationship between susceptibility to insecticides and fecundity across populations of cotton aphid (Homoptera: Aphididae). Journal of Economic Entomology, 90(1), 55-58.
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  Abstract: We measured the relationship between susceptibility to 4 insecticides and fecundity across 15 populations of cotton aphid, Aphis gossypii Glover, from Hawaii. Multiple regression showed that the relationship between the logarithm of LC50 and mean daily progeny production in the absence of insecticide exposure was negative for esfenvalerate (a pyrethroid), positive for methomyl (a carbamate), and not significant for endosulfan (an organochlorine) or oxydemeton-methyl (an organophosphate). The association between methomyl susceptibility and fecundity was not evident in a simple regression analysis, which suggests that major fitness costs of resistance to one insecticide can obscure relationships between fitness and resistance to other insecticides.
• Liu, Y. -., & Tabashnik, B. E. (1997). Experimental evidence that refuges delay insect adaptation to Bacillus thuringiensis. Proceedings of the Royal Society B: Biological Sciences, 264(1381), 605-610.
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  Abstract: Theoretical projections suggest that refuges from exposure can delay insect adaptation to environmentally benign insecticides derived from Bacillus thuringiensis, but experimental tests of this approach have been limited. We tested the refuge tactic by selecting two sets of two colonies of diamondback moth (Plutella xylostella) for resistance to B. thuringiensis subsp. aizawai in the laboratory. In each set, one colony was selected with no refuge and the other with a 10% refuge from exposure to B. thuringiensis subsp. aizawai. Bioassays conducted after nine selections were completed show that mortality caused by B. thuringiensis subsp. aizawai was significantly greater in the refuge colonies than in the no-refuge colonies. These results demonstrate that the refuges delayed the evolution of resistance. Relative to a susceptible colony, final resistance ratios were 19 and eight for the two no-refuge colonies compared to six and five for the refuge colonies. The mean realized heritability of resistance to B. thuringiensis subsp. aizawai was 0.046 for colonies without refuges, and -0.002 for colonies with refuges. Selection with B. thuringiensis subsp. aizawai decreased susceptibility to B, thuringiensis toxin CrylAb, but not to CrylC or B. thuringiensis subsp. kurstaki. Although the ultimate test of refuges will occur in the field, the experimental evidence reported here confirms modelling results indicating that refuges can slow the evolution of insect resistance to B. thuringiensis.
• Liu, Y., & Tabashnik, B. E. (1997). Inheritance of resistance to the Bacillus thuringiensis toxin Cry1C in the diamondback moth. Applied and Environmental Microbiology, 63(6), 2218-2223.
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  PMID: 16535623;PMCID: PMC1389178;Abstract: Laboratory selection increased resistance to the Bacillus thuringiensis toxin Cry1C in a strain of diamondback moth (Plutella xylostella). The selected strain was derived from a field population that had evolved high levels of resistance to Bacillus thuringiensis subsp. kurstaki and moderate resistance to Cry1C. Relative to the responses of a susceptible strain of diamondback moth, the resistance to Cry1C of the selected strain increased to 62-fold after six generations of selection. The realized heritability of resistance was 0.10. Analysis of F1 hybrid progeny from reciprocal crosses between the selected strain and a susceptible strain showed that resistance to Cry1C was autosomally inherited. The dominance of resistance to Cry1C depended on the concentration; inheritance was increasingly dominant as the concentration decreased. Responses of progeny from single-pair families showed that resistance to Cry1C and resistance to Cry1Ab were inherited independently, which enhances opportunities for managing resistance. However, compared with projections based on previously reported recessive inheritance of resistance to Cry1A toxins, the potentially dominant inheritance of resistance to Cry1C observed here could accelerate evolution of resistance.
• Liu, Y., & Tabashnik, B. E. (1997). Synergism of Bacillus thuringiensis by Ethylenediamine Tetraacetate in Susceptible and Resistant Larvae of Diamondback Moth (Lepidoptera: Plutellidae). Journal of Economic Entomology, 90(2), 287-292.
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  Abstract: We measured mortality of neonates and 3rd instars from susceptible and resistant colonies of diamondback moth, Plutella xylostella (L.), caused by Bacillus thuringiensis with and without ethylenediamine tetraacetate (EDTA) and trypsin. For susceptible larvae, the addition of EDTA + trypsin reduced the LC50 of B. thuringiensis by 5-fold in neonates but had no effect on 3rd instars. For resistant larvae, the addition of EDTA + trypsin significantly reduced LC50s of B. thuringiensis by 2- to 3-fold in neonates and 3rd instars. EDTA + trypsin without B. thuringiensis did not kill larvae. Further tests with susceptible and resistant neonates showed that EDTA significantly increased the toxicity of B. thuringiensis, whereas trypsin had no effect and no interaction occurred between EDTA and trypsin.
• Liu, Y., & Tabashnik, B. E. (1997). Visual determination of sex of diamondback moth larvae. Canadian Entomologist, 129(3), 585-586.
• Luo, K., Tabashnik, B. E., & Adang, M. J. (1997). Binding of Bacillus thuringiensis Cry1Ac toxin to aminopeptidase in susceptible and resistant diamondback moths (Plutella xylostella). Applied and Environmental Microbiology, 63(3), 1024-1027.
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  PMID: 16535536;PMCID: PMC1389130;Abstract: Bacillus thuringiensis Cry1Ac toxin bound to a 120-kDa protein isolated from the brush border membranes of bath susceptible and resistant larvae of Phaella xylostella, the diamondback moth. The 120-kDa protein was purified by Cry1Ac toxin affinity chromatogrophy. Like Cry1Ac-binding aminopeptidase N (EC 3.4.11.2) from other insects, this protein was eluted from the affinity column with 200 mM N-acetylgaloctosamine. The purified protein had aminopeptidase activity and bound Cry1Ac toxin on ligand blots. Purified aminopeptidase was recognized by antibodies to the cross-reacting determinant found on phosphotidylinositol-specific phospholipase C-solubilized proteins. The results show that the presence of Cry1Ac-binding aminopeptidase in the brush border membrane is not sufficient to confer susceptibility to Cry1Ac. Furthermore, the results do not support the hypothesis that resistance to Cry1Ac was caused by lack of a Cry1Ac-binding aminopeptidase.
• Tabashnik, B. (1997). Insect resistance to Bt revisited.. Nature biotechnology, 15(13), 1324-.
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  PMID: 9415873;
• Tabashnik, B. (1997). Pest adaptation [3]. Nature, 389(6653), 778-.
• Tabashnik, B. E. (1997). Seeking the root of insect resistance to transgenic plants. Proceedings of the National Academy of Sciences of the United States of America, 94(8), 3488-3490.
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  PMID: 11607727;PMCID: PMC34157;
• Tabashnik, B. E., Liu, Y., Finson, N., Masson, L., & Heckel, D. G. (1997). One gene in diamondback moth confers resistance to four Bacillus thuringiensis toxins. Proceedings of the National Academy of Sciences of the United States of America, 94(5), 1640-1644.
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  PMID: 9050831;PMCID: PMC19969;Abstract: Environmentally benign insecticides derived from the soil bacterium Bacillus thuringiensis (Bt) are the most widely used biopesticides, but their success will be short-lived if pests quickly adapt to them. The risk of evolution of resistance by pests has increased, because transgenic crops producing insecticidal proteins from Bt are being grown commercially. Efforts to delay resistance with two or more Bt toxins assume that independent mutations are required to counter each toxin. Moreover, it generally is assumed that resistance alleles are rare in susceptible populations. We tested these assumptions by conducting single-pair crosses with diamondback moth (Plutella xylostella), the first insect known to have evolved resistance to Bt in open field populations. An autosomal recessive gene conferred extremely high resistance to four Bt toxins (Cry1Aa, Cry1Ab, Cry1Ac, and Cry1F). The finding that 21% of the individuals from a susceptible strain were heterozygous for the multiple-toxin resistance gene implies that the resistance allele frequency was 10 times higher than the most widely cited estimate of the upper limit for the initial frequency of resistance alleles in susceptible populations. These findings suggest that pests may evolve resistance to some groups of toxins much faster than previously expected.
• Tabashnik, B. E., Liu, Y., Malvar, T., Heckel, D. G., Masson, L., Ballester, V., Granero, F., Ménsua, J. L., & Ferré, J. (1997). Global variation in the genetic and biochemical basis of diamondback moth resistance to Bacillus thuringiensis. Proceedings of the National Academy of Sciences of the United States of America, 94(24), 12780-12785.
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  PMID: 9371752;PMCID: PMC24215;Abstract: Insecticidal proteins from the soil bacterium Bacillus thuringiensis (Bt) are becoming n cornerstone of ecologically sound pest management. However, if pests quickly adapt, the benefits of environmentally benign Bt toxins in sprays and genetically engineered crops will be short-lived. The diamondback moth (Plutella xylostella) is the first insect to evolve resistance to Bt in open-field populations. Here we report that populations from Hawaii and Pennsylvania share a genetic locus at which a recessive mutation associated with reduced toxin binding confers extremely high resistance to four Bt toxins. In contrast, resistance in a population from the Philippines shows multilocus control, a narrower spectrum, and for some Bt toxins, inheritance that is not recessive and not associated with reduced binding. The observed variation in the genetic and biochemical basis of resistance to Bt, which is unlike patterns documented for some synthetic insecticides, profoundly affects the choice of strategies for combating resistance.
• Tabashnik, B., & Tabashnik, B. E. (1997). Seeking the root of insect resistance to transgenic plants. Proceedings of the National Academy of Sciences of the United States of America, 94(8).
• X., W., Tabashnik, B. E., Artelt, B., Malvar, T., Ballester, V., Ferré, J., & Roderick, G. K. (1997). Mitochondrial DNA sequence variation among geographic strains of diamondback moth (Lepidoptera: Plutellidae). Annals of the Entomological Society of America, 90(5), 590-595.
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  Abstract: We examined genetic variation among 6 geographic strains of diamondback moth, Plutella xylostella (L.), using 365 base pairs of the mitochondrial gene encoding cytochrome oxidase I (COI). No sequence variation was detected within 5 of the 6 strains; 1 strain contained 2 haplotypes that differed by a single base substitution (0.27%). Sequence differences between strains of diamondback moth from Hawaii, the Philippines, and Pennsylvania ranged from 0 to 0.82%. With one exception, base pair substitutions among strains resulted in synonymous codons and did not alter amino acid sequence. Genetic divergence between strains of diamondback moth was not correlated with geographic distances between the sources of each strain. Variation in COI among populations of diamondback moth was similar to that found in other species of Lepidoptera and was substantially less than variation between diamondback moth and 2 undescribed Hawaiian Plutella species (5 and 10%, respectively). The sequences reported here provide benchmark data for related work, such as analysis of global patterns of resistance to Bacillus thuringiensis in diamondback moth.
• Liu, Y., Tabashnik, B. E., & Pusztai-Carey, M. (1996). Field-evolved resistance to Bacillus thuringiensis toxin CryIC in diamondback moth (Lepidoptera: Plutellidae). Journal of Economic Entomology, 89(4), 798-804.
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  Abstract: Previous results have shown that diamondback moth, Plutella xylostella (L.), populations resistant to toxins from Bacillus thuringiensis subsp. kurstaki were susceptible to toxin CryIC. Use of commercial formulations of B. thuringiensis subsp. aizawai that contain CryIC has increased recently. Analysis of two commercial formulations by high pressure liquid chromatography showed that CryIC accounted for 26% of the CryI protein in the B. thuringiensis subsp. aizawai formulation, but did not occur in the B. thuringiensis subsp. kurstaki formulation. CryIAb was the most abundant CryI protein in the commercial formulations of B. thuringiensis subsp. aizawai and kurstaki. We found resistance to CryIC in a field population of diamondback moth from Hawaii that had been treated with B. thuringiensis subsp. aizawai. Leaf residue bioassays showed that, at 5 d after treatment with CryIC, LC50s for colonies derived from this population in 1993 and 1995 were ~20 times greater than the LC50 for a susceptible laboratory colony. For a nearby population that had not been treated with B. thuringiensis subsp. aizawai, responses to CryIC did not differ significantly from those of the susceptible laboratory colony. Resistance to Cry1Ab was lower in a Cry1C- resistant colony than in a Cry1C-susceptible colony that had been selected with B. thuringiensis subsp. kurstaki. These results suggest that the gene(s) conferring resistance to Cry1C segregate independently from the gene(s) conferring resistance to Cry1Ab. In contrast with previous results with colonies derived in 1989, resistance to B. thuringiensis subsp. kurstaki in a colony derived in 1993 from the same field population did not decline when exposure to B. thuringiensis stopped. Thus, stability of resistance is not necessarily a fixed character, even for a specific population and pesticide. Despite substantial resistance to CryIC and B. thuringiensis subsp. kurstaki, resistance to a spore-crystal formulation of B. thuringiensis subsp. aizawai was only 2- to 4-fold.
• Omer, A. D., Johnson, M. W., & Tabashnik, B. E. (1996). Demography of the leafminer parasitoid Ganaspidium utilis beardsley (Hymenoptera: Eucoilidae) at different temperatures. Biological Control, 6(1), 29-34.
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  Abstract: Demographic parameters, reproductive pattern, and heterogeneity for the leafminer parasitoid Ganaspidium utilis Beardsley (Hymenoptera: Eucoilidae) were determined in the laboratory at 16, 27, and 32°C using Liriomyza trifolii (Burgess) with 'Henderson' bush lima beans as the host. Development and progeny production did not occur at 16°C. Mean developmental time was significantly shorter (13.2 days) at 32°C than at 27°C (18.4 days). Preadult cumulative survival rates at 27°C (62%) and 32°C (43%) were significantly different. Male and female longevities were similar. Adult G. utilis lived a maximum of 23 days at 16°C, 13 days at 270C, and 8 days at 32°C. Total mean lifetime progeny production per female G. utilis ranged from 68.50 progeny at 32°C to 115.10 progeny at 27°C. Heaviest progeny production occurred during Days 2 and 3 of a female's life at 32 and 27°C, respectively. Of the total progeny produced, the female percentage ranged from 47 at 27°C to 49 at 32°C. Mean generation time was 15 days at 32°C and 21 days at 27°C. Intrinsic rate of increase varied from 0.19 at 27°C to 0.23 at 32°C. In the stable age distribution, egg percentage was highest (56-58) followed by larval stage (36-37), whereas pupal and adult stages each only represented 3 to 4. Age-by-parity relations demonstrated that timing and magnitude of reproduction were influenced by temperature.
• Rosenheim, J. A., Johnson, M. W., F., R., Welter, S. C., & Tabashnik, B. E. (1996). Biochemical preadaptations, founder events, and the evolution of resistance in arthropods. Journal of Economic Entomology, 89(2), 263-273.
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  Abstract: We conducted literature-based comparative analyses of pesticide resistance evolution to evaluate 2 aspects of microevolutionary response to strong novel selection pressures. First, it has been proposed that herbivorous arthropods are preadapted to evolve resistance by a system of detoxifying enzymes the evolution of which was elaborated in response to plant defensive chemicals. To test this hypothesis, we note that arthropods that feed on plant vascular tissues (phloem and xylem, which are less strongly chemically defended than other tissues) have lower levels of detoxifying enzyme activities than species that chew or feed on cell contents, and might therefore be expected to be less strongly preadapted to pesticides. The record of pesticide resistance evolution in 615 arthropod pests of North American agriculture is consistent with he biochemical preadaptation hypothesis; phloem-and xylem-feeding species have a significantly diminished ability to evolve resistance when compared with either chewing or cell content-feeding species. The same result is obtained in an analysis restricted to species within the order Homoptera. We caution, however, that feeding behavior is confounded with taxonomy in these analyses; thus, strong inferences of a causal relationship between feeding mode and resistance evolution would be premature. Second, conventional theory suggests that population bottlenecks that occur during the initiation of geographically isolated populations can have profound effects on the genetic structure of populations, including particularly the loss of allelic diversity. Arthropods that have been introduced to North America, and thus have undergone events, might therefore be expected to be depanperate for key resistance-conferring genetic variants. Our analysis find no support for this founder event hypothesis; native and introduced species do not differ significantly in resistance evolution. We evaluate the importance of possibly confounding variables in the interpretation of these results and emphasize the need to integrate experimental results with these comparative broad-scale results.
• Tabashnik, B. E., Groeters, F. R., Finson, N., Liu, Y., Johnson, M. W., Heckel, D. G., Luo, K., & Adang, M. J. (1996). Resistance to Bacillus thuringiensis in Plutella xylostella: The Moth Heard Round the World. ACS Symposium Series, 645, XXIII-140.
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  Abstract: Insecticidal crystal proteins from Bacillus thuringiensis (Bt) offer safe, specific, and effective insect control. Evolution of resistance by pests is the greatest threat to the long-term effectiveness of Bt. The first insect to evolve resistance to Bt in open field populations is the diamondback moth (Plutella xylostella), a global pest of vegetables. Resistance to Bt has been found in some populations of diamondback moth from Hawaii, the continental United States, and several Asian nations. Resistance to Bt in diamondback moth is inherited as an autosomal, recessive trait and is associated with reduced binding of toxin to sites on the larval midgut membrane. Resistance to CryIA toxins from Bt subsp. kurstaki caused cross-resistance to CryIF, but not to CryIB or CryIC. In laboratory populations containing susceptible and resistant individuals, fitness costs associated with resistance promoted reversal of resistance in the absence of exposure to Bt. In an isofemale line derived from an extensively selected strain, >5,000-fold resistance persisted for >20 generations without selection, which shows that at least one resistant genotype is not inherently unstable. Field-evolved resistance to Bt in diamondback moth has provided a warning and an opportunity to gain knowledge essential for sustainable use of Bt.
• Tabashnik, B. E., Malvar, T., Liu, Y., Finson, N., Borthakur, D., Shin, B., Park, S., Masson, L., A., R., & Bosch, D. (1996). Cross-resistance of the diamondback moth indicates altered interactions with domain II of Bacillus thuringiensis toxins. Applied and Environmental Microbiology, 62(8), 2839-2844.
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  PMID: 8702276;PMCID: PMC168069;Abstract: We compared responses to six insecticidal crystal proteins from Bacillus thuringiensis by a Cry1A-resistant strain (NO-QA) and a susceptible strain (LAB-P) of the diamondback moth, Plutella xylostella. The resistant strain showed >100-fold cross-resistance to Cry1J and to H04, a hybrid with domains I and II of Cry1Ab and domain III of Cry1C. Cross-resistance was sixfold to Cry1Bb and threefold to Cry1D. The potency of Cry1I did not differ significantly between the resistant and susceptible strains. Cry2B did not kill resistant or susceptible larvae. By combining these new data with previously published results, we classified responses to 14 insecticidal crystal proteins by strains NO-QA and LAB-P. NO-QA showed high levels of resistance to Cry1Aa, Cry1Ab, and Cry1Ac and high levels of cross-resistance to Cry1F, Cry1J, and H04. Cross-resistance was low or nil to Cry1Ba, Cry1Bb, Cry1C, Cry1D, Cry1I, and Cry2A. Cry1E and Cry2B showed little or no toxicity to susceptible or resistant larvae. In dendrograms based on levels of amino acid sequence similarity among proteins, Cry1F and Cry1J clustered together with Cry1A proteins for domain II, but not for domain I or III. High levels of cross-resistance to Cry1Ab-Cry1C hybrid H04 show that although Cry1C is toxic to NO-QA, domain III of Cry1C is not sufficient to restore toxicity when it is combined with domains I and II of Cry1Ab. Thus, diamondback moth strain NO-QA cross-resistance extends beyond the Cry1A family of proteins to at least two other families that exhibit high levels of amino sequence similarity with Cry1A in domain II (Cry1F and Cry1J) and to a protein that is identical to Cry1Ab in domain II (H04). The results of this study imply that resistance to Cry1A alters interactions between the insect and domain II.
• Escriche, B., Tabashnik, B., Finson, N., & Ferré, J. (1995). Immunohistochemical detection of binding of cryia crystal proteins of Bacillus thuringiensis in highly resistant strains of Plutella xylostella (L.) from Hawaii. Biochemical and Biophysical Research Communications, 212(2), 388-395.
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  PMID: 7626052;Abstract: We detected binding of insecticidal crystal proteins from Bacillus thuringiensis in one susceptible strain and six resistant strains of diamondback moth, Plutella xylostella, from Hawaii. Immunohistochemical tests with tissue sections from larval midguts showed specific binding of CryIA(a), CryIA(b), and CryIA(c) to brush border membranes. CryIE, which is not toxic to P. xylostella, did not bind to midgut tissues. Larvae from one of the resistant strains ingested extremely high concentrations of a commercial formulation containing the three CryIA proteins without suffering midgut cell damage or mortality. This same resistant strain had previously been found to have greatly reduced binding of radioactively-labeled CryIA(c) to vesicles prepared from brush border membranes. The finding that binding as detected in immunohistochemical tests was not sufficient for toxicity suggests that low levels of binding can occur without harmful effects; the tests did not reflect in vivo binding, or post-binding factors contribute to resistance. Comparison with resistant strains from Florida and the Philippines, which did not bind CryIA(b) in previously reported immunohistochemical tests, suggests that more than one mechanism of resistance to B. thuringiensis may occur within single species.
• Frankenhuyzen, K. v., Nystrom, C. W., & Tabashnik, B. E. (1995). Variation in tolerance to Bacillus thuringiensis among and within populations of the spruce budworm (Lepidoptera: Tortricidae) in Ontario.. Journal of Economic Entomology, 88(1), 97-105.
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  PMID: 7884080;Abstract: Variation in tolerance to Bacillus thuringiensis Berliner subsp. kurstaki (strain HD-1-S-1980) among and within populations of the spruce budworm, Choristoneura fumiferana (Clemens), was assessed in the laboratory. Force-feeding assays using offspring of females collected as pupae from nine locations throughout Ontario and from a laboratory colony (DCF) demonstrated limited variation in tolerance among populations. Variation among populations was comparable with the variation observed among repeated assays with different batches of larvae from the DCF colony. Population LC50s were not significantly associated with age of the outbreak, host-plant species, incidence of the microsporidian Nosema fumiferanae (Thomson), or size of the female parent. Upper limits for genetic variation in tolerance were estimated by examining variation among full-sibling families within same populations. Mortality of individual families ranged from 6.5 to 70.9% within five field populations and from 2.7 to 93.3% within two laboratory colonies in response to a dose that caused a mean mortality of 40%. Familial factors accounted for 32.8% of the phenotypic variation in response across field populations, as compared with 3% for population factors. These data suggest that the phenotypic variation in tolerance to B. thuringiensis has a substantial genetic component and may provide a basis for evolution of resistance given sufficient selection pressure.
• Heckel, D. G., Gahan, L. J., Tabashnik, B. E., & Johnson, M. W. (1995). Randomly amplified polymorphic DNA differences between strains of diamondback moth (Lepidoptera: Plutellidae) susceptible or resistant to Bacillus thuringiensis). Annals of the Entomological Society of America, 88(4), 531-537.
• Masson, L., Mazza, A., Brousseau, R., & Tabashnik, B. (1995). Kinetics of Bacillus thuringiensis toxin binding with brush border membrane vesicles from susceptible and resistant larvae of Plutella xylostella. Journal of Biological Chemistry, 270(20), 11887-11896.
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  PMID: 7744839;Abstract: An optical biosensor technology based on surface plasmon resonance was used to determine the kinetic rate constants for interactions between the CryIA(c) toxin from Bacillus thuringiensis and brush border membrane vesicles purified from susceptible and resistant larvae of diamondback moth (Plutella xylostella). CryIA(c) association and dissociation rate constants for vesicles from susceptible larvae were determined to be 4.5 x 103 M-1 s-1 and 3.2 x 10-5 s-1, respectively, resulting in a calculated affinity constant of 7 nM. CryIE toxin did not kill susceptible or resistant larvae and did not bind to brush border vesicles. Contrary to expectations based on previous studies of binding in resistant P. xylostella, the binding kinetics for CryIA(c) did not differ significantly between susceptible larvae and those that were resistant to CryIA(c). Determination of the number of CryIA(c) receptors revealed an approximately 3-fold decrease in total CryIA(c) receptor numbers for resistant vesicles. These results suggest that factors other than binding may be altered in our resistant diamond-back moth strain. They also support the view that binding is not sufficient for toxicity.
• Omer, A. D., Tabashinik, B. E., & Johnson, M. W. (1995). Inheritance of dicrotophos resistance in greenhouse whitefly. Entomologia Experimentalis et Applicata, 77(2), 177-181.
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  Abstract: We studied inheritance of resistance to dicrotophos in greenhouse whitefly, Trialeurodes vaporariorum Westwood (Homoptera, Aleyrodidae). Compared with females from a field-collected susceptible strains (S), females from a resistant strain (R) of T. vaporariorum derived from heavily treated cotton fields had a 28-fold greater LC50 to dicrotophos in laboratory bioassays. Concentration-mortality lines obtained from female progeny of reciprocal F1 crosses (R♀ XS♂ and S♀ XR♂) were similar, suggesting that inheritance of dicrotophos resistance was autosomal and not influenced by maternal effects. Responses of F1 female progeny were similar to those of the parental S strain, indicating that the resistance was partially recessive (degree of dominance, D, was -0.61). Mortality observed in female progeny obtained from a backcross (F1♀ XR♂) corresponded more closely to expectations derived from polygenic models than to expectations from a monogenic model. The estimated number of effective factors (sensu Lande, 1981) contributing equally to resistance was three. © 1995 Kluwer Academic Publishers.
• Rathman, R. J., Johnson, M. W., Tabashnik, B. E., & Spollen, K. M. (1995). Variation in susceptibility to insecticides in the leafminer parasitoid Ganaspidium utilis (Hymenoptera: Eucoilidae). Journal of Economic Entomology, 88(3), 475-479.
• Tabashnik, B. E. (1995). Array. Trends in Ecology & Evolution, 10(4), 164-165.
• Yong-Biao, L., Tabashnik, B. E., & Johnson, M. W. (1995). Larval age affects resistance to Bacillus thuringiensis in diamondback moth (Lepidoptera: Plutellidae). Journal of Economic Entomology, 88(4), 788-792.
• Gillespie, R. G., & Tabashnik, B. E. (1994). Foraging behavior of the Hawaiian happy face spider (Araneae: Theridiidae). Annals of the Entomological Society of America, 87(6), 815-822.
• Groeters, F. R., Tabashnik, B. E., Finson, N., & Johnson, M. W. (1994). Fitness costs of resistance to Bacillus thuringiensis in the diamondback moth (Plutella xylostella). Evolution, 48(1), 197-201.
• Hollingsworth, R. G., Tabashnik, B. E., Ullman, D. E., Johnson, M. W., & Messing, R. (1994). Resistance of Aphis gossypii (Homoptera: Aphididae) to insecticides in Hawaii: Spatial patterns and relation to insecticide use. Journal of Economic Entomology, 87(2), 293-300.
• Tabashnik, B. E. (1994). Delaying insect adaptation to transgenic plants: Seed mixtures and refugia reconsidered. Proceedings of the Royal Society B: Biological Sciences, 255(1342), 7-12.
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  Abstract: Genetically engineered plants expressing insecticidal proteins from the bacterium Bacillus thuringiensis can provide safe and effective pest control, but they will also select intensely for insect adaptation to the toxins. Mallet and Porter used a model to compare two planting strategies for delaying crop failure: 'seed mixtures' of toxic and toxin-free plants within fields, and 'refugia' of toxin-free fields. They concluded that 'if insects can move from plant to plant, seed mixtures may actually hasten insect resistance compared with pure stands of toxic plants'. In the present study, the model of Mallet and Porter was used to evaluate pure stands of toxic plants, seed mixtures, refugia, and mixtures + refugia. Across a broad range of conditions, seed mixtures delayed development of insect resistance compared with pure stands of toxic plants. The relative merits of seed mixtures and refugia depend upon untested assumptions about pest movement, mating, and inheritance of resistance. Minimizing exposure of pests to toxins in space and time is the surest way to preserve the efficacy of transgenic plants.
• Tabashnik, B. E. (1994). Evolution of resistance to Bacillus thuringiensis. Annual Review of Entomology, 39(1), 47-79.
• Tabashnik, B. E. (1994). Evolution of resistance to Bacillus thuringiensis. Annual review of entomology. Vol. 39, 47-79.
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  Abstract: Insecticides derived from Bacillus thuringiensis have become important for pest management, but recently resistance has been reported from field populations of diamondback moth Plutella xylostella, and laboratory populations of a number of species of Lepidoptera, Coleoptera and Diptera have shown similar characteristics. In this context, the author examines laboratory selection for resistance, resistance risk assessment, variation among conspecific populations, mechanisms, cross-resistance, genetics, stability, fitness costs, and management. -P.J.Jarvis
• Tabashnik, B. E., & McGaughey, W. H. (1994). Resistance risk assessment for single and multiple insecticides: Responses of indianmeal moth (Lepidoptera: Pyralidae) to Bacillus thuringiensis. Journal of Economic Entomology, 87(4), 834-841.
• Tabashnik, B. E., Finson, N., Groeters, F. R., Moar, W. J., Johnson, M. W., Luo, K., & Adang, M. J. (1994). Reversal of resistance to Bacillus thuringiensis in Plutella xylostella. Proceedings of the National Academy of Sciences of the United States of America, 91(10), 4120-4124.
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  PMID: 8183881;PMCID: PMC43736;Abstract: Continued success of the most widely used biopesticide, Bacillus thuringiensis, is threatened by development of resistance in pests. Experiments with Plutella xylostella (diamondback moth), the first insect with field populations resistant to B. thuringiensis, revealed factors that promote reversal of resistance. In strains of P. xylostella with 25- to 2800- fold resistance to B. thuringiensis compared with unselected strains, reversal of resistance occurred when exposure to B. thuringiensis was stopped for many generations. Reversal of resistance was associated with restoration of binding of B. thuringiensis toxin CryIA(c) to brush-border membrane vesicles and with increased biotic fitness. Compared with susceptible colonies, revertant colonies had a higher proportion of extremely resistant individuals. Revertant colonies responded rapidly to reselection for resistance. Understanding reversal of resistance will help to design strategies for extending the usefulness of this environmentally benign insecticide.
• Tabashnik, B. E., Finson, N., Johnson, M. W., & Heckel, D. G. (1994). Cross-resistance to Bacillus thuringiensis toxin CryIF in the diamondback moth (Plutella xylostella). Applied and Environmental Microbiology, 60(12), 4627-4629.
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  PMID: 16349471;PMCID: PMC202035;Abstract: Selection with Bacillus thuringiensis subsp. kurstaki, which contains CryIA and CryII toxins, caused a >200-fold cross-resistance to CryIF toxin from B. thuringiensis subsp. aizawai in the diamondback moth, Plutella xylostella. CryIE was not toxic, but CryIB was highly toxic to both selected and unselected larvae. The results show that extremely high levels of cross- resistance can be conferred across classes of CryI toxins of B. thuringiensis.
• Costa, H. S., Ullman, D. E., Johnson, M. W., & Tabashnik, B. E. (1993). Antibiotic oxytetracycline interferes with Bemisia tabaci (Homoptera: Aleyrodidae) oviposition, development, and ability to induce squash silverleaf. Annals of the Entomological Society of America, 86(6), 740-748.
• Costa, H. S., Ullman, D. E., Johnson, M. W., & Tabashnik, B. E. (1993). Association between Bemisia tabaci density and reduced growth, yellowing, and stem blanching of lettuce and kai choy. Plant Disease, 77(10), 969-972.
• Omer, A. D., Johnson, M. W., Tabashnik, B. E., & Ullman, D. E. (1993). Association between insecticide use and greenhouse whitefly (Trialeurodes vaporariorum Westwood) resistance to insecticides in Hawaii. Pesticide Science, 37(3), 253-259.
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  Abstract: Susceptibility to acephate, methomyl and permethrin was determined with bioassays for adults of greenhouse whitefly. Comparisons at LC50 showed up to 42-fold resistance to acephate, 36-fold resistance to methomyl, and 8-fold resistance to permethrin. Higher levels of resistance to acephate and methomyl than to permethrin are consistent with greater use of organophosphates and carbamates than pyrethroids by growers. Insecticide use varied from 1-98 insecticide sprays per site per season. Significant positive associations between LC50 for each insecticide and frequency of application of the same insecticide were found across sites. -from Authors
• Omer, A. D., Johnson, M. W., Tabashnik, B. E., Costa, H. S., & Ullman, D. E. (1993). Sweetpotato whitefly resistance to insecticides in Hawaii: Intra-island variation is related to insecticide use. Entomologia Experimentalis et Applicata, 67(2), 173-182.
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  Abstract: Susceptibility to acephate, methomyl, and permethrin was determined with laboratory bioassays of field-collected adults from 15 populations of the B biotype of sweetpotato whitefly, Bemisia tabaci (Gennadius) (Homoptera:Aleyrodidae), from Hawaii. Comparisons at the LC50 showed up to 24-fold resistance to acephate, 18-fold resistance to methomyl, and 4-fold resistance to permethrin. Analysis of variance showed significant intra-island variation in susceptibility to each insecticide, but no significant variation among islands. Insecticide use varied from 4 to 103 insecticide sprays per site per season. Acephate and methomyl were used more often than permethrin. The frequency of application and LC50 for each insecticide were positively correlated across sites. These results suggest that local variation in insecticide use was a primary cause of variation in susceptibility. If local insecticide use is a key determinant of resistance, as our results suggest, growers can retard resistance development locally by reducing their own insecticide use. © 1993 Kluwer Academic Publishers.
• Omer, A. D., Tabashnik, B. E., Johnson, M. W., & Leigh, T. F. (1993). Realized heritability of resistance to dicrotophos in greenhouse whitefly. Entomologia Experimentalis et Applicata, 68(3), 211-217.
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  Abstract: Realized heritability (h2) of resistance to dicrotophos in greenhouse whitefly, Trialeurodes vaporariorum Westwood, was estimated from a laboratory selection experiment. Five generations of selection increased the LC50 approximately 13-fold. Estimated h2 of resistance to dicrotophos was 0.40 when calculated with the method of Tabashnik (1992) and 0.35 with the method of Tanaka & Noppun (1989). These results suggest that 35 to 40% of the total phenotypic variation in resistance was caused by additive genetic variation. For thirteen previously reported estimates of h2 of insecticide resistance in other insect pests, the mean was 0.29. The relatively high h2 of dicrotophos resistance for T. vaporariorum is consistent with rapid resistance development in field populations. © 1993 Kluwer Academic Publishers.
• Rosenheim, J. A., & Tabashnik, B. E. (1993). Generation time and evolution [5]. Nature, 365(6449), 791-792.
• Tabashnik, B. E., Finson, N., Johnson, M. W., & Moar, W. J. (1993). Resistance to toxins from Bacillus thuringiensis subsp. kurstaki causes minimal cross-resistance to B. thuringiensis subsp. aizawai in the diamondback moth (Lepidoptera: Plutellidae). Applied and Environmental Microbiology, 59(5), 1332-1335.
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  PMID: 16348929;PMCID: PMC182085;Abstract: Repeated exposure in the field followed by laboratory selection produced 1,800- to >6,800-fold resistance to formulations of Bacillus thuringiensis subsp. kurstaki in larvae of the diamondback moth, Plutella xylostella. Four toxins from B. thuringiensis subsp. kurstaki [CryIA(a), CryIA(b), CryIA(c), and CryIIA] caused significantly less mortality in resistant larvae than in susceptible larvae. Resistance to B. thuringiensis subsp. kurstaki formulations and toxins did not affect the response to CryIC toxin from B. thuringiensis subsp. aizawai. Larvae resistant to B. thuringiensis subsp. kurstaki showed threefold cross-resistance to formulations of B. thuringiensis subsp. aizawai containing CryIC and CryIA toxins. This minimal cross-resistance may be caused by resistance to CryIA toxins shared by B. thuringiensis subsp. kurstaki and B. thuringiensis subsp. aizawai.
• Caprio, M. A., & Tabashnik, B. E. (1992). Allozymes used to estimate gene flow among populations of diamondback moth (Lepidoptera: Plutellidae) in Hawaii. Environmental Entomology, 21(4), 808-816.
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  Abstract: Estimates of genetic differentiation of Plutella xylostella subpopulations (FST) based on electrophoretic variation at four polymorphic loci varied from 0.038-0.028. Using Wright's method, estimates of the number of migrants exchanged per generation per population (Nem) from FST estimates based on four loci ranged from 6-9. There was little variation between islands, and only slightly more variation between Hawaiian populations and two populations from the continental United States. Local variation in insecticide resistance among Hawaiian diamondback moth populations is not an indication of restricted gene flow and is most probably a result of local variation in selection. -from Authors
• Groeters, F. R., Tabashnik, B. E., Finson, N., & Johnson, M. W. (1992). Oviposition preference of the diamondback moth (Plutella xylostelld) unaffected by the presence of conspecific eggs or Bacillus thuringiensis. Journal of Chemical Ecology, 18(12), 2353-2362.
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  PMID: 24254875;Abstract: Neither toxins from Bacillus thuringiensis Berliner nor conspecific eggs deterred oviposition by the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), in laboratory choice tests. The finding that toxins did not deter oviposition by moths from a susceptible line shows that oviposition preference and larval survival were not associated in this line. Selection for larval resistance to toxins did not significantly alter oviposition preference, which rules out a strong genetic correlation between larval performance and oviposition preference. Failure of conspecific eggs to deter oviposition may not represent lack of association of preference and performance because other evidence suggests that larval performance may not be greatly affected by larval density. These results suggest that the ability of refuges to slow evolution of physiological resistance to B. thuringiensis toxins will not be magnified by an oviposition preference for untreated foliage. © 1992 Plenum Publishing Corporation.
• Tabashnik, B. E. (1992). Evaluation of synergism among Bacillus thuringiensis toxins. Applied and Environmental Microbiology, 58(10), 3343-3346.
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  PMID: 1444368;PMCID: PMC183101;Abstract: A simple test for synergism among toxins is described and applied to previously reported data on independent and joint toxicities of insecticidal proteins from Bacillus thuringiensis. The analysis shows synergism between a 27-kDa (CytA) toxin and 130- or 65-kDa (CryIV) toxins from B. thuringiensis subsp. israelensis against Aedes aegypti larvae. No positive synergism between 130- and 65-kDa toxins or among three CryIA toxins tested against seven species of Lepidoptera occurred. Comparisons with the original interpretations of these data show one case in which synergism occurred but was reported previously as absent and two cases that were not synergistic but were reported previously as suggestive of synergism. These results show that lack of an appropriate test for synergism can produce misleading conclusions. The methods described here can be used to test for synergistic effects of any poisons.
• Rathman, R., Johnson, M., & Tabashnik, B. (1991). Production of Ganaspidium utilis (Hymenoptera: Eucoilidae) for biological control of Liriomyza spp. (Diptera: Agromyzidae). Biological Control, 1(3), 256-260.
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  Abstract: A method for rearing the Liriomyza leafminer parasitoid Ganaspidium utilis Beardsley (Hymenoptera: Eucoilidae) is described. 'Henderson' bush lima beans, Phaseolus limensis var. limenanus L. H. Bailey, were planted in vermiculite in six 20 × 14 × 35.5-cm2 plastic bags. After 10 to 11 days the plants were exposed to 500 to 600 adult Liriomyza trifolii (Burgess) (Diptera: Agromyzidae) for 24 h and then placed in a holding cage for 4 days at 22°C. Plants containing second instar L. trifolii were exposed to G. utilis for 48 h. Emergence of parasitoids occurred 24 to 33 days after exposure. The number of G. utilis emerging per bag of beans was 403 ± 29 (X ± SEM; n = 15); 51% of these were females. Recurring cost of production of 5000 parasitoids per day was estimated at $36 for G. utilis compared with $97 for another leafminer parasitoid, Diglyphus begini Ashmead (Hymenoptera: Eulophidae). © 1991.
• Rosenheim, J. A., & Tabashnik, B. E. (1991). Influence of generation time on the rate of response to selection. American Naturalist, 137(4), 527-541.
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  Abstract: Examined the influence of generation time on the rate of evolution of pesticide resistance in arthropod pests. Data integrated information on resistance evolution, generation time and other biological parameters for 682 North American arthropod pests. The data did not support a linear relationship between generations per year and the evolution of resistance, revealing instead a nonlinear and highly variable relationship, with peak rates of resistance evolution for species with intermediate generation times. Per-generation fitness values for genotypes conferring resistance to pesticides or genotypes conferring increased fitness in response to any density-independent selective agent are related exponentially to generation time, resulting in the independence of generation time and the rate of response to selection in the simplest-case model. -from Authors
• Schwartz, J. M., Tabashnik, B. E., & Johnson, M. W. (1991). Behavioral and physiological responses of susceptible and resistant diamondback moth larvae to Bacillus thuringiensis. Entomologia Experimentalis et Applicata, 61(2), 179-187.
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  Abstract: To determine whether field-selected resistance of diamondback moth (Plutella xylostella L.) (Lepidoptera: Plutellidae) to Bacillus thuringiensis is based on behavioral or physiological adaptation, we measured mortality, consumption, and movement of larvae from a susceptible and a resistant colony when placed on untreated and B. thuringiensis treated cabbage. Colonies did not differ in mortality, consumption, or movement on untreated cabbage. However, for a given amount of consumption of treated cabbage, resistant larvae had lower mortality than susceptible larvae, demonstrating that resistance had a physiological basis. The movement patterns could not account for the differences between colonies in survival. Resistant larvae did not avoid B. thuringiensis more than did susceptible larvae. Thus, we found no evidence for behavioral resistance. © 1991 Kluwer Academic Publishers.
• Tabashnik, B. E. (1991). Determining the mode of inheritance of pesticide resistance with backcross experiments.. Journal of Economic Entomology, 84(3), 703-712.
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  PMID: 1885840;Abstract: The most widely used method for evaluating the mode of inheritance of pesticide resistance is based on bioassays of individuals from a backcross between F1 (hybrid of resistant and susceptible strains) and parental resistant or susceptible strains. Monte Carlo simulations of the standard backcross method showed that the probability of incorrectly rejecting the null hypothesis of monogenic inheritance (Type I error) was generally more than double the conventional value of P = 0.05. Conversely, the null hypothesis of monogenic inheritance was likely to be accepted in a relatively large proportion of cases in which resistance is controlled by two or more loci. Expected differences in mortality of backcross offspring between monogenic and additive polygenic models approached zero as dose approached extremely low values, extremely high values, and the LD50 of the backcross generation. Thus, the effectiveness of the backcross method depended strongly on dose. The power of the standard backcross method to correctly reject the null hypothesis of monogenic inheritance increased as number of loci, slope of parental dose-mortality lines, magnitude of resistance, and sample size increased. Guidelines for improving the design and interpretation of backcross experiments are presented.
• Rathman, R. J., Johnson, M. W., Rosenheim, J. A., & Tabashnik, B. E. (1990). Carbamate and pyrethroid resistance in the leafminer parasitoid Diglyphus begini (Hymenoptera: Eulophidae). Journal of Economic Entomology, 83(6), 2153-2158.
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  Abstract: Populations of Diglyphus begini showed resistance to oxamyl, methomyl, fenvalerate, and permethrin. Resistance D. begini may be useful for controlling leafminers in management programs that integrate biological and chemical controls. -from Authors
• Rosenheim, J. A., & Tabashnik, B. E. (1990). Evolution of pesticide resistance: interactions between generation time and genetic, ecological, and operational factors. Journal of Economic Entomology, 83(4), 1184-1193.
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  PMID: 2212232;Abstract: The simplest model demonstrated that the time required for resistance to evolve can be independent of generation time, but interactions of generation time with genetic, biological, and operational factors resulted in positive, negative, and U-shaped relationships between the number of generations per year and the time required for resistance to evolve. Results preclude any generalizations concerning the influence of generation time on resistance evolution. -from Authors
• Tabashnik, B. E. (1990). Implications of gene amplification for evolution and management of insecticide resistance.. Journal of economic entomology, 83(4), 1170-1176.
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  PMID: 2212231;Abstract: Simulations were used to compare evolution of insecticide resistance predicted by a conventional two-allele model with predictions from three- and four-allele models that assume resistance is based on gene amplification. Results were similar between models when insecticide concentration was low or moderate. In contrast, when 10% of the population was not exposed to insecticide each generation, high insecticide concentrations slowed resistance development in the two-allele model, but caused rapid development of high levels of resistance in the three- and four-allele models. The presence of a third allele at an initial frequency as low as 10-7 doubled or tripled the rate of resistance development in some cases. Attempts to slow evolution of resistance by overwhelming it with high concentrations of insecticides are not likely to succeed if gene amplification or other mechanisms generate alleles that confer high levels of resistance.
• Tabashnik, B. E., Croft, B. A., & Rosenheim, J. A. (1990). Spatial scale of fenvalerate resistance in pear psylla (Homoptera: Psyllidae) and its relationship to treatment history. Journal of Economic Entomology, 83(4), 1177-1183.
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  Abstract: Data on fenvalerate susceptibility, pyrethroid use, and related information for 48 sites in British Columbia, Washington, Oregon and California were analyzed to clarify the spatial scale and causes of fenvalerate resistance in Psylla pyricola. -from Authors
• Tabashnik, B. E., Cushing, N. L., Finson, N., & Johnson, M. W. (1990). Field development of resistance to Bacillus thuringiensis in diamondback moth (Lepidoptera: Plutellidae). Journal of Enonomic Entomology, 83(5), 1671-1676.
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  Abstract: Foliar applications of commerical formulations of the insecticidal spore-crystal protein complex of Bacillus thuringiensis subsp. kurstaki caused development of resistance in field populations of Plutella xylostella. -from Authors
• Tabashnik, B. E. (1989). Managing resistance with multiple pesticide tactics: theory, evidence, and recommendations.. Journal of economic entomology, 82(5), 1263-1269.
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  PMID: 2689487;Abstract: Sequences, mixtures, rotations, and mosaics are potential strategies for using more than one pesticide to manage pest populations and for slowing the evolution of pesticide resistance. Results from theoretical models suggest that, under certain conditions, mixtures might be especially effective for resistance management. The assumptions of such models, however, are probably not widely applicable. Potential disadvantages associated with mixtures that are usually not considered in modeling studies include disruption of biological control, promotion of resistance in secondary pests, and intense selection for cross-resistance. Results from limited experimental work suggest that pesticide combinations do not consistently suppress resistance development. More thorough evaluation of tactics that seek to optimize benefits of more than one insecticide will require rigorous experiments with the particular pest and pesticide combinations. Because of the difficulty in generalizing results across systems and the potential negative effects of multiple insecticide use, emphasis on minimizing insecticide use is recommended.
• Yudin, L. S., Tabashnik, B. E., Cho, J. J., & Mitchell, W. C. (1988). Colonization of weeds and lettuce by thrips (Thysanoptera: Thripidae). Environmental Entomology, 17(3), 522-526.
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  Abstract: In Lactuca sativa fields at Kula, Hawaii, Frankliniella occidentalis, a vector of tomato spotted wilt virus, was the most abundant thrips species (82.4% of total) of the 12 species collected. Nonflowering lettuce was less attractive to thrips than flowering plants of the weed species. Among weeds in flower, Verbesina encelioides, Malva parviflora and Amaranthus hybridus were most attractive to thrips, but among nonflowering stages, V. encelioides attracted the fewest thrips and only A. hybridus and M. parviflora were more attractive than lettuce. -from Authors
• Tabashnik, B. E. (1987). Plant secondary compounds as oviposition deterrents for cabbage butterfly, Pieris rapae (Lepidoptera: Pieridae). Journal of Chemical Ecology, 13(2), 309-316.
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  PMID: 24301809;Abstract: Oviposition by Pieris rapae butterflies was deterred by spraying the plant secondary compounds coumarin and rutin on cabbage plants in greenhouse choice tests. In no-choice tests ranging from 5 min to 24 hr, acceptance of rutin-treated plants for oviposition increased with trial duration. Both coumarin and rutin deterred oviposition primarily by affecting prealighting rather than postalighting behavior, indicating that deterrence was mediated by noncontact cues. © 1987 Plenum Publishing Corporation.
• Tabashnik, B. E. (1986). Insect resistance [4]. Science, 234(4778), 802-.
• Tabashnik, B. E., & Croft, B. A. (1985). Evolution of pesticide resistance in apple pests and their natural enemies. Entomophaga, 30(1), 37-49.
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  Abstract: Evolution of pesticide resistance in 24 apple pest and natural enemy species was simulated with a computer model. Population ecology parameters were varied among species while physiological, biochemical and genetic assumptions were held constant. There was good agreement between the model's predictions and observed historical patterns of azinphosmethyl resistance among pests and natural enemies. Correspondence between predicted and observed was improved by assuming that natural enemies evolved resistance only after their prey/hosts became resistant, but not by assuming greater initial susceptibility in natural enemies. Results suggest that ecological factors may be important in determining rates of resistance evolution. © 1985 Lavoisier Abonnements.
• Tabashnik, B. E., Mattson, W. J., & Miller, J. R. (1985). Host acceptance behavior of the red pine cone beetle (Conophthorus resinosae). Entomologia Experimentalis et Applicata, 37(1), 3-7.
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  Abstract: Adult female red pine cone beetles (Conophthorus resinosae Hopkins) (Coleoptera:Scolytidae) left cones of white pine (Pinus strobus L.) more readily than cones of red pine (Pinus resinosa Ait.) in the first 3 min after being placed on cones. Slices of white pine cones, however, did not elicit different beetle behavior than slices of red pine cones. Beetles bored into white pine as readily as into red pine. We conclude that beetles initially reject white pine due to structural traits, but eventually accept it during prolonged exposure and may utilize white pine in nature. © 1985 Dr W. Junk Publishers.
• Tabashnik, B. E. (1983). Host range evolution. The shift from native legume hosts to alfalfa by the butterfly, Colias philodice eriphyle ( Medicago sativa).. Evolution, 37(1), 150-162.
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  Abstract: Colias philodice eriphyle has pest populations which feed in the field on alfalfa Medicago sativa and non-pest populations which feed on several legumes, but not on alfalfa. Non-pest females laid a substantial fraction of their eggs on alfalfa in preference tests and non-pest larvae were able to grow on alfalfa without a severe disadvantage relative to their growth on native hosts. Genetic changes in host use characters would thus not be required for a shift to alfalfa. Oviposition choice tests showed no differences between pest and non-pest females. On alfalfa, pest larvae grew better than non-pest larvae; but on native hosts, non-pest larvae grew better than pest larvae. Results suggest that pest C. p. eriphyle have improved their ability to grow on alfalfa, while losing some of their ability to grow on native hosts. Shifts to closely related plant species are not likely to lead to rapid divergence. -from Author
• Tabashnik, B. E. (1983). Oviposition specificity in single vs. cluster egg-laying butterflies: a discrimination phase in Colias eurytheme?. Oecologia, 58(2), 278-279.
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  Abstract: Oviposition preferences and rates of the pierid butterfly Colias eurytheme were examined in laboratory tests. Oviposition rates with only a non-preferred host plant present were not lower than rates observed when both preferred and non-preferred hosts were present. There was no correlation between strength of preference in simultaneous choice tests and the extent to which oviposition rate declined when the preferred host was absent. The results are consistent with the hypothesis that butterflies which lay their eggs singly will accept non-preferred hosts for oviposition more readily than cluster egg-layers. © 1983 Springer-Verlag.
• Tabashnik, B. E. (1982). Responses of pest and non-pest Colias butterfly larvae to intraspecific variation in leaf nitrogen and water content. Oecologia, 55(3), 389-394.
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  Abstract: This study examined the effects of intraspecific variation in leaf nitrogen and water content on the growth, consumption, conversion efficiency and nitrogen use of Colias butterfly larvae. Pest and non-pest Colias philodice eriphyle larvae and Colias eurytheme larvae were fed field-collected alfalfa (Medicago sativa) and vetch (Vicia americana) leaves in laboratory experiments. In all treatments, at least one indicator of larval growth performance was positively correlated with leaf nitrogen content, which supports the view that nitrogen is a limiting nutrient for larval growth. The benefits associated with eating leaves with high nitrogen content included higher growth rates, conversion efficiencies, nitrogen accumulation rates and larval nitrogen contents. Over the ranges examined in this study, variation in leaf nitrogen content (2.8-7.0% dry wt) affected larval growth more than variation in leaf water content (66-79% fresh wt). Pest and non-pest C. p. eriphyle responded alike to variation in the leaf nitrogen content of vetch, but there were differences between populations on alfalfa. Pest larvae were more sensitive to variation in leaf water content than non-pest larve. The differences between these populations may be due to specific adaptations resulting from the shift to alfala by pest Colias. It is suggested that herbivores' responses to intraspecific variation in leaf nitrogen content may have important consequences for the evolution of plant defenses and nutrient allocation patterns, and for agricultural pest management. © 1982 Springer-Verlag.
• Tabashnik, B. E., Wheelock, H., Rainbolt, J. D., & Watt, W. B. (1981). Individual variation in oviposition preference in the butterfly, Colias eurytheme. Oecologia, 50(2), 225-230.
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  Abstract: We examined intra-population variation in oviposition preference in the pierid butterfly, Colias eurytheme. Females' preferences were tested in the laboratory, using two-way choice tests between the potential hosts, alfalfa and vetch. There were consistent differences in oviposition preference among females within a population. Larval and adult experience had little or no effect on females' preference. These results suggest that the intra-population variation in oviposition preference is genetically based, but the results of experiments designed to estimate the heritability of oviposition preference were not conclusive. We suggest that intra-population variation in host selection characters may play a key role in shifts to new host plants. © 1981 Springer-Verlag.
• Tabashnik, B. E. (1980). Population structure of pierid butterflies - III. Pest populations of Colias philodice eriphyle. Oecologia, 47(2), 175-183.
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  Abstract: Mark-release-recapture techniques were used to study alfalfa pest populations of Colias philodice eriphyle. Two new methods for estimating relative catchability and residence rates were used to compare males to females. The results show that: (1) both sexes had limited dispersal, with mean individual ranges less than 100 m; (2) males were more abundant than females; (3) males and females had similar residence times; (4) males were more catchable than females in uncut fields, but not in cut fields. Explanations for the differences between the sexes are considered. Females may be less catchable in uncut fields because they spend less time in flight than males. Males may be more abundant than females because they develop faster, and may have lower pre-adult mortality. The differences between the sexes are discussed with respect to reproductive strategy. Comparisons with non-pest C.p. eriphyle show differences between pest and non-pest populations. Pest C.p. eriphyle were more sedentary. The residence times were similar for pest and non-pest populations, but pest C.p. eriphyle probably have longer "reproductive life-spans". Mid-summer broods of the pest population were partially overlapping; the non-pest population has discrete broods. Pest population density varied less between years than non-pest population density. The differences between pest and non-pest C.p. eriphyle support the idea of ecological diversity among conspecific populations. © 1980 Springer-Verlag.
• Watt, W. B., Han, D., & Tabashnik, B. E. (1979). Population structure of pierid butterflies - II. A "Native" population of Colias philodice eriphyle in Colorado. Oecologia, 44(1), 44-52.
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  Abstract: The structure of a bivoltine, discrete-generation population of Colias philodice eriphyle, occurring in relatively undisturbed habitat, has been examined by mark-release-recapture techniques. The population's general ecology is briefly discussed. Males eclose before females as in other Colias, and a measure of physical wear on adults is related to age of individuals and to the overall position of a sample in the flight period, again as in other Colias. Densities of adults fluctuate drastically, with the first (overwintering) brood always being less dense than the second brood. Dispersal radius of those dispersing does not vary with brood, sex, or year, although the proportion of dispersants does: more males than females disperse in the first brood, while the reverse is true in the second. A tentative behavioral explanation for this is proposed. Adult mortality is unusually high compared to other Colias. The population displays area continuity with adjacent population areas. The Wrightian neighborhood size varies in geographic extent, due to change in dispersant proportions, from 70 to 260 hectares. In adult numbers, it varies from 4-500 (or possibly fewer in very severe first-brood conditions) to upwards of 20,000 in some second broods (though not all adults present always reproduce successfully). Two possible models for the dispersal behavior of Colias are presented. One, the "excited state" model, is so far supported over the other, "continuous activity" model, by the present data and by earlier work on C. alexandra. The adult mortality level is consistent with the conclusion that this population is ecologically marginal for the species. Possible selective pressures preventing further extension of the species' distribution, including possible competition with other Colias, are discussed. © 1979 Springer-Verlag.

Presentations

• Tabashnik, B. E. (2021). Invited presentation, Global patterns of resistance to Bt crops: What went right and wrong?. ENTO’21. Royal Entomological Society, United Kingdom (online)..
• Tabashnik, B. E. (2021). Invited presentation, The role of science in sustainable agriculture.. Roundtable, Agroecology Forum 2021, Barcelona, Spain..
• Tabashnik, B. E. (2021). and Yves Carriere, Invited presentation, Managing resistance to Bt crops: Lessons from failures and successes.. Entomological Society of America Annual Meeting (online)..
• Tabashnik, B. E. (2020, December). Invited webinar, Battling bollworm resistance to Bt cotton. International Cotton Advisory Committee, Inaugural International Webinar Series, https://www.youtube.com/watch?v=_lxAoA02NIE.
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  392 participants from 19 countries representing 5 continents
• Tabashnik, B., Carriere, Y., & Fabrick, J. (2020, November). Invited symposium presentation, Pink bollworm resistance to Bt cotton: A tale of three countries. Entomological Society of America Annual Meeting (online)..
• Tabashnik, B. E. (2019, August). Lessons from global patterns of resistance to Bt crops for sustainable pest management.. Food System Impacts of Pests and Pathogens in a Changing Climate. Aspen, Colorado: Aspen Global Change Institute.
• Tabashnik, B. E. (2019, March). Lessons from global patterns of resistance to Bt crops for managing pink bollworm in India.. Indian Seed Congress.. Hyderabad, India.
• Tabashnik, B. E. (2018, April). Challenges of trying to be an honest broker on insect resistance to GE crops. Invited Genetic Engineering & Society Colloquium PresentationNorth Carolina State University.
• Tabashnik, B. E. (2018, April). Insect resistance to transgenic crops: Lessons from the first billion acres. Invited seminar presentationDepartment of Entomology, North Carolina State University.
• Tabashnik, B. E. (2018, January). Invited Workshop Presentation. Managing resistance to transgenic crops: from theory to practice.. Annual Meeting of the National Alliance of Independent Crop Consultants. Tucson, Arizona..
• Tabashnik, B. E., & Carriere, Y. (2018, November). Invited Symposium Presentation. Global surge in pest resistance to Bt crops and prospects for sustainability.. Joint meeting of the Entomological Societies of America and Canada.. Vancouver, Canada..
• Tabashnik, B. E. (2017, August). Insect resistance to transgenic crops: Lessons from the first billion acres.. Invited Plenary Speaker, Ward Watt Festschrift. Rocky Mountain Biological Laboratory, Colorado.
• Tabashnik, B. E. (2017, December). Insect resistance to transgenic crops: Lessons from the first billion acres. Invited Plenary Speaker, International Society for Evolution, Ecology & Cancer. Tempe, Arizona.
• Tabashnik, B. E. (2017, November). Pyramiding RNAi and Bt counters insect resistance. Invited Symposium Presentation. Denver: Entomological Society of America.
• Mathew, L. G., Carriere, Y., Li, X., Tabashnik, B. E., & Fabrick, J. A. (2016, September/2016). Genetic basis of resistance to Bt toxin Cry2Ab in pink bollworm. 2016 International Congress of Entomology (ICE2016), Orlando, Florida. Orlando, Florida: Entomological Society of America (ESA).
• Tabashnik, B. E. (2014, July). Keynote address: Resistance management for Bt crops: Successes and failures.. First International Symposium on Insecticide Toxicology. Guangzhou, China.
• Tabashnik, B. E. (2014, October). Invited Symposium Presentation: Resistance management for Bt crops: Successes and failures.. Biotechnology Summit, Huatulco, Mexico.
• Tabashnik, B. E., & Carriere, Y. (2014, April). Invited Symposium Presentation: Insect resistance to transgenic crops: lessons from the first billion acres. Annual Meeting, Pacific Branch, Entomological Society of America, Tucson, AZ.
• Carriere, Y. -., & Tabashnik, B. E. (2013, November). Invited symposium presentation: Landscape-based approach for sustaining efficacy of Bt crops (coauthor). Annual Meeting of the Entomological Society of America.
• Tabashnik, B. E. (2013, December). Suppressing pink bollworm with Bt cotton and sterile insect releases. Visiting delegation of the Chinese Academies of Engineering & Science.
• Tabashnik, B. E., & Carriere, Y. -. (2013, November). Invited syposium presentation: Where and why resistance management has and hasn't succeeded for Bt crops (speaker). Annual Meeting of the Entomological Society of America.
• Tabashnik, B. (2012). Field-evolved resistance to Bt crops. Biotechnology Summit. Merida, Mexico: Biotechnology Summit.
• Tabashnik, B. (2012). Using the sterile insect technique to manage resistance of pink bollworm to Bt cotton. Food & Agriculture Organization of the United Nations and International Atomic Energy Agency Workshop. Tucson, Arizona: Food & Agriculture Organization of the United Nations and International Atomic Energy Agency.
• Tabashnik, B., & , Y. (2012). Field-evolved resistance to Bt crops: Theory and data. Annual Meeting of the Entomological Society of America. Knoxville, Tennessee: Entomological Society of America.
• Tabashnik, B. (2011). Suppressing resistance to Bt cotton with sterile insect releases. Beijing, Nanjing, Wuhan and Chengdu; China.
• Tabashnik, B., Huang, F., Ghimire, M., Leonard, B., Siegfried, B., Rangasamy, M., Yang, Y., Wu, Y., Heckel, D., Bravo, A., & , M. (2011). Got resistance? Try modified Bt toxins. Annual Meeting of the Entomological Society of America. Reno, Nevada: Entomological Society of America.
• Tabashnik, B., Li, X., & Wu, Y. (2011). Field-evolved resistance to Bt crops. Annual Meeting of the Society for Invertebrate Pathology. Halifax, Canada: Society for Invertebrate Pathology.

Reviews

• Gassmann, A. J., Carriere, Y., & Tabashnik, B. E. (2011. Fitness Costs of Insect Resistance to Bacillus thuringiensis(pp 147-163).
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  Evolution of resistance by iusect pests threatens the continued effectiveness of Bacillus thuringiensis (Bt) toxins in sprays and trausgenic crops. Fitness costs of Bt resistance occur when, in the absence of Bt toxins, fitness is lower for resistant insects than for susceptible insects. Modeling results show that fitness costs call delay resistance by selecting against Bt-resistant genotypes in refuges where insects are not exposed to Bt toxins. Ill 77 studies including 18 species, fitness costs were detected in 62% of experiments testing for declines in resistance and in 34% of fitness component comparisons. Mean fitness costs were 15.5% for survival, 7.4% for development time, and 2.5% for mass. Although most fitness costs were recessive, nonrecessive costs call select more strongly against resistance. Because fitness costs vary with ecological conditions, refuges designed to increase the dominance or magnitude of fitness costs could be especially useful for delaying pest resistance.
• Carriere, Y., Crowder, D. W., & Tabashnik, B. E. (2009. Evolutionary ecology of insect adaptation to Bt crops(pp 561-573).
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  Transgenic crops producing Bacillus thuringiensis (Bt) toxins are used worldwide to control major pests of corn and cotton. Development of strategies to delay the evolution of pest resistance to Bt crops requires an understanding of factors affecting responses to natural selection, which include variation in survival on Bt crops, heritability of resistance, and fitness advantages associated with resistance mutations. The two main strategies adopted for delaying resistance are the refuge and pyramid strategies. Both can reduce heritability of resistance, but pyramids can also delay resistance by reducing genetic variation for resistance. Seasonal declines in the concentration of Bt toxins in transgenic cultivars, however, can increase the heritability of resistance. The fitness advantages associated with resistance mutations can be reduced by agronomic practices, including increasing refuge size, manipulating refuges to increase fitness costs, and manipulating Bt cultivars to reduce fitness of resistant individuals. Manipulating costs and fitness of resistant individuals on transgenic insecticidal crops may be especially important for thwarting evolution of resistance in haplodiploid and parthenogenetic pests. Field-evolved resistance to Bt crops in only five pests during the last 14 years suggests that the refuge strategy has successfully delayed resistance, but the accumulation of resistant pests could accelerate.

Others

• Tabashnik, B. E. (2017, December). GRANT UPDATES.
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  USDA proposal (Assessing Risk of Western Corn Rootworm Resistance...) submitted in 2016 was not funded.USDA grant submitted in Summer 2017 (Genetics and Mechanisms of Pest Resistance to Second Generation Bt Crops) has been recommended for $460K in funding.
• Tabashnik, B. E., & Carriere, Y. (2015, March). Comment on the EPA Proposal to Improve Corn Rootworm Resistance Management.. US EPA: http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPP-2014-0805-0031.
• Tabashnik, B. E., Brevault, T., & Carriere, Y. (2015, January). Insect Resistance to Genetically Engineered Crops: Successes and Failures. Information Systems for Biotechnology News Report.
• Tabashnik, B., & Wu, Y. (2012, Fall). Cotton bollworm adapting to Bt cotton in China via diverse mutations. Information Systems for Biotechnology News Report. October 2012: 4-6.
• Heuberger, S., Ellers-Kirk, C., Tabashnik, B., & , Y. (2011, Fall). A spatially-explicit analysis of crop-to-crop gene flow in cotton. Information Systems for Biotechnology News Report, March 2011: 5-8.
• Tabashnik, B. (2011, Fall). A brief history of the role of Bt cotton in Arizona.
• Tabashnik, B. (2011, Fall). Pest control with Bt cotton and sterile insect releases. Information Systems for Biotechnology News Report. February 2011: 4-6.