Early Warning of Resistance to Bt Toxin Vip3Aa in Helicoverpa zea

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.

Genome-wide DNA methylation profile and its function in regulating Vip3Aa tolerance in fall armyworm (Spodoptera frugiperda)

BACKGROUND

Vegetative insecticidal proteins (Vips) are widely used in pest management, but Vip tolerance poses a significant threat. DNA methylation plays important roles in regulating the response of biological organisms to environmental stress, and it may also regulate fall armyworm (FAW, Spodoptera frugiperda) Vip3Aa tolerance.

RESULTS

In this study, a DNA methylation map was developed for FAW, and its function in regulating FAW Vip3Aa tolerance was explored. The FAW genome-wide DNA methylation map showed that exons were preferred regions for DNA methylation and housekeeping genes were highly methylated. FAW was screened using Vip3Aa for ten generations, and bioassays indicated that Vip3Aa tolerance increased trans-generationally. A comparison of DNA methylation maps between Vip3Aa-tolerant and -susceptible strains showed that gene body methylation was positively correlated with gene expression level. FAW exhibits significant variation in DNA methylation among individuals, and Vip3Aa screening induces epigenetic variation based on DNA methylation. Moreover, the study demonstrated that a reduction in methylation density within the gene body of a 3'5'-cyclic nucleotide phosphodiesterase gene resulted in decreased expression and increased tolerance of FAW to Vip3Aa, which was validated through RNA interference experiments.

CONCLUSION

The DNA methylation map and mechanism of Vip3Aa tolerance improve our understanding of DNA methylation and its function in Lepidoptera and provide a new perspective for developing pest management strategies. © 2024 Society of Chemical Industry.

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.

Retrotransposon-mediated disruption of a chitin synthase gene confers insect resistance to Bacillus thuringiensis Vip3Aa toxin

The vegetative insecticidal protein Vip3Aa from Bacillus thuringiensis (Bt) has been produced by transgenic crops to counter pest resistance to the widely used crystalline (Cry) insecticidal proteins from Bt. To proactively manage pest resistance, there is an urgent need to better understand the genetic basis of resistance to Vip3Aa, which has been largely unknown. We discovered that retrotransposon-mediated alternative splicing of a midgut-specific chitin synthase gene was associated with 5,560-fold resistance to Vip3Aa in a laboratory-selected strain of the fall armyworm, a globally important crop pest. The same mutation in this gene was also detected in a field population. Knockout of this gene via CRISPR/Cas9 caused high levels of resistance to Vip3Aa in fall armyworm and 2 other lepidopteran pests. The insights provided by these results could help to advance monitoring and management of pest resistance to Vip3Aa.

Yield analysis and corn earworm feeding in Bt and non-Bt corn hybrids across diverse locations

Corn, Zea mays L. (Poales: Poaceae), growers in the US Cotton Belt are required to plant 20% of total corn acres to non-Bt hybrids for resistance management (non-Bt refuge). Most growers do not meet this requirement, in part, because they perceive non-Bt hybrids to yield less than Bt hybrids. We planted multiple non-Bt and Bt hybrids from a single company in small-plot replicated trials at a single location from 2019 to 2023, as well as in small-plot replicated trials at multiple locations during 2022 and 2023. In the single location, we measured kernel injury from corn earworm, Helicoverpa zea Boddie (Lepidoptera: Noctuidae), and we recorded yield at all locations. In the single location trial, yields only separated among hybrids in 3 out of 5 years. In the multiple location trial, yields were variable between both years. We found that Bt hybrids tended to yield higher than non-Bt hybrids overall, but this was influenced by the inclusion of non-Bt hybrids that had a lower overall genetic yield potential in the environments we tested them in. In both tests, when hybrids were analyzed during each year, both Bt and non-Bt hybrids were among the statistically highest yielders. Our study demonstrates the importance of comparing multiple Bt and non-Bt hybrids to draw yield comparisons. This highlights the need for corn seed company breeders to put effort into improving yield for non-Bt hybrids. Hopefully this effort will translate into increased planting of non-Bt refuge corn for growers in the US Cotton Belt.

RNAi silencing CHS1 gene shortens the mortality time of Plutella xylostella feeding Bt-transgenic Brassica napus

BACKGROUND

Insect-resistance genetically modified (GM) plants derived from Bacillus thuringiensis (Bt) have been cultivated to control pests, but continuous cultivation of Bt-transgenic plants at large-scale regions leads to the resistance evolution of target insects to transgenic plants. RNA interference (RNAi) technology is considered an effective strategy in delaying the resistance evolution of target insects.

RESULTS

We here developed a single transgenic oilseed rape (Brassica napus) line with hairpin RNA of the chitin-synthase 1 gene (CHS1) of Plutella xylostella (hpPxCHS1) and a pyramid transgenic B. napus line harboring hpPxCHS1 and Bt gene (Cry1Ac). Escherichia coli HT115 delivered hpPxCHS1 showed negative effects on the growth of P. xylostella. The single transgenic and pyramid transgenic B. napus significantly reduced the larval weight and length of P. xylostella and increased its lethality rate, with down-regulation expression of the PxCHS1 gene in insects.

CONCLUSION

Compared to Bt-transgenic B. napus, pyramid-transgenic B. napus shorted the mortality time of P. xylostella, indicating that RNAi technology synergistic with Bt protein improves the effectiveness of controlling target insects. Our results proved that RNAi can delay the resistance evolution of target insects to Bt-transgenic plants. © 2024 Society of Chemical Industry.

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.

Sublethal effects of a commercial Bt product and Bt cotton flowers on the bollworm (Helicoverpa zea) with impacts to predation from a lady beetle (Hippodamia convergens)

Insecticidal Bacillus thuringiensis Berliner (Bt) toxins produced by transgenic cotton (Gossypium hirsutum L.) plants have become an essential component of cotton pest management. Bt toxins are the primary management tool in transgenic cotton for lepidopteran pests, the most important of which is the bollworm (Helicoverpa zea Boddie) (Lepidoptera: Noctuidae) in the United States (U.S.). However, bollworm larvae that survive after consuming Bt toxins may experience sublethal effects, which could alter interactions with other organisms, such as natural enemies. Experiments were conducted to evaluate how sublethal effects of a commercial Bt product (Dipel) incorporated into artificial diet and from Bt cotton flowers impact predation from the convergent lady beetle (Hippodamia convergens Guérin-Méneville) (Coleoptera: Coccinellidae), common in cotton fields of the mid-southern U.S. Sublethal effects were detected through reduced weight and slower development in bollworm larvae which fed on Dipel incorporated into artificial diet, Bollgard II, and Bollgard 3 cotton flowers. Sublethal effects from proteins incorporated into artificial diet were found to significantly alter predation from third instar lady beetle larvae. Predation of bollworm larvae also increased significantly after feeding for three days on a diet incorporated with Bt proteins. These results suggest that the changes in larval weight and development induced by Bt can be used to help predict consumption of bollworm larvae by the convergent lady beetle. These findings are essential to understanding the potential level of biological control in Bt cotton where lepidopteran larvae experience sublethal effects.

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.

Susceptibility of Field and Laboratory Bt-Susceptible and Resistant Strains of Helicoverpa zea (Boddie) to HearNPV

During 2021 and 2022, eight field-collected and five laboratory Helicoverpa zea strains with varying susceptibility to different Bt proteins were evaluated for their responses against HearNPV using diet-overlay bioassays. The five laboratory strains included SS (susceptible to all Bt proteins), CRY-RR (resistant to Cry1 and Cry2), VIP-RR-70 (resistant to Vip3Aa), VIP-RR-15 (resistant to Vip3Aa), and TRE-RR (resistant to Cry1, Cry2, and Vip3Aa). Our findings showed that the susceptibility of TRE-RR, VIP-RR-70, and VIP-RR-15 strains to HearNPV was similar to that of the SS strain. However, the field and Cry-RR strains were more resistant to HearNPV compared to the SS strain. Because most feral H. zea strains in the southern U.S. have developed practical resistance to Cry Bt proteins but remain susceptible to Vip3Aa, the results suggest that the reduced susceptibility to HearNPV in H. zea may be associated with the resistance to Cry Bt proteins but not with the resistance to Vip3Aa. Correlation analysis confirmed that there was a significant positive relationship between Cry resistance and HearNPV resistance, but not between the Vip3Aa resistance and HearNPV resistance in H. zea. Our findings provide valuable insights into the relationship between susceptibility to HearNPV and resistance to Bt proteins in H. zea.

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.

Downregulation of a transcription factor associated with resistance to Bt toxin Vip3Aa in the invasive fall armyworm

Transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt) have revolutionized control of some major pests. However, more than 25 cases of field-evolved practical resistance have reduced the efficacy of transgenic crops producing crystalline (Cry) Bt proteins, spurring adoption of alternatives including crops producing the Bt vegetative insecticidal protein Vip3Aa. Although practical resistance to Vip3Aa has not been reported yet, better understanding of the genetic basis of resistance to Vip3Aa is urgently needed to proactively monitor, delay, and counter pest resistance. This is especially important for fall armyworm (Spodoptera frugiperda), which has evolved practical resistance to Cry proteins and is one of the world's most damaging pests. Here, we report the identification of an association between downregulation of the transcription factor gene SfMyb and resistance to Vip3Aa in S. frugiperda. Results from a genome-wide association study, fine-scale mapping, and RNA-Seq identified this gene as a compelling candidate for contributing to the 206-fold resistance to Vip3Aa in a laboratory-selected strain. Experimental reduction of SfMyb expression in a susceptible strain using RNA interference (RNAi) or CRISPR/Cas9 gene editing decreased susceptibility to Vip3Aa, confirming that reduced expression of this gene can cause resistance to Vip3Aa. Relative to the wild-type promoter for SfMyb, the promoter in the resistant strain has deletions and lower activity. Data from yeast one-hybrid assays, genomics, RNA-Seq, RNAi, and proteomics identified genes that are strong candidates for mediating the effects of SfMyb on Vip3Aa resistance. The results reported here may facilitate progress in understanding and managing pest resistance to Vip3Aa.

Inheritance and fitness cost of laboratory-selected resistance to Vip3Aa in Helicoverpa zea (Lepidoptera: Noctuidae)

The polyphagous pest Helicoverpa zea (Lepidoptera: Noctuidae) has evolved practical resistance to transgenic corn and cotton producing Cry1 and Cry2 crystal proteins from Bacillus thuringiensis (Bt) in several regions of the United States. However, the Bt vegetative insecticidal protein Vip3Aa produced by Bt corn and cotton remains effective against this pest. To advance knowledge of resistance to Vip3Aa, we selected a strain of H. zea for resistance to Vip3Aa in the laboratory. After 28 generations of continuous selection, the resistance ratio was 267 for the selected strain (GA-R3) relative to a strain not selected with Vip3Aa (GA). Resistance was autosomal and almost completely recessive at a concentration killing all individuals from GA. Declines in resistance in heterogeneous strains containing a mixture of susceptible and resistant individuals reared in the absence of Vip3Aa indicate a fitness cost was associated with resistance. Previously reported cases of laboratory-selected resistance to Vip3Aa in lepidopteran pests often show partially or completely recessive resistance at high concentrations and fitness costs. Abundant refuges of non-Bt host plants can maximize the benefits of such costs for sustaining the efficacy of Vip3Aa against target pests.

Impact of seed blend and structured maize refuge on Helicoverpa zea (Lepidoptera: Noctuidae) potential phenological resistance development parameters in pupae and adults

BACKGROUND

Helicoverpa zea, an economic pest in the south-eastern United States, has evolved practical resistance to Bacillus thuringiensis (Bt) Cry toxins in maize and cotton. Insect resistance management (IRM) programs have historically required planting of structured non-Bt maize, but because of its low adoption, the use of seed blends has been considered. To generate knowledge on target pest biology and ecology to help improve IRM strategies, nine field trials were conducted in 2019 and 2020 in Florida, Georgia, North Carolina, and South Carolina to evaluate the impact of Bt (Cry1Ab + Cry1F or Cry1Ab + Cry1F + Vip3A) and non-Bt maize plants in blended and structured refuge treatments on H. zea pupal survival, weight, soil pupation depth, adult flight parameters, and adult time to eclosion.

RESULTS

From a very large sample size and geography, we found a significant difference in pupal mortality and weight among treatments in seed blends with Vip3A, implying that cross-pollination occurred between Bt and non-Bt maize ears. There was no treatment effect for pupation depth, adult flight distance, and eclosion time.

CONCLUSION

Results of this study demonstrate the potential impact of different refuge strategies on phenological development and survival of an important pest species of regulatory concern. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

Global Patterns of Insect Resistance to Transgenic Bt Crops: The First 25 Years

Crops genetically engineered to produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) have improved pest management and reduced reliance on insecticide sprays. However, evolution of practical resistance by some pests has reduced the efficacy of Bt crops. We analyzed global resistance monitoring data for 24 pest species based on the first 25 yr of cultivation of Bt crops including corn, cotton, soybean, and sugarcane. Each of the 73 cases examined represents the response of one pest species in one country to one Bt toxin produced by one or more Bt crops. The cases of practical resistance rose from 3 in 2005 to 26 in 2020. Practical resistance has been documented in some populations of 11 pest species (nine lepidopterans and two coleopterans), collectively affecting nine widely used crystalline (Cry) Bt toxins in seven countries. Conversely, 30 cases reflect no decrease in susceptibility to Bt crops in populations of 16 pest species in 10 countries. The remaining 17 cases provide early warnings of resistance, which entail genetically based decreases in susceptibility without evidence of reduced field efficacy. The early warnings involve four Cry toxins and the Bt vegetative insecticidal protein Vip3Aa. Factors expected to favor sustained susceptibility include abundant refuges of non-Bt host plants, recessive inheritance of resistance, low resistance allele frequency, fitness costs, incomplete resistance, and redundant killing by multi-toxin Bt crops. Also, sufficiently abundant refuges can overcome some unfavorable conditions for other factors. These insights may help to increase the sustainability of current and future transgenic insecticidal crops.

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.

Removal of an Aminopeptidase N From Midgut Brush Border Does Not Affect Susceptibility of Spodoptera litura (Lepidoptera: Noctuidae) Larvae to Four Insecticidal Proteins of Bacillus thuringiensis (Bacillales: Bacillaceae)

Spodoptera litura is one of the most destructive lepidopteran insects of cabbages and cauliflowers in the world. Cry1 and Vip3 toxins from Bacillus thuringiensis have been reported to show toxicity in multiple lepidopteran insects. Binding of toxic molecules to specific receptors on the midgut epithelial cells is known to be a key step in the action mode of Bt toxins. Aminopeptidase N (APN) -like proteins have been reported to be binding sites of multiple Cry toxins in the midgut of Cry susceptible insects. In the present study, we identified six midgut APNs by analysis of the genome and midgut transcriptome of S. litura. CRISPR/Cas9 mediated gene-knockout system was utilized to mutate the GPI-anchor signal peptide at the C terminus of SlAPN1. SlAPN1 was verified to be removed from the midgut brush border membrane vesicles of a homozygous knockout strain of S. litura (SlAPN1-KO). Bioassay results indicated that susceptibility of the SlAPN1-KO strain to Cry1Aa, Cry1Ac, Cry1Ca, and Vip3Aa toxins was close to that of the wild-type strain of S. litura. RT-qPCR results showed that the transcriptional level of SlAPN2-6 was not up-regulated after knockout of the SlAPN1. Results in this study indicated that the SlAPN1 did not play a critical role in the pathway of toxicity of Cry1Aa, Cry1Ac, Cry1Ca, and Vip3Aa toxins in S. litura.

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.

Management of Insect Pests with Bt Crops in the United States

Genetically engineered corn and cotton that produce insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) have been used to manage insect pests in the United States and elsewhere. In some cases, this has led to regional suppression of pest populations and pest eradication within the United States, and these outcomes were associated with reductions in conventional insecticides and increased profits for farmers. In other instances, pests evolved resistance to multiple Bt traits, compromising the capacity of Bt crops to manage pests and leading to increased feeding injury to crops in the field. Several aspects of pest biology and pest-crop interactions were associated with cases where pests remained susceptible versus instances where pests evolved resistance. The viability of future transgenic traits can be improved by learning from these past outcomes. In particular, efforts should be made to delay resistance by increasing the prevalence of refuges and using integrated pest management.

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.

Impact of Caterpillar Increased Feeding Rates on Reduction of Bt Susceptibility

The use of insect-resistant transgenic crops producing Bacillus thuringiensis protein Cry toxins (Bt) to control caterpillars is wide-spread. Development of a mechanism to prevent Bt from reaching its target site in the digestive system could result in Bt resistance and resistance to other insecticides active per os. Increased feeding rates by increasing temperature in tobacco budworms, Chloridea virescens, and bollworms, Helicoverpa zea, decreased Bt Cry1Ac susceptibility and mortality. The same was found in C. virescens for Bollgard II plant extract containing Bt Cry1Ac and Cry2Ab2 toxins. Furthermore, H. zea from the same inbred laboratory colony that fed faster independent of temperature manipulation were less susceptible to Bt intoxication. A laboratory derived C. virescens Bt resistant strain demonstrated a higher feeding rate on non-Bt artificial diet than the parental, Bt susceptible strain. A laboratory-reared Bt resistant fall armyworm, Spodoptera frugiperda, strain also fed faster on non-Bt diet compared to Bt susceptible caterpillars of the same species, both originally collected from corn. The studies in toto and the literature reviewed support the hypothesis that increased feeding rate is a behavioral mechanism for reducing caterpillar susceptibility to Bt. Its possible role in resistance needs further study.

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.

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.

Helicoverpa zea (Lepidoptera: Noctuidae) feeding incidence and survival on Bt maize in relation to maize in the landscape

BACKGROUND

Characterizing Helicoverpa zea (Boddie) damage to maize (Zea mays L.) in relation to the spatiotemporal composition of Bt crops is essential to understand how landscape composition affects H. zea abundance. To examine this relationship, paired Bt (expressing Cry1A.105 + Cry2Ab2) and non-Bt maize plots were sampled across North and South Carolina during 2017-2019. Kernel damage and larval exit holes were measured following larval development. To understand how maize abundance surrounding sample sites related to feeding damage and larval development, we quantified maize abundance in a 1 km buffer surrounding the sample site and examined the relationship between local maize abundance and kernel damage and larval exit holes.

RESULTS

Across the years and locations, damage in Bt maize was widespread but significantly lower than in non-Bt maize, indicating that despite the widespread occurrence of resistance to Cry toxins in maize, Bt maize still provides a measurable reduction in damage. There were negative relationships between kernel injury and ears with larval exit holes in both Bt and non-Bt maize and the proportion of maize in the landscape during the current year.

CONCLUSION

Despite the widespread occurrence of resistance to Cry toxins in maize, this resistance is incomplete, and on average Bt maize continues to provide a measurable reduction in damage. We interpret the negative relationship between abundance of maize within 1 km of the sample location and maize infestation levels, as measured by kernel damage and larval exit holes, to reflect dispersion of the ovipositing moth population over available maize within the local landscape. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Best Management Practices to Delay the Evolution of Bt Resistance in Lepidopteran Pests Without High Susceptibility to Bt Toxins in North America

Canadian and United States (US) insect resistance management (IRM) programs for lepidopteran pests in Bacillus thuriengiensis (Bt)-expressing crops are optimally designed for Ostrinia nubilalis Hübner in corn (Zea mays L.) and Chloridea virescens Fabricius in cotton (Gossypium hirsutum L.). Both Bt corn and cotton express a high dose for these pests; however, there are many other target pests for which Bt crops do not express high doses (commonly referred to as nonhigh dose pests). Two important lepidopteran nonhigh dose (low susceptibility) pests are Helicoverpa zea Boddie (Lepidoptera: Noctuidae) and Striacosta albicosta Smith (Lepidoptera: Noctuidae). We highlight both pests as cautionary examples of exposure to nonhigh dose levels of Bt toxins when the IRM plan was not followed. Moreover, IRM practices to delay Bt resistance that are designed for these two ecologically challenging and important pests should apply to species that are more susceptible to Bt toxins. The purpose of this article is to propose five best management practices to delay the evolution of Bt resistance in lepidopteran pests with low susceptibility to Bt toxins in Canada and the US: 1) better understand resistance potential before commercialization, 2) strengthen IRM based on regional pest pressure by restricting Bt usage where it is of little benefit, 3) require and incentivize planting of structured corn refuge everywhere for single toxin cultivars and in the southern US for pyramids, 4) integrate field and laboratory resistance monitoring programs, and 5) effectively use unexpected injury thresholds.

Characterization of Long Non-Coding RNAs in the Bollworm, Helicoverpa zea, and Their Possible Role in Cry1Ac-Resistance

Multiple insect pest species have developed field resistance to Bt-transgenic crops. There has been a significant amount of research on protein-coding genes that contribute to resistance, such as the up-regulation of protease activity or altered receptors. However, our understanding of the role of non-protein-coding mechanisms in Bt-resistance is minimal, as is also the case for resistance to chemical pesticides. To address this problem relative to Bt, RNA-seq was used to examine statistically significant, differential gene expression between a Cry1Ac-resistant (~100-fold resistant) and Cry1Ac-susceptible strain of Helicoverpa zea, a prevalent caterpillar pest in the USA. Significant differential expression of putative long non-coding RNAs (lncRNAs) was found in the Cry1Ac-resistant strain (58 up- and 24 down-regulated gene transcripts with an additional 10 found only in resistant and four only in susceptible caterpillars). These lncRNAs were examined as potential pseudogenes and for their genomic proximity to coding genes, both of which can be indicative of regulatory relationships between a lncRNA and coding gene expression. A possible pseudogenic lncRNA was found with similarities to a cadherin. In addition, putative lncRNAs were found significantly proximal to a serine protease, ABC transporter, and CYP coding genes, potentially involved in the mechanism of Bt and/or chemical insecticide resistance. Characterization of non-coding genetic mechanisms in Helicoverpa zea will improve the understanding of the genomic evolution of insect resistance, improve the identification of specific regulators of coding genes in general (some of which could be important in resistance), and is the first step for potentially targeting these regulators for pest control and resistance management (using molecular approaches, such as RNAi and others).