Occurrence and Ear Damage of Helicoverpa zea on Transgenic Bacillus thuringiensis Maize in the Field in Texas, U.S. and Its Susceptibility to Vip3A Protein

Longitudinal trials illustrate interactive effects between declining Bt efficacy against Helicoverpa zea (Lepidoptera: Noctuidae) and planting dates of corn

Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) has evolved resistance to insecticidal toxins from Bacillus thuringiensis (Bt) Berliner (Bacillales: Bacillaceae) expressed in genetically engineered corn, Zea mays L. This study provides an overview of field trials from Georgia, North Carolina, and South Carolina evaluating Bt and non-Bt corn hybrids from 2009 to 2022 to show changes in susceptibility in H. zea to Bt corn. The reduction in kernel injury relative to a non-Bt hybrid averaged across planting dates generally declined over time for Cry1A.105 + Cry2Ab2 corn. In addition, there was a significant interaction with planting date used as a covariate. The reduction in kernel injury remained above 80% and did not vary with planting date from 2009 to 2014, whereas a significant decline with planting date was found in this reduction from 2015 to 2022. For Cry1Ab + Cry1F corn, the reduction in kernel injury relative to a non-Bt hybrid averaged across planting dates did not vary among years. The reduction in kernel injury significantly declined with planting date from 2012 to 2022. Kernel injury as a proxy for H. zea pressure was greater in late-planted trials in non-Bt corn hybrids. Our study showed that Bt hybrids expressing Cry1A.105 + Cry2Ab2 are now less effective in later planted trials in reducing H. zea injury; however, this was not the case during the earlier years of adoption of corn expressing these 2 toxins when resistance alleles were likely less frequent in H. zea populations. The implications for management of H. zea and for insect resistance management are discussed.

Continued decline in sublethal effects of Bt toxins on Helicoverpa zea (Lepidoptera: Noctuidae) in field corn

The majority of field corn, Zea mays L., in the southeastern United States has been genetically engineered to express insecticidal toxins produced by the soil bacterium, Bacillus thuringiensis (Bt). Field corn is the most important mid-season host for corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), which has developed resistance to all Cry toxins in Bt corn. From 2020 to 2023, corn earworm pupae were collected from early- and late-planted pyramided hybrids expressing Bt toxins and non-Bt near-isolines in North and South Carolina (16 trials). A total of 5,856 pupae were collected across all trials, with 55 and 88% more pupae collected in later-planted trials relative to early plantings in North and South Carolina, respectively. Only 20 pupae were collected from hybrids expressing Cry1F + Cry1Ab + Vip3A20 across all trials. Averaged across trials, Cry1A.105 + Cry2Ab2 hybrids reduced pupal weight by 6 and 9% in North and South Carolina, respectively, relative to the non-Bt near-isoline. Cry1F + Cry1Ab hybrids reduced pupal weight on average by 3 and 8% in North and South Carolina, respectively, relative to the non-Bt near-isoline. The impact of the Bt toxins on pupal weight varied among trials. When combined with data from 2014 to 2019 from previous studies, a significant decline in the percent reduction in pupal weight over time was found in both states and hybrid families. This study demonstrates a continued decline in the sublethal impacts of Bt toxins on corn earworm, emphasizing the importance of insect resistance management practices.

Diverse genetic basis of Vip3Aa resistance in five independent field-derived strains of Helicoverpa zea in the US

BACKGROUND

Practical resistance of Helicoverpa zea to Cry proteins has become widespread in the US, making Vip3Aa the only effective Bacillus thuringiensis (Bt) protein for controlling this pest. Understanding the genetic basis of Vip3Aa resistance in H. zea is essential in sustaining the long-term efficacy of Vip3Aa. The objectives of this study were to characterize the inheritance of Vip3Aa resistance in four distinct field-derived H. zea strains (M1-RR, AC4-RR, R2-RR and R15-RR), and to test for shared genetic basis among these strains and a previously characterized Texas resistant strain (LT#70-RR).

RESULTS

Maternal effects and sex linkage were absent, and the effective dominance level (DML) was 0.0 across Vip3Aa39 concentrations ranging from 1.0 to 31.6 μg cm-2, in all H. zea resistant strains. Mendelian monogenic model tests indicated that Vip3Aa resistance in each of the four strains was controlled by a single gene. However, interstrain complementation tests indicated that three distinct genetic loci are involved in Vip3Aa resistance in the five resistant H. zea strains: one shared by M1-RR and LT#70-RR; another shared by R2-RR and R15-RR; and a distinct one for AC4-RR.

CONCLUSION

Results of this study indicate that Vip3Aa resistance in all H. zea strains was controlled by a single, recessive and autosomal gene. However, there were three distinct genetic loci associated with Vip3Aa resistance in the five resistant H. zea strains. The information generated from this study is valuable for exploring mechanisms of Vip3Aa resistance, monitoring the evolution of Vip3Aa resistance, and devising effective strategies for managing Vip3Aa resistance in H. zea. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Cross-pollination in seed-blended refuge and selection for Vip3A resistance in a lepidopteran pest as detected by genomic monitoring

The evolution of pest resistance to management tools reduces productivity and results in economic losses in agricultural systems. To slow its emergence and spread, monitoring and prevention practices are implemented in resistance management programs. Recent work suggests that genomic approaches can identify signs of emerging resistance to aid in resistance management. Here, we empirically examined the sensitivity of genomic monitoring for resistance management in transgenic Bt crops, a globally important agricultural innovation. Whole genome resequencing of wild North American Helicoverpa zea collected from non-expressing refuge and plants expressing Cry1Ab confirmed that resistance-associated signatures of selection were detectable after a single generation of exposure. Upon demonstrating its sensitivity, we applied genomic monitoring to wild H. zea that survived Vip3A exposure resulting from cross-pollination of refuge plants in seed-blended plots. Refuge seed interplanted with transgenic seed exposed H. zea to sublethal doses of Vip3A protein in corn ears and was associated with allele frequency divergence across the genome. Some of the greatest allele frequency divergence occurred in genomic regions adjacent to a previously described candidate gene for Vip3A resistance. Our work highlights the power of genomic monitoring to sensitively detect heritable changes associated with field exposure to Bt toxins and suggests that seed-blended refuge will likely hasten the evolution of resistance to Vip3A in lepidopteran pests.

The Efficacy of Entomopathogenic Nematodes Plus an Adjuvant against Helicoverpa zea and Chrysodeixis includens in Aboveground Applications

In the southern United States, corn earworm, Helicoverpa zea (Boddie), and soybean looper, Chrysodeixis includens (Walker) are economically important crop pests. Although Bt crops initially provided effective control of target pests such as H. zea, many insect pests have developed resistance to these Bt crops. Alternative approaches are needed, including biological control agents such as entomopathogenic nematodes (EPNs). However, the effectiveness of EPNs for aboveground applications can be limited due to issues such as desiccation and ultraviolet radiation. Effective adjuvants are needed to overcome these problems. Ten strains of EPNs were tested for virulence against eggs, first to fourth instars, fifth instars, and pupae of H. zea and C. includens in the laboratory. These 10 EPN strains were Heterorhabditis bacteriophora (HP88 and VS strains), H. floridensis (K22 strain), Hgkesha (Kesha strain), Steinernema carpocapsae (All and Cxrd strains), S. feltiae (SN strain), S. rarum (17c+e strain), and S. riobrave (355 and 7-12 strains). EPNs could infect eggs of H. zea or C. includens in the laboratory, but the infection was low. The mortality caused by 10 EPN strains in seven days was significantly higher for the first to fourth instars of H. zea compared to the control, as was the fifth instars of H. zea. Similarly, for the first to fourth and fifth instars of C. includens, the mortality was significantly higher compared to the controls, respectively. However, only S. riobrave (355) had significantly higher mortality than the control for the pupae of H. zea. For the pupae of C. includens, except for H. bacteriophora (HP88), S. rarum (17c+e), and H. floridensis (K22), the mortality of the other seven strains was significantly higher than the control. Subsequently, S. carpocapsae (All) and S. riobrave (7-12) were chosen for efficacy testing in the field with an adjuvant 0.066% Southern Ag Surfactant (SAg Surfactant). In field experiments, the SAg Surfactant treatment significantly increased the mortality and EPN infection for S. carpocapsae (All) on first instars of H. zea in corn plant whorls. On soybean plants, with the SAg Surfactant, S. carpocapsae (All) was more effective than S. riobrave (7-12) on fifth instars of C. includens. This study indicates that EPNs can control H. zea and C. includens, and SAg Surfactant can enhance EPN efficacy.

N-Terminal α-Helices in Domain I of Bacillus thuringiensis Vip3Aa Play Crucial Roles in Disruption of Liposomal Membrane

Bacillus thuringiensis Vip3 toxins form a tetrameric structure crucial for their insecticidal activity. Each Vip3Aa monomer comprises five domains. Interaction of the first four α-helices in domain I with the target cellular membrane was proposed to be a key step before pore formation. In this study, four N-terminal α-helix-deleted truncations of Vip3Aa were produced and, it was found that they lost both liposome permeability and insecticidal activity against Spodoptera litura. To further probe the role of domain I in membrane permeation, the full-length domain I and the fragments of N-terminal α-helix-truncated domain I were fused to green fluorescent protein (GFP), respectively. Only the fusion carrying the full-length domain I exhibited permeability against artificial liposomes. In addition, seven Vip3Aa-Cry1Ac fusions were also constructed by combination of α-helices from Vip3Aa domains I and II with the domains II and III of Cry1Ac. Five of the seven combinations were determined to show membrane permeability in artificial liposomes. However, none of the Vip3Aa-Cry1Ac combinations exhibited insecticidal activity due to the significant reduction in proteolytic stability. These results indicated that the N-terminal helix α1 in the Vip3Aa domain I is essential for both insecticidal activity and liposome permeability and that domain I of Vip3Aa preserved a high liposome permeability independently from domains II-V.

Reduced toxin binding associated with resistance to Vip3Aa in the corn earworm (Helicoverpa zea)

IMPORTANCE

Helicoverpa zea is a major crop pest in the United States that is managed with transgenic corn and cotton that produce insecticidal proteins from the bacterium, Bacillus thuringiensis (Bt). However, H. zea has evolved widespread resistance to the Cry proteins produced in Bt corn and cotton, leaving Vip3Aa as the only plant-incorporated protectant in Bt crops that consistently provides excellent control of H. zea. The benefits provided by Bt crops will be substantially reduced if widespread Vip3Aa resistance develops in H. zea field populations. Therefore, it is important to identify resistance alleles and mechanisms that contribute to Vip3Aa resistance to ensure that informed resistance management strategies are implemented. This study is the first report of reduced binding of Vip3Aa to midgut receptors associated with resistance.

Cross-resistance and redundant killing of Vip3Aa resistant populations of Helicoverpa zea on purified Bt proteins and pyramided Bt crops

BACKGROUND

Pyramiding Bt proteins is a key strategy to delay insect resistance development. However, the durability of pyramided Bt crops for controlling insect pests is threatened by cross-resistance among Bt proteins, which can ultimately contribute to resistance development. The corn earworm, Helicoverpa zea, is a major agricultural pest of pyramided Bt crops. Previous studies have examined cross-resistance and redundant killing of Cry resistance in H. zea, but such information is lacking for Vip3Aa resistance in this pest. Here, we evaluated cross-resistance and redundant killing of Vip3Aa-resistant H. zea to purified Bt proteins, as well as Bt corn and Bt cotton.

RESULTS

Diet bioassays demonstrated high susceptibility of Vip3Aa-resistant H. zea to Cry1Ac, Cry1A.105, and Cry2Ab2 purified proteins. No Vip3Aa-susceptible, -heterozygous, or -resistant H. zea could survive on pyramided Bt corn containing Cry1 and/or Cry2 proteins. Complete redundant killing was observed in pyramided Bt corn containing Cry1 and/or Cry2 proteins against Vip3Aa resistance in H. zea. Vip3Aa-susceptible, -heterozygous, and -resistant H. zea exhibited survival rates ranging from 0.0% to 22.5% on pyramided Bt cotton with Cry1 and/or Cry2 proteins. Incomplete to complete redundant killing was observed for Vip3Aa-resistant H. zea on pyramided Bt cotton containing Cry1 and/or Cry2 proteins.

CONCLUSION

Our findings indicate that Vip3Aa-resistant H. zea does not exhibit positive cross-resistance to Cry1 or Cry2 proteins. In addition, most pyramided Bt crops showed complete or nearly complete redundant killing of Vip3Aa-resistant H. zea. These results indicate that a pyramiding strategy would often be effective for managing Vip3Aa resistance in regions of the United States where H. zea has not evolved resistance to Cry1 and Cry2 toxins. © 2023 Society of Chemical Industry.

Functional analysis of transgenic cry1Ah-1 maize

Maize is an important food crop in the world, but the yield and quality of maize have been significantly reduced due to the impact of insect pests. In order to address this issue, the cry1Ah gene was subjected to error-prone PCR for mutagenesis, and subsequently, the mutant cry1Ah-1 gene was introduced into maize inbred line GSH9901 callus using the Agrobacterium-mediated method. The T2 generation transformed plants were obtained by subculture, and 9 transgenic positive plants were obtained by molecular detection which was carried out by PCR, qRT-PCR, Bt gold-labeled immunoassay test strips, Western blot and ELISA. It was found that the Cry1Ah-1 gene could be transcribed normally in maize leaves, of which OE1 and OE3 had higher relative expression levels and could successfully express proteins of 71.94 KD size. They were expressed in different tissues at the 6-leaf stage, heading stage and grain-filling stage, and could ensure the protection of maize from corn borer throughout the growth period. The biological activities of OE1 and OE3 were tested indoors and in the field, and the results showed that in indoors, the corn borer that fed on OE1 and OE3 corn leaves had a mortality rate of 100 % after 3 days; in the field, OE1 and OE3 had strong insecticidal activity against corn borer, reaching a high resistance level. In conclusion, the transgenic cry1Ah-1 maize has a strong insecticidal effect on corn borer, and has a good prospect of commercialization.

Engineered Expression of Vip3A in Green Tissues as a Feasible Approach for the Control of Insect Pests in Maize

Genetic engineering technology offers opportunities to improve many important agronomic traits in crops, including insect-resistance. However, genetically modified (GM) exogenous proteins in edible tissues of transgenic crops has become an issue of intense public concern. To advance the application of GM techniques in maize, a Cre/loxP-based strategy was developed for manipulating the transgenes in green tissues while locking them in non-green tissues. In the strategy, the site-specific excision can be used to switch on or off the expression of transgenes at specific tissues. In this work, two basic transgenic maize, named KEY, carrying the Cre gene, and LOCK, containing the Vip3A gene with a blocked element, were obtained based on their separate fusion gene cassettes. The expression level and concentration of Vip3A were observed with a high specific accumulation in the green tissues (leaf and stem), and only a small amount was observed in the root and kernel tissues in the KEY × LOCK hybrids. The insect resistance of transgenic maize against two common lepidopteran pests, Ostrinia furnacalis and Spodoptera frugiperda, was assessed in the laboratory and field. The results indicate that the hybrids possessed high resistance levels against the two pests, with mortality rates above 73.6% and damage scales below 2.4 compared with the control group. Our results suggest that the Cre/loxP-mediated genetic engineering approach has a competitive advantage in GM maize. Overall, the findings from this study are significant for providing a feasible strategy for transgenes avoiding expression in edible parts and exploring novel techniques toward the biosafety of GM plants.

An Extended Investigation of Unexpected Helicoverpa zea (Boddie) Survival and Ear Injury on a Transgenic Maize Hybrid Expressing Cry1A/Cry2A/Vip3A Toxins

The wide occurrence of resistance to Cry1A and Cry2A insecticidal toxins from Bacillus thuringiensis (Bt) in the corn earworm/bollworm Helicoverpa zea (Boddie) leaves the Vip3A toxin produced during the vegetative stage of Bt as the only fully active toxin expressed in transgenic crops to control H. zea in the U.S.A. During 2021, the first unexpected survival of H. zea and injury (UXI) on a maize hybrid expressing Cry1A.105, Cry2Ab2, and Vip3Aa in Louisiana, U.S.A. were observed in two sentinel plots used for resistance monitoring. A follow-up intensive investigation was conducted with two H. zea populations established from larvae collected from the two UXI plots. The main goal of this study was to reveal if the unexpected damage was due to resistance development in the insect to the Bt toxins expressed in the maize hybrid. Diet-overlay bioassays showed that the two populations were highly resistant to Cry1A.105, moderately resistant to Cry2Ab2, but still highly susceptible to Vip3Aa when compared to a reference susceptible strain. In 10 d assays with detached ears, the larvae of the two UXI populations exhibited survival on ears expressing only Cry toxins but presented near 100% mortality on maize hybrids containing both cry and vip3A transgenes. Multiple field trials over three years demonstrated that natural H. zea populations in Louisiana were highly resistant to maize expressing only Cry toxins but remained susceptible to all tested hybrids containing cry and vip3A genes. Altogether, the results of this study suggest that the observed UXIs in Louisiana were associated with a resistance to Cry toxins but were not due to a resistance to Vip3A. The possible causes of the UXIs are discussed. The results generated and procedures adopted in this study help in determining thresholds for defining UXIs, assessing resistance risks, and documenting field resistance.

Extended Sentinel Monitoring of Helicoverpa zea Resistance to Cry and Vip3Aa Toxins in Bt Sweet Corn: Assessing Changes in Phenotypic and Allele Frequencies of Resistance

Transgenic corn and cotton that produce Cry and Vip3Aa toxins derived from Bacillus thuringiensis (Bt) are widely planted in the United States to control lepidopteran pests. The sustainability of these Bt crops is threatened because the corn earworm/bollworm, Helicoverpa zea (Boddie), is evolving a resistance to these toxins. Using Bt sweet corn as a sentinel plant to monitor the evolution of resistance, collaborators established 146 trials in twenty-five states and five Canadian provinces during 2020-2022. The study evaluated overall changes in the phenotypic frequency of resistance (the ratio of larval densities in Bt ears relative to densities in non-Bt ears) in H. zea populations and the range of resistance allele frequencies for Cry1Ab and Vip3Aa. The results revealed a widespread resistance to Cry1Ab, Cry2Ab2, and Cry1A.105 Cry toxins, with higher numbers of larvae surviving in Bt ears than in non-Bt ears at many trial locations. Depending on assumptions about the inheritance of resistance, allele frequencies for Cry1Ab ranged from 0.465 (dominant resistance) to 0.995 (recessive resistance). Although Vip3Aa provided high control efficacy against H. zea, the results show a notable increase in ear damage and a number of surviving older larvae, particularly at southern locations. Assuming recessive resistance, the estimated resistance allele frequencies for Vip3Aa ranged from 0.115 in the Gulf states to 0.032 at more northern locations. These findings indicate that better resistance management practices are urgently needed to sustain efficacy the of corn and cotton that produce Vip3Aa.

A Modified F2 Screen for Estimating Cry1Ac and Cry2Ab Resistance Allele Frequencies in Helicoverpa zea (Lepidoptera: Noctuidae)

Evaluating the frequency of resistance alleles is important for resistance management and sustainable use of transgenic crops that produce insecticidal proteins from Bacillus thuringiensis. Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) is a major crop pest in the United States that has evolved practical resistance to the crystalline (Cry) proteins in Bt corn and cotton. The standard F2 screen for estimating resistance allele frequency does not work well for H. zea because successful single-pair matings are rare. In this study, we developed and implemented a modified F2 screen for H. zea that generates F1 progeny by crossing three laboratory susceptible female moths with one feral male moth instead of single-pair crosses. During 2019-2020, we used this modified method to establish 192 F2 families from 623 matings between susceptible females and feral males from Arkansas, Louisiana, Mississippi, and Tennessee. From each F2 family, we screened 128 neonates against discriminating concentrations of Cry1Ac and Cry2Ab in diet overlay bioassays. Based on these discriminating concentration bioassays, families were considered positive for resistance if at least five larvae survived to second instar, including at least one to third instar. The percentage of positive families was 92.7% for Cry1Ac and 38.5% for Cry2Ab, which yields an estimated resistance allele frequency (with 95% confidence interval) of 0.722 (0.688-0.764) for Cry1Ac and 0.217 (0.179-0.261) for Cry2Ab. The modified F2 screen developed and implemented here may be useful for future resistance monitoring studies of H. zea and other pests.

Relative fitness of susceptible and Cry1A.105/Cry2Ab2-single-/dual-protein-resistant Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) on non-Bt diet and a diet containing a low concentration of two proteins

Helicoverpa zea (Boddie) is a destructive agricultural pest species that is targeted by both Bacillus thuringiensis (Bt) maize and cotton in the United States. Cry1A.105 and Cry2Ab2 are two Bt proteins expressed in a widely planted maize event MON 89034. In this study, two tests (Test-I and Test-II) were conducted to evaluate the relative fitness of Bt-susceptible and -resistant H. zea on non-Bt diet (Test-I and Test-II) and a diet containing a mix of Cry1A.105 and Cry2Ab2 at a low concentration (Test-II only). Insect populations evaluated in Test-I were two Bt-susceptible strains and three Bt-resistant strains (a single-protein Cry1A.105-, a single-protein Cry2Ab2-, and a dual-protein Cry1A.105/Cry2Ab2-resistant strains). Test-II analyzed the same two susceptible strains, three backcrossed-and-reselected Cry1A.105/Cry2Ab2-single-/dual-protein-resistant strains, and three F₁ heterozygous strains. Measurements of life table parameters showed that neither the single- nor dual-protein Cry1A.105/Cry2Ab2 resistance in H. zea was associated with fitness costs under the test conditions. The single Cry protein resistances at a concentration of a mix of Cry1A.105 and Cry2Ab2 that resulted in a zero net reproductive rate for the two susceptible strains were functionally incomplete recessive or codominant, and the dual-protein resistance was completely dominant. The lack of fitness costs could be a factor contributing to the rapid revolution of resistance to the Cry proteins in this species. Data generated from this study should aid our understanding of Cry protein resistance evolution and help in refining IRM programs for H. zea.

A Chromosome-Scale Genome Assembly of a Helicoverpa zea Strain Resistant to Bacillus thuringiensis Cry1Ac Insecticidal Protein

Helicoverpa zea (Lepidoptera: Noctuidae) is an insect pest of major cultivated crops in North and South America. The species has adapted to different host plants and developed resistance to several insecticidal agents, including Bacillus thuringiensis (Bt) insecticidal proteins in transgenic cotton and maize. Helicoverpa zea populations persist year-round in tropical and subtropical regions, but seasonal migrations into temperate zones increase the geographic range of associated crop damage. To better understand the genetic basis of these physiological and ecological characteristics, we generated a high-quality chromosome-level assembly for a single H. zea male from Bt-resistant strain, HzStark_Cry1AcR. Hi-C data were used to scaffold an initial 375.2 Mb contig assembly into 30 autosomes and the Z sex chromosome (scaffold N50 = 12.8 Mb and L50 = 14). The scaffolded assembly was error-corrected with a novel pipeline, polishCLR. The mitochondrial genome was assembled through an improved pipeline and annotated. Assessment of this genome assembly indicated 98.8% of the Lepidopteran Benchmark Universal Single-Copy Ortholog set were complete (98.5% as complete single copy). Repetitive elements comprised approximately 29.5% of the assembly with the plurality (11.2%) classified as retroelements. This chromosome-scale reference assembly for H. zea, ilHelZeax1.1, will facilitate future research to evaluate and enhance sustainable crop production practices.

Resistance Allele Frequency of Helicoverpa zea to Vip3Aa Bacillus thuringiensis Protein in the Southeastern U.S

Helicoverpa zea is a major target pest of Bt crops expressing Cry and/or Vip3Aa proteins in the U.S.A. Widespread practical resistance of H. zea to the Cry1 and Cry2 proteins makes Vip3Aa the only effective Bt protein against this pest. Understanding the frequency of resistance alleles against Vip3Aa in field populations of H. zea is crucial for resistance management and the sustainability of Vip3Aa technology. Using a modified F₂ screen method by crossing susceptible laboratory female moth with feral male moth of H. zea, we successfully screened a total of 24,576 neonates from 192 F₂ families of H. zea collected from Arkansas, Louisiana, Mississippi, and Tennessee during 2019-2020. We found five F₂ families containing ≥3rd instar survivors on the diagnostic concentration of 3.0 µg/cm² Vip3Aa39. Dose-response bioassays confirmed the high levels of Vip3Aa resistance in these F₂ families, with an estimated resistance ratio of >909.1-fold relative to the susceptible strain. The estimated resistance allele frequency against Vip3Aa in H. zea for these four southern states is 0.0155 with a 95% CI of 0.0057-0.0297. These data should provide critical information for understanding the risks of Vip3Aa resistance in H. zea and help design appropriate resistance management strategies for the sustainability of the Vip3Aa technology.

Practical resistance to Cry toxins and efficacy of Vip3Aa in Bt cotton against Helicoverpa zea

BACKGROUND

Crops genetically engineered to make insect-killing proteins from Bacillus thuringiensis (Bt) have revolutionized management of some pests. However, the benefits of such transgenic crops are reduced when pests evolve resistance to Bt toxins. We evaluated resistance to Bt toxins and Bt cotton plants using laboratory bioassays and complementary field trials focusing on Helicoverpa zea, one of the most economically important pests of cotton and other crops in the United States.

RESULTS

The data from 235 laboratory bioassays demonstrate resistance to Cry1Ac, Cry1Fa, and Cry2Ab occurred in most of the 95 strains of H. zea derived from Arkansas, Louisiana, Mississippi, Tennessee, and Texas during 2016 to 2021. Complementary field data show efficacy decreased for Bt cotton producing Cry1Ac + Cry1Fa or Cry1Ac + Cry2Ab, but not Cry1Ac + Cry1Fa + Vip3Aa. Moreover, analysis of data paired by field site and year shows higher survival in bioassays was generally associated with lower efficacy of Bt cotton.

CONCLUSIONS

The results confirm and extend previous evidence showing widespread practical resistance of H. zea in the United States to the Cry toxins produced by Bt cotton and corn, but not to Vip3Aa. Despite deployment in combination with Cry toxins in Bt crops, Vip3Aa effectively acts as a single toxin against H. zea larvae that are highly resistant to Cry toxins. Furthermore, Vip3Aa adoption is increasing and previous work provided an early warning of field-evolved resistance. Thus, rigorous resistance management measures are needed to preserve the efficacy of Vip3Aa against this highly adaptable pest. © 2022 Society of Chemical Industry.

Utility and challenges of using whole-genome resequencing to detect emerging insect and mite resistance in agroecosystems

Arthropods that invade agricultural ecosystems systematically evolve resistance to the control measures used against them, and this remains a significant and ongoing challenge for sustainable food production systems. Early detection of resistance evolution could prompt remedial action to slow the spread of resistance alleles in the landscape. Historical approaches used to detect emerging resistance included phenotypic monitoring of agricultural pest populations, as well as monitoring of allele frequency changes at one or a few candidate pesticide resistance genes. In this article, I discuss the successes and limitations of these traditional monitoring approaches and then consider whether whole-genome scanning could be applied to samples collected from agroecosystems over time for resistance monitoring. I examine the qualities of agroecosystems that could impact application of this approach to pesticide resistance monitoring and describe a recent retrospective analysis where genome scanning successfully detected an oligogenic response to selection by pesticides years prior to pest management failure. I conclude by considering areas of further study that will shed light on the feasibility of applying whole-genome scanning for resistance risk monitoring in agricultural pest species.

Effective dominance and redundant killing of single- and dual-gene resistant populations of Helicoverpa zea on pyramided Bt corn and cotton

BACKGROUND

Pyramided Bacillus thuringiensis (Bt) crops producing multiple Bt proteins with different modes of action are widely planted in the United States. Helicoverpa zea is a major target pest of pyramided Bt crops and has evolved practical resistance to both Cry1 and Cry2 proteins in some regions of U.S. However, little information is available regarding redundant killing and the dominance of resistance for insects possessing multiple resistance on pyramided Bt crops. In this study, we evaluated redundant killing and the dominance of resistance for H. zea strains resistant to Cry1 or Cry1 + Cry2 on pyramided Bt corn and cotton.

RESULTS

We found that the Cry1-resistant H. zea was incompletely dominant on Cry1Ac + Cry1F cotton. Pyramided crops producing Cry2 and/or Vip3Aa proteins showed a complete redundant killing against the Cry1-resistant H. zea. The Cry1 + Cry2-resistant H. zea displayed incompletely recessive to completely dominant resistance on pyramided Bt crops containing Cry1 and/or Cry2 proteins. The redundant killing was complete for the Cry1 + Cry2-resistant H. zea on pyramided Bt crops producing Vip3Aa protein.

CONCLUSION

The dominant resistance of Cry1 and Cry2 in H. zea on pyramided Bt crops deviates from the assumption of functionally recessive resistance underlying the high-dose refuge strategy. However, the assumptions of complete redundant killing are achieved for both Cry1- and Cry1 + Cry2-resistant H. zea on pyramided Bt crops. These results suggest that the pyramided strategy could be valuable for increasing the durability of Bt technology for managing H. zea, a pest with inherently low susceptibility against Cry proteins. © 2022 Society of Chemical Industry.

Responses to Bt toxin Vip3Aa by pink bollworm larvae resistant or susceptible to Cry toxins

BACKGROUND

Transgenic crops that make insecticidal proteins from Bacillus thuringiensis (Bt) have revolutionized management of some pests. However, evolution of resistance to Bt toxins by pests diminishes the efficacy of Bt crops. Resistance to crystalline (Cry) Bt toxins has spurred adoption of crops genetically engineered to produce the Bt vegetative insecticidal protein Vip3Aa. Here we used laboratory diet bioassays to evaluate responses to Vip3Aa by pink bollworm (Pectinophora gossypiella), one of the world's most damaging pests of cotton.

RESULTS

Against pink bollworm larvae susceptible to Cry toxins, Vip3Aa was less potent than Cry1Ac or Cry2Ab. Conversely, Vip3Aa was more potent than Cry1Ac or Cry2Ab against laboratory strains highly resistant to those Cry toxins. Five Cry-susceptible field populations were less susceptible to Vip3Aa than a Cry-susceptible laboratory strain (APHIS-S). Relative to APHIS-S, significant resistance to Vip3Aa did not occur in strains selected in the laboratory for > 700-fold resistance to Cry1Ac or both Cry1Ac and Cry2Ab.

CONCLUSIONS

Resistance to Cry1Ac and Cry2Ab did not cause strong cross-resistance to Vip3Aa in pink bollworm, which is consistent with predictions based on the lack of shared midgut receptors between these toxins and previous results from other lepidopterans. Comparison of the Bt toxin concentration in plants relative to the median lethal concentration (LC₅₀ ) from bioassays may be useful for estimating efficacy. The moderate potency of Vip3Aa against Cry1Ac- and Cry2Ab-resistant and susceptible pink bollworm larvae suggests that Bt cotton producing this toxin together with novel Cry toxins might be useful as one component of integrated pest management. © 2022 Society of Chemical Industry.

Inheritance of Resistance to Cry1A.105 in Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae)

Cry1A.105 is a bioengineered Bacillus thuringiensis (Bt) insecticidal protein consisting of three domains derived from Cry1Ac, Cry1Ab, and Cry1F. It is one of the two pyramided Bt toxins expressed in the MON 89034 event, a commonly planted Bt maize trait in the Americas. Recent studies have documented that field resistance of the corn earworm, Helicoverpa zea (Boddie), to the Cry1A.105 toxin in maize plants has become widespread in the United States. To investigate the inheritance of resistance to Cry1A.105 in H. zea, two independent tests, each with various genetic crosses among susceptible and Cry1A.105-resistant populations, were performed. The responses of these susceptible, resistant, F₁, F₂, and backcrossed insect populations to Cry1A.105 were assayed using a diet overlay method. The bioassays showed that the resistance to Cry1A.105 in H. zea was inherited as a single, autosomal, nonrecessive gene. The nonrecessive nature of the resistance could be an important factor contributing to the widespread resistance of maize hybrids containing Cry1A.105 in the United States. The results indicate that resistance management strategies for Bt crops need to be refined to ensure that they are effective in delaying resistance evolution for nonrecessive resistance (nonhigh dose).

Effects of cross-pollination among non-Bt and pyramided Bt corn expressing cry proteins in seed mixtures on resistance development of dual-gene resistant Helicoverpa zea

BACKGROUND

Seed mixture strategy can guarantee the compliance of planting non-Bt crops to host the susceptible insects for resistance management. However, pollen movement between Bt and non-Bt corn in the mixed plantings could reduce the efficacy of this strategy for ear-feeding insects. Few studies have evaluated the effects of cross-pollination among non-Bt and pyramided Bt corn in seed mixtures on the resistance development of insects possessing multiple resistances. Here, we provided the first study to investigate whether cross-pollination in mixed plantings of pyramided Bt corn producing Cry1A.105 and Cry2Ab2 would increase the dominance of resistance of dual-gene resistant populations of Helicoverpa zea, a target of pyramided Bt corn and cotton in the USA.

RESULTS

We compared the survival and development of susceptible, dual-gene resistant (resistance to both Cry1 and Cry2 proteins) and heterozygous genotypes of H. zea in the laboratory on non-Bt and pyramided Bt corn ears collected from mixed plantings and structured plantings in the field. We found higher fitness for F₁ heterozygous insects than for the susceptible insects of H. zea on both pyramided Bt corn and non-Bt corn in the mixed plantings.

CONCLUSION

These results suggest that cross-pollination in mixed plantings will significantly increase the dominance of resistance by supporting survival of heterozygous insects for dual-gene resistant populations of H. zea, and therefore accelerate evolution of resistance to pyramided Bt crops. © 2022 Society of Chemical Industry.

Resistance Allele Frequency to Cry1Ab and Vip3Aa20 in Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) in Louisiana and Three Other Southeastern U.S. States

The corn earworm/bollworm, Helicoverpa zea (Boddie), is a pest species that is targeted by both Bacillus thuringiensis (Bt) maize and cotton in the United States. Cry1Ab and Vip3Aa20 are two common Bt toxins that are expressed in transgenic maize. The objective of this study was to determine the resistance allele frequency (RAF) to Cry1Ab and Vip3Aa20 in H. zea populations that were collected during 2018 and 2019 from four southeastern U.S. states: Louisiana, Mississippi, Georgia, and South Carolina. By using a group-mating approach, 104 F₂ iso-lines of H. zea were established from field collections with most iso-lines (85) from Louisiana. These F₂ iso-lines were screened for resistance alleles to Cry1Ab and Vip3Aa20, respectively. There was no correlation in larval survivorship between Cry1Ab and Vip3Aa20 when the iso-lines were exposed to these two toxins. RAF to Cry1Ab maize was high (0.256) and the RAFs were similar between Louisiana and the other three states and between the two sampling years. In contrast, no functional major resistance allele (RA) that allowed resistant insects to survive on Vip3Aa20 maize was detected and the expected RAF of major RAs with 95% probability was estimated to 0 to 0.0073. However, functional minor RAs to Vip3Aa20 maize were not uncommon; the estimated RAF for minor alleles was 0.028. The results provide further evidence that field resistance to Cry1Ab maize in H. zea has widely occurred, while major RAs to Vip3Aa20 maize are uncommon in the southeastern U.S. region. Information that was generated from this study should be useful in resistance monitoring and refinement of resistance management strategies to preserve Vip3A susceptibility in H. zea.

Status of Cry1Ac and Cry2Ab2 resistance in field populations of Helicoverpa zea in Texas, USA

Helicoverpa zea is a major target pest of Bt corn and Bt cotton. Field-evolved resistance of H. zea to Cry1 and Cry2 proteins has been widely reported in the United States. Understanding the frequency of resistance alleles in a target insect is critical for Bt resistance management. Despite multiple cases of practical resistance to Cry proteins having been documented in H. zea, there are no data on the current status of alleles conferring resistance to Cry1Ac and Cry2Ab2 in field populations of this pest. During 2018-2019, a total of 106 F₂ families for Cry1Ac and 120 F₂ families for Cry2Ab2 were established using mass mating and light trap strategy. We screened 13,568 and 15,360 neonates using a discriminatory dose of Cry1Ac and Cry2Ab2, respectively. The results showed that 93.4% and 35.0% of the F₂ families could survive on the discriminatory dose of Cry1Ac and Cry2Ab2, respectively. The estimated resistance allele frequency for Cry1Ac in H. zea ranged from 0.4150 to 0.4975 and for Cry2Ab2 ranged from 0.1097 and 0.1228. These data indicate that the frequency of alleles conferring resistance to Cry1 and Cry2 proteins in H. zea in Texas are high. In addition, our data suggest the resistance to Cry1Ac and Cry2Ab2 in the screened families of H. zea varies from recessive to dominant. The information in this study provides precise estimates of Cry resistance allele frequencies in H. zea and increases our understanding of the risks to the sustainability of Bt crops.

Genome evolution in an agricultural pest following adoption of transgenic crops

Evolution of resistance to management approaches in agricultural landscapes is common and results in economic losses. Early detection of pest resistance prior to significant crop damage would benefit the agricultural community. It has been hypothesized that new genomic approaches could track molecular signals of emerging resistance and trigger efforts to preempt widespread damage. We tested this hypothesis by quantifying genomic changes in the pest Helicoverpa zea over a 15-y period concurrent with commercialization of transgenic Bacillus thuringiensis–expressing crops and their subsequent loss of efficacy. Our results demonstrate the complex nature of evolution in agricultural ecosystems and provide insight into the potential and pitfalls of using genomic approaches for resistance monitoring.

Replacing synthetic insecticides with transgenic crops for pest management has been economically and environmentally beneficial, but these benefits erode as pests evolve resistance. It has been proposed that novel genomic approaches could track molecular signals of emerging resistance to aid in resistance management. To test this, we quantified patterns of genomic change in Helicoverpa zea, a major lepidopteran pest and target of transgenic Bacillus thuringiensis (Bt) crops, between 2002 and 2017 as both Bt crop adoption and resistance increased in North America. Genomic scans of wild H. zea were paired with quantitative trait locus (QTL) analyses and showed the genomic architecture of field-evolved Cry1Ab resistance was polygenic, likely arising from standing genetic variation. Resistance to pyramided Cry1A.105 and Cry2Ab2 toxins was controlled by fewer loci. Of the 11 previously described Bt resistance genes, 9 showed no significant change over time or major effects on resistance. We were unable to rule out a contribution of aminopeptidases (apns), as a cluster of apn genes were found within a Cry-associated QTL. Molecular signals of emerging Bt resistance were detectable as early as 2012 in our samples, and we discuss the potential and pitfalls of whole-genome analysis for resistance monitoring based on our findings. This first study of Bt resistance evolution using whole-genome analysis of field-collected specimens demonstrates the need for a more holistic approach to examining rapid adaptation to novel selection pressures in agricultural ecosystems.

Development of Economic Thresholds Toward Bollworm (Lepidoptera: Noctuidae), Management in Bt Cotton, and Assessment of the Benefits From Treating Bt Cotton With Insecticide

Widespread field-evolved resistance of bollworm [Helicoverpa zea (Boddie)] to Cry1 and Cry2 Bt proteins has threatened the utility of Bt cotton for managing bollworm. Consequently, foliar insecticide applications have been widely adopted to provide necessary additional control. Field experiments were conducted across the Mid-South and in Texas to devise economic thresholds for foliar insecticide applications targeting bollworm in cotton. Bt cotton technologies including TwinLink (TL; Cry1Ab+Cry2Ae), TwinLink Plus (TLP; Cry1Ab+Cry2Ae+Vip3Aa), Bollgard II (BG2; Cry1Ac+Cry2Ab), Bollgard 3 (BG3; Cry1Ac+Cry2Ab+Vip3Aa), WideStrike (WS; Cry1Ac+Cry1F), WideStrike 3 (WS3; Cry1Ac+Cry1F+Vip3Aa), and a non-Bt (NBT) variety were evaluated. Gain threshold, economic injury level, and economic thresholds were determined. A 6% fruiting form injury threshold was selected and compared with preventive treatments utilizing chlorantraniliprole. Additionally, the differences in yield from spraying bollworms was compared among Bt cotton technologies. The 6% fruiting form injury threshold resulted in a 25 and 75% reduction in insecticide applications relative to preventive sprays for WS and BG2, respectively. All Bt technologies tested in the current study exhibited a positive increase in yield from insecticide application. The frequency of yield increase from spraying WS was comparable to that of NBT. Significant yield increases due to insecticide application occurred less frequently in triple-gene Bt cotton. However, their frequencies were close to the dual-gene Bt cotton, except for WS. The results of our study suggest that 6% fruiting form injury is a viable threshold, and incorporating a vetted economic threshold into an Integrated Pest Management program targeting bollworm should improve the sustainability of cotton production.

Landscape-level variation in Bt crops predict Helicoverpa zea (Lepidoptera: Noctuidae) resistance in cotton agroecosystems

BACKGROUND

Helicoverpa zea (Boddie) damage to Bt cotton and maize has increased as a result of widespread Bt resistance across the USA Cotton Belt. Our objective was to link Bt crop production patterns to cotton damage through a series of spatial and temporal surveys of commercial fields to understand how Bt crop production relates to greater than expected H. zea damage to Bt cotton. To do this, we assembled longitudinal cotton damage data that spanned the Bt adoption period, collected cotton damage data since Bt resistance has been detected, and estimated local population susceptibility using replicated on-farm studies that included all Bt pyramids marketed in cotton.

RESULTS

Significant year effects of H. zea damage frequency in commercial cotton were observed throughout the Bt adoption period, with a recent damage increase after 2012. Landscape-level Bt crop production intensity over time was positively associated with the risk of H. zea damage in two- and three-toxin pyramided Bt cotton. Helicoverpa zea damage also varied across Bt toxin types in spatially replicated on-farm studies.

CONCLUSIONS

Landscape-level predictors of H. zea damage in Bt cotton can be used to identify heightened Bt resistance risk areas and serves as a model to understand factors that drive pest resistance evolution to Bt toxins in the southeastern United States. These results provide a framework for more effective insect resistance management strategies to be used in combination with conventional pest management practices that improve Bt trait durability while minimizing the environmental footprint of row crop agriculture. © 2021 Society of Chemical Industry. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Computer vision for detecting field-evolved lepidopteran resistance to Bt maize

BACKGROUND

Resistance evolution of lepidopteran pests to Bacillus thuringiensis (Bt) toxins produced in maize and cotton is a significant issue worldwide. Effective toxin stewardship requires reliable detection of field-evolved resistance to enable the implementation of mitigation strategies. Currently, visual estimates of maize injury are used to document changing susceptibility. In this study, we evaluated an existing maize injury monitoring protocol used to estimate Bt resistance levels in Helicoverpa zea (Lepidoptera: Noctuidae).

RESULTS

We detected high interobserver variability across multiple injury metrics, suggesting that the precision and accuracy of maize injury detection could be improved. To do this, we developed a computer vision-based algorithm to measure H. zea injury. Algorithm estimates were more accurate and precise than a sample of human observers. Moreover, observer estimates tended to overpredict H. zea injury, which may increase the false-positive rate, leading to prophylactic insecticide application and unnecessary regulatory action.

CONCLUSIONS

Automated detection and tracking of lepidopteran resistance evolution to Bt toxins are critical for genetically engineered crop stewardship to prevent the use of additional insecticides to combat resistant pests. Advantages of this computerized screening are: (i) standardized Bt injury metrics in space and time, (ii) preservation of digital data for cross-referencing when thresholds are reached, and (iii) the ability to increase sample sizes significantly. This technological solution represents a significant step toward improving confidence in resistance monitoring efforts among researchers, regulators and the agricultural biotechnology industry.

Early Warning of Resistance to Bt Toxin Vip3Aa in Helicoverpa zea

Evolution of resistance by pests can reduce the benefits of crops genetically engineered to produce insecticidal proteins from Bacillus thuringiensis (Bt). Because of the widespread resistance of Helicoverpa zea 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 H. zea 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 H. zea that supports calls for urgent action to preserve the efficacy of this toxin.

Vegetative Insecticidal Protein (Vip): A Potential Contender From Bacillus thuringiensis for Efficient Management of Various Detrimental Agricultural Pests

Bacillus thuringiensis (Bt) bacterium is found in various ecological habitats, and has natural entomo-pesticidal properties, due to the production of crystalline and soluble proteins during different growth phases. In addition to Cry and Cyt proteins, this bacterium also produces Vegetative insecticidal protein (Vip) during its vegetative growth phase, which is considered an excellent toxic candidate because of the difference in sequence homology and receptor sites from Cry proteins. Vip proteins are referred as second-generation insecticidal proteins, which can be used either alone or in complementarity with Cry proteins for the management of various detrimental pests. Among these Vip proteins, Vip1 and Vip2 act as binary toxins and have toxicity toward pests belonging to Hemiptera and Coleoptera orders, whereas the most important Vip3 proteins have insecticidal activity against Lepidopteran pests. These Vip3 proteins are similar to Cry proteins in terms of toxicity potential against susceptible insects. They are reported to be toxic toward pests, which can’t be controlled with Cry proteins. The Vip3 proteins have been successfully pyramided along with Cry proteins in transgenic rice, corn, and cotton to combat resistant pest populations. This review provides detailed information about the history and importance of Vip proteins, their types, structure, newly identified specific receptors, and action mechanism of this specific class of proteins. Various studies conducted on Vip proteins all over the world and the current status have been discussed. This review will give insights into the significance of Vip proteins as alternative promising candidate toxic proteins from Bt for the management of pests in most sustainable manner.

Resistance of the fall armyworm, Spodoptera frugiperda, to transgenic Bacillus thuringiensis Cry1F corn in the Americas: lessons and implications for Bt corn IRM in China

The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a major target pest of Bt crops (e.g., corn, cotton, and soybean) in North and South America. This pest has recently invaded Africa and Asia including China and the invasion has placed a great threat to the food security in many countries of these two continents. Due to the extensive use of Bt crops, practical resistance of S. frugiperda to Cry1F corn (TC 1507) with field control problems has widely occurred in Puerto Rico, Brazil, Argentina, and the mainland United States. Analyzing data generated from decade-long studies showed that several factors might have contributed to the wide development of the resistance. These factors include (1) limited modes of action of Bt proteins used in Bt crops; (2) cross-resistance among Cry1 proteins; (3) use of nonhigh dose Bt crop traits; (4) that the resistance is complete on Bt corn plants; (5) abundant in initial Cry1F resistance alleles; and (6) lack of fitness costs/recessive fitness costs of the resistance. The long-term use of Bt crop technology in the Americas suggests that Bt corn can be an effective tool for controlling S. frugiperda in China. IRM programs for Bt corn in China should be as simple as possible to be easily adopted by small-scale growers. The following aspects may be considered in its Bt corn IRM programs: (1) use of only "high dose" traits for both S. frugiperda and stalk borers; (2) developing and implementing a combined resistance monitoring program; (3) use "gene pyramiding" as a primary IRM strategy; and (4) if possible, Bt corn may not be planted in the areas where S. frugiperda overwinters. Lessons and experience gained from the global long-term use of Bt crops should have values in improving IRM programs in the Americas, as well as for a sustainable use of Bt corn technology in China.

No positive cross-resistance to Cry1 and Cry2 proteins favors pyramiding strategy for management of Vip3Aa resistance in Spodoptera frugiperda

BACKGROUND

Pyramided Bacillus thuringiensis (Bt) crops could delay insect resistance development by expressing multiple distinct Bt proteins to manage the same insect pest. The efficacy of pyramiding strategy for resistance management could be jeopardized by positive cross-resistance, which is defined as insects showing resistance to one Bt protein also exhibiting resistance to other Bt proteins. The fall armyworm, Spodoptera frugiperda, is a destructive agricultural pest and target of Vip3Aa. In this study, we evaluate the cross-resistance of Vip3Aa resistance in S. frugiperda to Bt cotton and corn plants, as well as purified Bt proteins.

RESULTS

Diet bioassay showed that Vip3Aa-resistant (RR), -heterozygous (RS), and -susceptible (SS) insects of S. frugiperda performed similarly against Cry2Ab2 purified protein. The data also indicated that genotypes RR and RS were more susceptible to Cry1F and Cry2Ae purified proteins relative to SS. The diet bioassays suggested that resistance to Vip3Aa does not confer any positive cross-resistance to Cry1F, Cry2Ae or Cry2Ab2 in S. frugiperda. The plant bioassay indicated that the S. frugiperda resistance to Vip3Aa conferred no cross-resistance to corn and no cross-crop resistance to cotton plants expressing Cry1 and/or Cry2 proteins.

CONCLUSION

These findings indicate that a lack of positive cross-resistance to Cry1 and Cry2 proteins favors pyramiding strategy for managing S. frugiperda resistance to Vip3Aa protein. © 2020 Society of Chemical Industry.

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

BACKGROUND

Using natural populations of Helicoverpa zea from Arizona, we tested the hypotheses that gene flow between Bacillus thuringiensis (Bt) plants 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.

RESULTS

Gene flow was not detected in blocks of non-Bt or Bt corn but was extensive in seed mixtures. Analyses of larval weight and abundance over a period of 3 to 4 weeks did not indicate consistent effects of gene flow on development rate and survival. However for non-Bt plants, the ear area damaged and percentage of ears with exit holes were significantly lower in the seed mixtures than blocks. By contrast, the percentage of ears with exit holes and ear damage did not differ significantly between the seed mixtures and blocks for Bt plants. Nearly 100% of the ears were damaged and the damaged area was substantial, showing that H. zea is a major ear-feeding pest in Arizona. Relative to non-Bt corn, the pyramided Bt corn did not significantly reduce the percentage of damaged ears and only reduced the ear area damaged by 21 to 39%, indicating that H. zea may have evolved resistance to Cry1A.105, Cry2Ab, or both.

CONCLUSIONS

Our results indicate that gene flow between Bt and non-Bt plants in seed mixtures reduced effective refuge size, and that block refuges may be needed to manage the evolution of H. zea resistance to Bt corn in Arizona. © 2020 Society of Chemical Industry.

Genetic basis of resistance to the Vip3Aa Bt protein in Helicoverpa zea

BACKGROUND

Helicoverpa zea is a destructive pest and target of maize and cotton expressing Cry and Vip3Aa proteins in North America. The efficacy of Cry proteins against H. zea in the USA has been largely compromised by resistance. A rapid shift towards planting Bt cotton and maize producing Vip3Aa will accelerate evolution of resistance to Vip3Aa in H. zea. Research on the genetic basis of Vip3Aa resistance in H. zea is urgently needed, and can provide fundamental information for managing resistance in this pest. Here, we characterize the inheritance of Vip3Aa resistance in H. zea.

RESULTS

Susceptibility of a Vip3Aa-susceptible strain (SS), a resistant strain (RR), and progeny from different crosses against Vip3Aa39 was determined. RR was established from an F₂ screening of a population from Texas sampled in 2019. RR had a resistance ratio of 45194.1-fold against Vip3Aa39 relative to SS. Maternal effects and sex linkage were absent in RR. The dominance D value, calculated based on median lethal concentration (LC₅₀ ) values, was -1.0 and the effective dominance (D_ML ), calculated based on a given Vip3Aa39 concentration, was ≤0.0 at concentrations of 0.1-31.6 μg cm^-2 . The test using a monogenic mode of inheritance showed that resistance to Vip3Aa in H. zea was largely due to a single gene.

CONCLUSION

Results of this study indicate that Vip3Aa resistance in H. zea is monogenic, autosomal, and recessive. This information is valuable for studying the mechanism of Vip3Aa resistance, monitoring of resistance development, and designing appropriate strategies for preventive management of Vip3Aa resistance. © 2020 Society of Chemical Industry.

Extended investigation of field-evolved resistance of the corn earworm Helicoverpa zea(Lepidoptera: Noctuidae) to Bacillus thuringiensis Cry1A.105 and Cry2Ab2 proteins in thesoutheastern United States

Previous studies have reported that the corn earworm/bollworm, Helicoverpa zea (Boddie), has developed field resistance to pyramided Bacillus thuringiensis (Bt) Cry1A/Cry2A maize and cotton in certain areas of the southeastern United States. The objective of the current study was to determine the current status and distribution of the resistance to Cry1A.105 and Cry2Ab2 in H. zea. In the study, 31 H. zea populations were collected from major maize planting areas across seven southeastern states of the United States during 2018 and 2019 and assayed against the two Bt proteins. Diet over-lay bioassays showed that most of the populations collected during the two years were significantly resistant to the Cry1A.105 protein. Most of the populations collected during 2019 were also resistant to Cry2Ab2, while significant variances were observed in the susceptibility of the populations collected during 2018 to Cry2Ab2. The results showed that Cry1A.105 and Cry2Ab2 resistance in H. zea is widely distributed in the regions sampled. The resistance to Cry1A.105 appeared to have plateaued, while selection for Cry2Ab2 resistance is likely still occurring. Thus, effective measures for mitigating the Cry1A/Cry2A resistance need to be developed and implemented to ensure the sustainable use of Bt crop biotechnology.

Response Mechanisms of Invertebrates to Bacillus thuringiensis and Its Pesticidal Proteins

Extensive use of chemical insecticides adversely affects both environment and human health. One of the most popular biological pest control alternatives is bioinsecticides based on Bacillus thuringiensis This entomopathogenic bacterium produces different protein types which are toxic to several insect, mite, and nematode species. Currently, insecticidal proteins belonging to the Cry and Vip3 groups are widely used to control insect pests both in formulated sprays and in transgenic crops. However, the benefits of B. thuringiensis-based products are threatened by insect resistance evolution. Numerous studies have highlighted that mutations in genes coding for surrogate receptors are responsible for conferring resistance to B. thuringiensis Nevertheless, other mechanisms may also contribute to the reduction of the effectiveness of B. thuringiensis-based products for managing insect pests and even to the acquisition of resistance. Here, we review the relevant literature reporting how invertebrates (mainly insects and Caenorhabditis elegans) respond to exposure to B. thuringiensis as either whole bacteria, spores, and/or its pesticidal proteins.

Sweet Corn Sentinel Monitoring for Lepidopteran Field-Evolved Resistance to Bt Toxins

As part of an insect resistance management plan to preserve Bt transgenic technology, annual monitoring of target pests is mandated to detect susceptibility changes to Bt toxins. Currently Helicoverpa zea (Boddie) monitoring involves investigating unexpected injury in Bt crop fields and collecting larvae from non-Bt host plants for laboratory diet bioassays to determine mortality responses to diagnostic concentrations of Bt toxins. To date, this monitoring approach has not detected any significant change from the known range of baseline susceptibility to Bt toxins, yet practical field-evolved resistance in H. zea populations and numerous occurrences of unexpected injury occur in Bt crops. In this study, we implemented a network of 73 sentinel sweet corn trials, spanning 16 U.S. states and 4 Canadian provinces, for monitoring changes in H. zea susceptibility to Cry and Vip3A toxins by measuring differences in ear damage and larval infestations between isogenic pairs of non-Bt and Bt hybrids over three years. This approach can monitor susceptibility changes and regional differences in other ear-feeding lepidopteran pests. Temporal changes in the field efficacy of each toxin were evidenced by comparing our current results with earlier published studies, including baseline data for each Bt trait when first commercialized. Changes in amount of ear damage showed significant increases in H. zea resistance to Cry toxins and possibly lower susceptibility to Vip3a. Our findings demonstrate that the sentinel plot approach as an in-field screen can effectively monitor phenotypic resistance and document field-evolved resistance in target pest populations, improving resistance monitoring for Bt crops.

Populations of Helicoverpa zea (Boddie) in the Southeastern United States are Commonly Resistant to Cry1Ab, but Still Susceptible to Vip3Aa20 Expressed in MIR 162 Corn

The corn earworm, Helicoverpa zea (Boddie), is a major pest targeted by pyramided Bacillus thuringiensis (Bt) corn and cotton in the U.S. Cry1Ab is one of the first insecticidal toxins used in Bt crops, while Vip3A is a relatively new toxin that has recently been incorporated into Cry corn with event MIR 162 and Cry cotton varieties to generate pyramided Bt traits targeting lepidopteran pests including H. zea. The objectives of this study were to determine the current status and distribution of the Cry1Ab resistance, and evaluate the susceptibility to Vip3Aa20 expressed in MIR 162 corn in H. zea in the southeastern U.S. During 2018 and 2019, 32 H. zea populations were collected from non-Bt corn (19 populations), Cry corn (12), and Cry/Vip3A cotton (1) across major corn areas in seven southeastern states of the U.S. Susceptibility of these populations to Cry1Ab and Vip3Aa20 was determined using diet-overlay bioassays. Compared to a known susceptible insect strain, 80% of the field populations were 13- to >150-fold resistant to Cry1Ab, while their response to Vip3Aa20 ranged from >11-fold more susceptible to 9-fold more tolerant. Mean susceptibility to each Bt toxin was not significantly different between the two groups of the populations collected from non-Bt and Bt crops, as well as between the two groups of the populations collected during 2018 and 2019. The results show that resistance to Cry1Ab in H. zea is widely distributed across the region. However, the Cry1Ab-resistant populations are not cross-resistant to Vip3Aa20, and H. zea in the region is still susceptible to the Vip3Aa20 toxin. Vip3Aa20 concentrations between 5 and 10 µg/cm2 may be used as diagnostic concentrations for susceptibility monitoring in future. Additional studies are necessary to elucidate the impact of the selection with Bt corn on resistance evolution in H. zea to Vip3A cotton in the U.S.

Contrasting susceptibility of lepidopteran pests to diamide and pyrethroid insecticides in a region of overwintering and migratory intersection

BACKGROUND

Pesticide resistance is a growing issue worldwide, and susceptibility of pest populations should be monitored in migratory intersection regions for successful resistance management. We determined the susceptibility of eight noctuid species from the Florida Panhandle to bifenthrin (pyrethroid) and chlorantraniliprole (diamide). Larvae from field and laboratory populations were exposed to commercial insecticide formulations using the leaf-dip method in concentration-mortality bioassays.

RESULTS

The field populations of Helicoverpa zea (Boddie), Spodoptera frugiperda (Smith), S. eridania (Stoll), S. exigua (Hubner) and Chloridea virescens (Fabricius) had reduced susceptibility to bifenthrin compared with the laboratory populations. Resistance ratios to bifenthrin were as high as 10 071-fold in S. exigua and 436-fold in S. frugiperda, while there was no reduced susceptibility in Agrotis ipsilon (Hufnagel). The susceptibility to chlorantraniliprole was similar between the field and laboratory populations studied, except for S. exigua that exhibited 630-fold resistance to the diamide. The probit regression equations indicated that the larval mortality of S. exigua and S. frugiperda populations was <80% with bifenthrin at the concentration equivalent to the label rate. Likewise, the estimated mortality of S. exigua larvae with chlorantraniliprole at the label rate concentration was <80%.

CONCLUSIONS

The lepidopteran pest populations tested were variable in susceptibility to bifenthrin by contrast to more consistent susceptibility to chlorantraniliprole. These results help in the choice of effective insecticides for integrated pest management and resistance management in cropping systems colonized by migratory lepidopteran pests from the U.S. Gulf Coast region. © 2020 Society of Chemical Industry.

Inheritance of Bacillus thuringiensis Cry2Ab2 protein resistance in Helicoverpa zea (Lepidoptera: Noctuidae)

BACKGROUND

The corn earworm, Helicoverpa zea (Boddie), is a major target pest of pyramided Bt corn and cotton in the United States. Field-evolved practical resistance to Cry1 and Cry2 proteins in H. zea has been documented in multiple locations in the United States. Understanding the genetic basis of Bt resistance is essential in developing insect resistance management (IRM) strategies for the sustainable use of the Bt crop technology. In this study, we characterized the genetic bases of Cry2Ab2 resistance in H. zea using diet-overlay bioassays with two different forms of Cry2Ab2 protein.

RESULTS

Laboratory bioassays using a Cry2Ab2-resistant (RR) strain, a susceptible (SS) strain, as well as cross and backcross strains, revealed that resistance to Cry2Ab2 was autosomally inherited and controlled by more than one locus. In diet bioassays, the dominance of Cry2Ab2 resistance in H. zea varied from incompletely recessive to incompletely dominant across all tested Cry2Ab2 concentrations of either Bt corn leaf powder or solubilized protein. On leaf tissue of TwinLink cotton (expressing Cry1Ab and Cry2Ae), Cry2Ab2 resistance in H. zea was completely dominant.

CONCLUSION

These results have significant implications for understanding the widespread field-evolved resistance of H. zea against Cry1 and Cry2 proteins in Bt corn and cotton and should be useful in developing effective IRM strategies for H. zea. © 2020 Society of Chemical Industry.

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)

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.

Decline in Sublethal Effects of Bt Corn on Corn Earworm (Lepidoptera: Noctuidae) Linked to Increasing Levels of Resistance

As part of an ongoing longitudinal study in North and South Carolina, this study reports the recovery of Helicoverpa zea (Boddie) pupae in field trials with genetically engineered corn, Zea mays L., hybrids that produce insecticidal toxins from Bacillus thuringiensis (Bt) in 2017-2019. In total, 10,400 corn ears were collected, which led to 3,927 H. zea pupae (2,215 in South Carolina and 1,712 in North Carolina). Late-planted corn led to a 3.39-fold increase in recovery of pupae compared to early-planted corn. Bt corn expressing Cry1F + Cry1Ab and Cry1A.105 + Cry2Ab2 had 1.67-fold and 2.51-fold fewer pupae than non-Bt near-isolines, respectively. Only six pupae were recovered from the hybrid expressing Cry1F + Cry1Ab + Vip3Aa20. Averaged across trials, Bt corn expressing either Cry1A.105 + Cry2Ab2 or Cry1F + Cry1Ab significantly reduced pupal weight compared to non-Bt near-isolines in North and South Carolina. Combining our data with a previous study at the same locations (Bilbo et al. 2018), reduction in pupal weight between Bt and non-Bt near-isolines significantly declined from 2014 to 2019 for Cry1Ab + Cry1F in North and South Carolina. This decline in levels of a sublethal effect of Bt corn expressing Cry1Ab + Cry1F on H. zea at both locations is likely correlated with resistance development.

Geographic Distribution of Bacillus thuringiensis Cry1F Toxin Resistance in Western Bean Cutworm (Lepidoptera: Noctuidae) Populations in the United States

The western bean cutworm (WBC), Striacosta albicosta (Lepidoptera: Noctuidae), can be a severe pest of transgenic corn in the western Plains and Great Lakes regions of North America, including on hybrids expressing the Bacillus thuringiensis (Bt) Cry1F toxin. The level and geographic distribution of Cry1F resistance are not completely known. Neonate S. albicosta from 10 locations between Nebraska and New York state were subjected to dose-response trypsin-activated native Cry1F toxin overlay bioassays. In 2017, the mean estimated lethal concentration causing 50% larval mortality (LC50) ranged from 15.1 to 18.4 µg Cry1F cm-2, and were not significantly different among locations. In 2018, LC50 estimates at Scottsbluff, NE (22.0 µg Cry1F cm-2) and Watertown, NY (21.7 µg Cry1F cm-2) were significantly higher when compared to locations in Michigan (15.8 µg Cry1F cm-2). Significantly lower 14-day larval weight among survivors was correlated with higher Cry1F dose. Results from this study indicate that S. albicosta survivorship on purified Bt Cry1F toxin shows a relatively even distribution across the native and range expansion areas where seasonal field infestations typically occur.

Multiple Known Mechanisms and a Possible Role of an Enhanced Immune System in Bt-Resistance in a Field Population of the Bollworm, Helicoverpa zea: Differences in Gene Expression with RNAseq

Several different agricultural insect pests have developed field resistance to Bt (Bacillus thuringiensis) proteins (ex. Cry1Ac, Cry1F, etc.) expressed in crops, including corn and cotton. In the bollworm, Helicoverpa zea, resistance levels are increasing; recent reports in 2019 show up to 1000-fold levels of resistance to Cry1Ac, a major insecticidal protein in Bt-crops. A common method to analyze global differences in gene expression is RNA-seq. This technique was used to measure differences in global gene expression between a Bt-susceptible and Bt-resistant strain of the bollworm, where the differences in susceptibility to Cry1Ac insecticidal proteins were 100-fold. We found expected gene expression differences based on our current understanding of the Bt mode of action, including increased expression of proteases (trypsins and serine proteases) and reduced expression of Bt-interacting receptors (aminopeptidases and cadherins) in resistant bollworms. We also found additional expression differences for transcripts that were not previously investigated, i.e., transcripts from three immune pathways-Jak/STAT, Toll, and IMD. Immune pathway receptors (ex. PGRPs) and the IMD pathway demonstrated the highest differences in expression. Our analysis suggested that multiple mechanisms are involved in the development of Bt-resistance, including potentially unrecognized pathways.

Mutations in a Novel Cadherin Gene Associated with Bt Resistance in Helicoverpa zea

Transgenic corn and cotton produce crystalline (Cry) proteins derived from the soil bacterium Bacillus thuringiensis (Bt) that are toxic to lepidopteran larvae. Helicoverpa zea, 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 H. zea. We scanned the genomes of Cry1Ac-selected and unselected H. zea lines, and identified twelve genes on five scaffolds that differed between lines, including cadherin-86C (cad-86C), a gene from a family that is involved in Cry1A resistance in other lepidopterans. Although this gene was expressed in the H. zea 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 cad-86C is a likely target of Cry1Ac selection in H. zea. 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 H. zea 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 H. zea larvae that ingest Cry1Ac.

Evaluating Cross-resistance Between Vip and Cry Toxins of Bacillus thuringiensis

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.

First documentation of major Vip3Aa resistance alleles in field populations of Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) in Texas, USA

The corn earworm, Helicoverpa zea, is a major target pest of the insecticidal Vip3Aa protein used in pyramided transgenic Bt corn and cotton with Cry1 and Cry2 proteins in the U.S. The widespread resistance to Cry1 and Cry2 proteins in H. zea will challenge the long-term efficacy of Vip3Aa technology. Determining the frequency of resistant alleles to Vip3Aa in field populations of H. zea is critically important for resistance management. Here, we provided the first F2 screen study to estimate the resistance allele frequency for Vip3Aa in H. zea populations in Texas, U.S. In 2019, 128 H. zea neonates per isofamily for a total of 114 F2 families were screened with a diagnostic concentration of 3.0 μg/cm2 of Vip3Aa39 protein in diet-overlay bioassays. The F2 screen detected two families carrying a major Vip3Aa resistance allele. The estimated frequency of major resistance alleles against Vip3Aa39 in H. zea in Texas from this study was 0.0065 with a 95% CI of 0.0014-0.0157. A Vip3Aa-resistant strain (RR) derived from the F2 screen showed a high level of resistance to Vip3Aa39 protein, with a resistance ratio of >588.0-fold relative to a susceptible population (SS) based on diet-overlay bioassays. We provide the first documentation of a major resistance allele conferring high levels of Vip3Aa resistance in a field-derived strain of H. zea in the U.S. Data generated from this study contribute to development of management strategies for the sustainable use of the Vip3Aa technology to control H. zea in the U.S.

Extended evaluation of Bt protein cross-pollination in seed blend plantings on survival, growth, and development of Helicoverpa zea feeding on refuge ears

BACKGROUND

A seed blend, also called refuge in a bag (RIB), is used as a strategy to provide refuge for planting pyramided Bt corn in the U.S. Corn Belt. A major concern for the RIB strategy is cross-pollination between Bt and refuge plants, which may reduce susceptible populations of ear-feeding insects like Helicoverpa zea and affect the evolution of resistance. Previous studies showed a 5:95% (Non-Bt: Bt) RIB would be unlikely to provide sufficient refuge populations of H. zea for resistance management. In this study, we extended our research and conducted multiple trials to determine if three RIB plantings with greater refuge sizes (10, 20, and 30% refuge) could provide sufficient H. zea susceptible populations to delay resistance development.

RESULTS

Experimental results showed that cross-pollination in 10:90%, 20:80%, and 30:70% RIB plantings still significantly reduced larval survival, delayed larval development and decreased the pupal size of H. zea on refuge plants. Regression analysis showed that the percent reduction of neonate-to-adult survivorship, relative to the survivorship on pure non-Bt ears, was significantly negatively correlated with the refuge percentage.

CONCLUSION

These findings suggest that, approximately a 15% non-Bt refuge plants in RIB plantings could produce a similar number of susceptible adult moths as a 5% structured refuge planting, while an approximately 30% non-Bt refuge plant in RIB plantings could provide a similar number of susceptible moths as a 20% structured refuge planting. Information generated from this study should be useful in refining resistance management strategies for Bt crop technologies. © 2019 Society of Chemical Industry.

Bt resistance alleles in field populations of pink bollworm from China: Similarities with the United States and decreased frequency from 2012 to 2015

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. © 2019 Society of Chemical Industry.