Search

Background: Transgenic maize (Zea mays L.) event TC1507 (Herculex ® I insect protection), expressing Cry1F δ-endotoxin derived from Bacillus thuringiensis var. aizawai, was commercialized in 2003 in the Americas. Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) susceptibility to Cry1F was monitored annually across several regions in Argentina using diagnostic concentration bioassays. Reduced performance of TC1507 maize against S. frugiperda was reported in 2013. A resistant population was established in the laboratory and the dominance of Cry1F resistance was characterized., Results: During 2012-2015, high-survivorship of several populations was observed in the resistance monitoring program. Reciprocal crosses of a Cry1F-resistant population with a Cry1F-susceptible population were evaluated to calculate effective dominance (D ML ) based on mortality levels observed at 100 µg/ml Cry1F. Two additional dominance levels (D LC and D EC ) were calculated using lethal (LC 50 ) or effective concentration (EC 50 ) derived from concentration-response bioassays. Estimates indicated that Cry1F resistance in S. frugiperda in Argentina was either highly recessive (D ML = 0.005) or incompletely recessive (D LC < 0.26 and D EC < 0.19)., Conclusion: This study is the first documented confirmation and characterization of S. frugiperda Cry1F field-evolved resistance in Argentina. The resistance to Cry1F in S. frugiperda populations collected in Argentina, is autosomal and incompletely recessive similar to the resistance reported in Brazil. © 2017 The Authors. Pest Management Science published by John Wiley © Sons Ltd on behalf of Society of Chemical Industry., (© 2017 The Authors. Pest Management Science published by John Wiley © Sons Ltd on behalf of Society of Chemical Industry.)

Background: The presence of fitness costs of resistance to Bacillus thuringiensis (Bt) insecticidal proteins in insect populations may delay or even reverse the local selection of insect resistance to Bt transgenic crops, and deserves rigorous investigation. Here we assessed the fitness costs associated with Cry1Fa resistance in two strains of fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), derived from field collections in different Brazilian regions and further selected in the laboratory for high levels of resistance to Cry1Fa using leaves of TC1507 corn., Results: Fitness components were compared using paired resistant and susceptible strains with similar genetic backgrounds and F 1 generations from reciprocal crosses, all of them reared on non-transgenic corn leaves. No apparent life history costs in the larval stage were observed in the Bt-resistant strains. Moreover, the resistance remained stable for seven generations in the absence of selection, with no decrease in the proportion of resistant individuals. Larval respiration rates were also similar between resistant and susceptible homozygotes, and heterozygotes displayed respiration rates and demographic performance equal or superior to those of susceptible homozygotes., Conclusion: In combination, these results indicate the lack of strong fitness costs associated with resistance to Cry1Fa in the fall armyworm strains studied. These findings suggest that Cry1Fa resistance in S. frugiperda populations is unlikely to be counterselected in Cry1Fa-free environments. © 2016 Society of Chemical Industry., (© 2016 Society of Chemical Industry.)

Refuge is mandated in the United States where genetically modified maize (Zea mays L.) expressing insecticidal proteins derived from Bacillus thuringiensis Berliner (Bt) are cultivated. Currently, refuge is deployed in different ways including blocks, field strips, or seed blends containing Bt and non-Bt maize. Seed blends provide practical advantages for refuge implementation. However, concerns related to the movement of insect larvae, potential differential survival of heterozygous resistant larvae, reduction in insect production, and cross-pollination of ears resulting in sublethal selection, have delayed seed blend use for Lepidoptera in the southern United States, where maize plantings are used as refuge for Helicoverpa zea (Boddie). In this study, we evaluated the relative survival of H. zea in Bt events and in seed blends compared with pure stand refuge and the relative survival of H. zea on the individual components of the pyramid 1507xMON810xMIR162. The results showed variation on the production of H. zea in refuge plants from seed blends compared with pure stand refuge plants. The relative survival of H. zea on the events 1507, MON810, MIR162, and 1507xMON810xMIR162 ranked similarly across the three locations tested. These results can be used in computer simulation modeling efforts to evaluate the feasibility of seed blends as a refuge deployment strategy with the pyramid 1507xMON810xMIR162. Because the reduction on survival of H. zea due to blending was variable, a sensitivity analysis that includes all possible scenarios of reduction in survival should be considered., (© The Authors 2015. Published by Oxford University Press on behalf of Entomological Society of America. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Transgenic maize, Zea maize L., expressing the Cry1F protein from Bacillus thuringiensis has been registered for Spodoptera frugiperda (J. E. Smith) control since 2003. Unexpected damage to Cry1F maize was reported in 2006 in Puerto Rico and Cry1F resistance in S. frugiperda was documented. The inheritance of Cry1F resistance was characterized in a S. frugiperda resistant strain originating from Puerto Rico, which displayed >289-fold resistance to purified Cry1F. Concentration-response bioassays of reciprocal crosses of resistant and susceptible parental populations indicated that resistance is recessive and autosomal. Bioassays of the backcross of the F1 generation crossed with the resistant parental strain suggest that a single locus is responsible for resistance. In addition, cross-resistance to Cry1Aa, Cry1Ab, Cry1Ac, Cry1Ba, Cry2Aa and Vip3Aa was assessed in the Cry1F-resistant strain. There was no significant cross-resistance to Cry1Aa, Cry1Ba and Cry2Aa, although only limited effects were observed in the susceptible strain. Vip3Aa was highly effective against susceptible and resistant insects indicating no cross-resistance with Cry1F. In contrast, low levels of cross-resistance were observed for both Cry1Ab and Cry1Ac. Because the resistance is recessive and conferred by a single locus, an F1 screening assay was used to measure the frequency of Cry1F-resistant alleles from populations of Florida and Texas in 2010 and 2011. A total frequency of resistant alleles of 0.13 and 0.02 was found for Florida and Texas populations, respectively, indicating resistant alleles could be found in US populations, although there have been no reports of reduced efficacy of Cry1F-expressing plants.

The refuge strategy can delay resistance of insect pests to transgenic maize producing toxins from Bacillus thuringiensis (Bt). This is important for the western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), because of its history of adaptation to several management practices. A 2-yr study across four locations was conducted to measure the effects of integrated refuge (i.e., blended refuge) on western corn rootworm survival to adulthood, fitness characteristics, and susceptibility to Bt maize in the subsequent generation. The treatments tested in this study were as follows: a pure stand of Bt maize (event DAS-59122-7, which produces Bt toxins Cry34Ab1/Cry35Ab1), a pure stand of refuge (non-Bt maize), and two variations on an integrated refuge consisting of 94.4% Bt maize and 5.6% non-Bt maize. Within the two integrated refuge treatments, refuge seeds received a neonicotinoid insecticidal seed treatment of either 1.25 mg clothianidin per kernel or 0.25 mg thiamethoxam per kernel. Insects in the pure stand refuge treatment had greater survival to adulthood and earlier emergence than in all other treatments. Although fecundity, longevity, and head capsule width were reduced in treatments containing Bt maize for some site by year combinations, Bt maize did not have a significant effect on these factors when testing data across all sites and years. We found no differences in susceptibility of larval progeny to Bt maize in bioassays using progeny of adults collected from the four treatments.

Background: The high dose plus refuge is one of the major components of the resistance management plan mandated for transgenic corn expressing Cry toxins from Bacillus thuringiensis Berliner (Bt) that targets the European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae). This strategy was based on assumptions such as functional recessive inheritance, which has not been previously tested for O. nubilalis. The authors used a field-derived resistant strain of O. nubilalis to define the nature of resistance to Cry1Ab toxin by examining the inheritance and on-plant survival of susceptible and resistant insects and their F(1) progeny., Results: The resistant strain exhibited > 800-fold resistance to Cry1Ab. Resistance was primarily autosomal and controlled by more than one locus or multiple alleles at one locus. The degree of dominance D calculated on the basis of LC(50) values was - 0.45(h' = 0.27), indicating that resistance was incompletely recessive. No survivors were found on vegetative-stage Bt corn, although both resistant larvae and their F(1) progeny were able to survive on reproductive corn 15 days after infestation., Conclusions: A field derived O. nubilalis strain exhibited high levels of resistance to Cry1Ab and survived on transgenic corn by feeding on tissues with low Cry1Ab expression. The Cry1Ab resistance was primarily autosomal, incompletely recessive and polygenic. Tissue and on-plant survival data indicated that dominance varies depending on plant stage., (Copyright 2009 Society of Chemical Industry.)

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 F1 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.