Crop rotation mitigates impacts of corn rootworm resistance to transgenic Bt corn.

Transgenic crops that produce insecticidal proteins from Bacillus thuringiensis (Bt) can suppress pests and reduce insecticide sprays, but their efficacy is reduced when pests evolve resistance. Although farmers plant refuges of non-Bt host plants to delay pest resistance, this tactic has not been sufficient against the western corn rootworm, Diabrotica virgifera virgifera In the United States, some populations of this devastating pest have rapidly evolved practical resistance to Cry3 toxins and Cry34/35Ab, the only Bt toxins in commercially available corn that kill rootworms. Here, we analyzed data from 2011 to 2016 on Bt corn fields producing Cry3Bb alone that were severely damaged by this pest in 25 crop-reporting districts of Illinois, Iowa, and Minnesota. The annual mean frequency of these problem fields was 29 fields (range 7 to 70) per million acres of Cry3Bb corn in 2011 to 2013, with a cost of $163 to $227 per damaged acre. The frequency of problem fields declined by 92% in 2014 to 2016 relative to 2011 to 2013 and was negatively associated with rotation of corn with soybean. The effectiveness of corn rotation for mitigating Bt resistance problems did not differ significantly between crop-reporting districts with versus without prevalent rotation-resistant rootworm populations. In some analyses, the frequency of problem fields was positively associated with planting of Cry3 corn and negatively associated with planting of Bt corn producing both a Cry3 toxin and Cry34/35Ab. The results highlight the central role of crop rotation for mitigating impacts of D. v. virgifera resistance to Bt corn.

Self-limiting fall armyworm: a new approach in development for sustainable crop protection and resistance management.

BACKGROUND: The fall armyworm, Spodoptera frugiperda, is a significant and widespread pest of maize, sorghum, rice, and other economically important crops. Successful management of this caterpillar pest has historically relied upon application of synthetic insecticides and through cultivation of genetically engineered crops expressing insecticidal proteins (Bt crops). Fall armyworm has, however, developed resistance to both synthetic insecticides and Bt crops, which risks undermining the benefits delivered by these important crop protection tools. Previous modelling and empirical studies have demonstrated that releases of insecticide- or Bt-susceptible insects genetically modified to express conditional female mortality can both dilute insecticide resistance and suppress pest populations. RESULTS: Here, we describe the first germline transformation of the fall armyworm and the development of a genetically engineered male-selecting self-limiting strain, OX5382G, which exhibits complete female mortality in the absence of an additive in the larval diet. Laboratory experiments showed that males of this strain are competitive against wild-type males for copulations with wild-type females, and that the OX5382G self-limiting transgene declines rapidly to extinction in closed populations following the cessation of OX5382G male releases. Population models simulating the release of OX5382G males in tandem with Bt crops and non-Bt 'refuge' crops show that OX5382G releases can suppress fall armyworm populations and delay the spread of resistance to insecticidal proteins. CONCLUSIONS: This article describes the development of self-limiting fall armyworm designed to control this pest by suppressing pest populations, and population models that demonstrate its potential as a highly effective method of managing resistance to Bt crops in pest fall armyworm populations. Our results provide early promise for a potentially valuable future addition to integrated pest management strategies for fall armyworm and other pests for which resistance to existing crop protection measures results in damage to crops and impedes sustainable agriculture.

Host plant resistance for fall armyworm management in maize: relevance, status and prospects in Africa and Asia.

KEY MESSAGE: Sustainable control of fall armyworm (FAW) requires implementation of effective integrated pest management (IPM) strategies, with host plant resistance as a key component. Significant opportunities exist for developing and deploying elite maize cultivars with native genetic resistance and/or transgenic resistance for FAW control in both Africa and Asia. The fall armyworm [Spodoptera frugiperda (J.E. Smith); FAW] has emerged as a serious pest since 2016 in Africa, and since 2018 in Asia, affecting the food security and livelihoods of millions of smallholder farmers, especially those growing maize. Sustainable control of FAW requires implementation of integrated pest management strategies, in which host plant resistance is one of the key components. Significant strides have been made in breeding elite maize lines and hybrids with native genetic resistance to FAW in Africa, based on the strong foundation of insect-resistant tropical germplasm developed at the International Maize and Wheat Improvement Center, Mexico. These efforts are further intensified to develop and deploy elite maize cultivars with native FAW tolerance/resistance and farmer-preferred traits suitable for diverse agro-ecologies in Africa and Asia. Independently, genetically modified Bt maize with resistance to FAW is already commercialized in South Africa, and in a few countries in Asia (Philippines and Vietnam), while efforts are being made to commercialize Bt maize events in additional countries in both Africa and Asia. In countries where Bt maize is commercialized, it is important to implement a robust insect resistance management strategy. Combinations of native genetic resistance and Bt maize also need to be explored as a path to more effective and sustainable host plant resistance options. We also highlight the critical gaps and priorities for host plant resistance research and development in maize, particularly in the context of sustainable FAW management in Africa and Asia.

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.

Determination of Cotton as a Larval Feeding Source for Lepidopteran Moths Using a Derivative from Cotton Metabolites as a Marker by LC-MS/MS Method.

Determination of the feeding history of polyphagous insect pests, such as noctuid moths (Lepidoptera: Noctuidae), is a critical element in developing population and resistance management strategies for such pests. To identify reliable markers for larval host plant determination and to develop simple extraction and detection methods, a metabolomics approach was implemented after acid hydrolysis of adult moth samples. We identified a derivative from cotton metabolites as a marker in adult moths that were fed cotton tissues as a larval diet, and we propose that the marker is tricycloheliocide H4 based on NMR and mass fragmentation analysis. Using this derivative from cotton metabolites as a marker, a targeted LC-MS/MS method reliably identified cotton as a larval diet in extracts of three noctuid moth species: Helicoverpa zea (cotton bollworm), Chloridea (Heliothis) virescens (tobacco budworm) and Chrysodeixis includens (soybean looper). We are using similar approaches to identify markers for other host plants including soybean.

Bacillus thuringiensis Cry1Da_7 and Cry1B.868 Protein Interactions with Novel Receptors Allow Control of Resistant Fall Armyworms, Spodoptera frugiperda (J.E. Smith).

Two new modified Bacillus thuringiensis (Bt) proteins, Cry1Da_7 and Cry1B.868, with activity against fall armyworms (FAW), Spodoptera frugiperda (J.E. Smith), were evaluated for their potential to bind new insect receptors compared to proteins currently deployed as plant-incorporated protectants (PIPs) in row crops. Results from resistant insect bioassays, disabled insecticidal protein (DIP) bioassays, and cell-based assays using insect cells expressing individual receptors demonstrate that receptor utilizations of the newly modified Cry1Da_7 and Cry1B.868 proteins are distinct from each other and from those of commercially available Bt proteins such as Cry1F, Cry1A.105, Cry2Ab, and Vip3A. Accordingly, these two proteins target different insect proteins in FAW midgut cells and when pyramided together should provide durability in the field against this economically important pest.IMPORTANCE There is increased concern with the development of resistance to insecticidal proteins currently expressed in crop plants, especially against high-resistance-risk pests such as fall armyworm (FAW), Spodoptera frugiperda, a maize pest that already has developed resistance to Bacillus thuringiensis (Bt) proteins such as Cry1F. Lepidopteran-specific proteins that bind new insect receptors will be critical in managing current Cry1F-resistant FAW and delaying future resistance development. Results from resistant insect assays, disabled insecticidal protein (DIP) bioassays, and cell-based assays using insect cells expressing individual receptors demonstrate that target receptors of the Cry1Da_7 and Cry1B.868 proteins are different from each other and from those of commercially available Bt proteins such as Cry1F, Cry1A.105, Cry2Ab, and Vip3A. Therefore, pyramiding these two new proteins in maize will provide durable control of this economically important pest in production agriculture.

Field-evolved resistance of Helicoverpa zea (Boddie) to transgenic maize expressing pyramided Cry1A.105/Cry2Ab2 proteins in northeast Louisiana, the United States.

The corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), is a major target pest of pyramided Bt maize and cotton in the U.S. In 2017 and 2018, notable ear damage and larval survival of H. zea were observed on pyramided Cry1A.105/Cry2Ab2 maize in some fields in northeast Louisiana, U.S. The objective of this study was to determine if the field control problem was due to resistance development to the Bt proteins in plants. A total of 15 H. zea populations were collected from Bt and non-Bt maize plants in 2017 and 2018 in multiple locations in Louisiana, Florida, and Georgia. Diet-overlay bioassays showed that LC50s of Cry1A.105 and Cry2Ab2 for populations collected from the areas with control problems in northeast Louisiana were as much as >1623- and 88-fold greater than that of a susceptible strain, respectively. In addition, two field trials in 2018 validated that Cry1A.105/Cry2Ab2 maize failed in managing natural H. zea populations, while Bt maize containing Vip3A was effective in northeast Louisiana. Results of the study documented that the observed field control problems of Cry1A.105/Cry2Ab2 maize against H. zea in northeast Louisiana were due to resistance development of the insect to the Bt proteins in plants. This is the first documentation of field-evolved resistance to pyramided Bt maize in a target insect species in southern U.S. However, susceptibility levels to Cry1A.105 and Cry2Ab2 varied greatly among populations collected from the three states, suggesting uneven distributions of the resistance in the region.

Performance of Bt-susceptible and -heterozygous genotypes of Spodoptera frugiperda (J.E. Smith) possessing single- or dual-gene resistance alleles in sequential feedings of non-Bt and Cry1A.105/Cry2Ab2 maize leaf tissues.

We simulated larval feeding behavior in seed blends of non-Bt and Bt maize to determine if seed blends create more favorable conditions for heterozygous-resistant insects over their Bt-susceptible counterparts. Survival, growth, development, and progeny production of four genotypes of the fall armyworm, Spodoptera frugiperda, Bt-susceptible (aabb), Cry1A.105 heterozygous resistant (Aabb), Cry2Ab2 heterozygous resistant (aaBb), and Cry1A.105/Cry2Ab2 heterozygous resistant (AaBb), were evaluated in eight feeding sequences (Seq 1-8) of non-Bt and MON89034 Bt maize leaf tissue expressing the Cry1A.105 and Cry2Ab2 proteins. We report variation in the performance of the four genotypes across the feeding sequences and biological parameters measured. Three heterozygous genotypes generally outperformed the susceptible genotype in larval survival, pupation rate, pupal weight, and progeny production. The performance was greater for Aabb over aaBb, AaBb over Aabb or aaBb, in two of the feeding sequences. The findings of this study could have important implications in assessing the risk of seed blends as refuge plantings for Bt crop resistance management where resistance in the target pest is not functionally recessive.

Fitness costs and inheritance of Cry2Ab2 resistance in Spodoptera frugiperda (J.E. Smith).

Cry2Ab2 is a Bacillus thuringiensis (Bt) protein expressed in transgenic corn and cotton targeting above-ground lepidopteran pests including the fall armyworm, Spodoptera frugiperda (J.E. Smith). The objective of this study was to characterize fitness costs and inheritance of Cry2Ab2 resistance in S. frugiperda. To determine if fitness costs were associated with the resistance, life history parameters (larval survival, growth, development and egg production) of Cry2Ab2-resistant, -susceptible, and two reciprocal F1 colonies of S. frugiperda were assayed on non-toxic diet and non-Bt corn leaf tissue. The results showed that there were no significant differences among the four insect colonies for all the biological parameters measured with few exceptions, suggesting that the resistance in the colony was not associated with significant fitness costs in the test conditions. To examine the inheritance of resistance, susceptibilities of the resistant and susceptible parents, as well as eight additional colonies generated from various genetic crosses, were assayed using Cry2Ab2-treated diet and Cry2Ab2 corn leaf tissue. The Cry2Ab2 resistance in S. frugiperda in the colony was inherited as a single autosomal recessive or incompletely recessive gene. The results of the study suggest a potential risk of resistance development in S. frugiperda to the Cry2Ab2 protein and thus effective management strategies should be implemented for the sustainable use of the Bt corn technology for pest management.

F2 screen for resistance to Bacillus thuringiensis Cry2Ab2-maize in field populations of Spodoptera frugiperda (Lepidoptera: Noctuidae) from the southern United States.

The fall armyworm, Spodoptera frugiperda (J. E. Smith) (Lepidoptera: Noctuidae), is a target pest of transgenic maize and cotton expressing Bacillus thuringiensis (Bt) proteins in both North and South America. In 2013 and 2014, a total of 215 F2 two-parent families of S. frugiperda were established using single-pair mating of field individuals collected from seven locations in four states of the southern U.S.: Texas, Louisiana, Georgia, and Florida. The objective of the investigation was to detect resistance alleles in field populations to Cry2Ab2, a common Bt protein produced in transgenic maize and cotton. For each F2 family, 128 F2 neonates were screened on leaf tissue of Cry2Ab2 maize plants in the laboratory. A conservative estimate of the frequency of major Cry2Ab2 resistance alleles in S. frugiperda from the four states was 0.0023 with a 95% credibility interval of 0.0003-0.0064. In addition, six families were considered to likely possess minor resistance alleles at a frequency of 0.0082 with a 95% credibility interval of 0.0033-0.0152. One F2 family from Georgia (GA-15) was confirmed to possess a major resistance allele to the Cry2Ab2 protein. Larvae from this family survived well on whole maize plants expressing Cry2Ab2 protein and demonstrated a significant level (>15-fold) of resistance when fed with the same protein incorporated in a meridic diet. The detection of the major resistance allele along with the relatively abundant minor resistance alleles revealed in this study may have important implications for resistance management.

Effects of Pyramided Bt Corn and Blended Refuges on Western Corn Rootworm and Northern Corn Rootworm (Coleoptera: Chrysomelidae).

The western corn rootworm, Diabrotica virgifera virgifera LeConte, and the northern corn rootworm, Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae), are major pests of corn (Zea mays L). Several transgenic corn events producing insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt) kill corn rootworm larvae and reduce injury to corn roots. However, planting of Bt corn imposes selection on rootworm populations to evolve Bt resistance. The refuge strategy and pyramiding of multiple Bt toxins can delay resistance to Bt crops. In this study, we assessed the impact of four treatments--1) non-Bt corn, 2) Cry3Bb1 corn, 3) corn pyramided with Cry3Bb1 and Cry34/35Ab1, and 4) pyramided corn with a blended refuge--on survival, time of adult emergence, and size of western and northern corn rootworm. All treatments with Bt corn led to significant reductions in the number of adults that emerged per plot. However, at one location, we identified Cry3Bb1-resistant western corn rootworm. In some cases Bt treatments reduced size of adults and delayed time of adult emergence, with effects most pronounced for pyramided corn. For both species, the number of adults that emerged from pyramided corn with a blended refuge was significantly lower than expected, based solely on emergence from pure stands of pyramided corn and non-Bt corn. The results of this study indicate that pyramided corn with a blended refuge substantially reduces survival of both western and northern corn rootworm, and as such, should be a useful tool within the context of a broader integrated pest management strategy.

Simulated herbicide mixtures delay both specialist monogenic and generalist polygenic resistance evolution in weeds.

BACKGROUND: Evolution of herbicide-resistant weed populations is a major challenge to world food production. Using different herbicides in rotation and/or using different herbicides together as mixtures are strategies that may delay the selection of resistance. This study used simulation modelling to investigate whether mixtures and rotations can delay the selection of both generalist polygenic and specialist monogenic herbicide resistance, and whether these strategies are more likely to lead to the selection of generalist resistance in weed types with varying biological characteristics. RESULTS: Our simulations suggest that well-designed effective herbicide mixtures should delay evolution of both polygenic and monogenic resistance better than rotations and single herbicides across all weed types. Both mixture and rotation strategies increased the likelihood of polygenic resistance compared to single-herbicide use, and the likelihood of polygenic resistance increased as the fecundity and competitiveness of the weed increased. Whether monogenic or polygenic resistance occurred in each case depended most on the relative initial allele frequencies. We did not find that herbicide mixtures were more likely than rotations to lead to the selection of generalist polygenic resistance. The simulated efficacy of mixtures over rotations decreased if components were used at reduced rates or when individual components had already been used solo. CONCLUSION: Herbicide rotations and particularly well-designed mixtures should delay evolution of both polygenic and monogenic resistance, especially if used as part of an effective integrated weed management programme. However, herbicide mixtures and rotations may also increase the risk that resistance will be generalist polygenic rather than specialist monogenic. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Resistance management and integrated pest management insights from deployment of a Cry3Bb1+ Gpp34Ab1/Tpp35Ab1 pyramid in a resistant western corn rootworm landscape.

In Nebraska USA, many populations of western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, now exhibit some level of resistance to all corn rootworm-active Bacillus thuringiensis Berliner (Bt) proteins expressed in commercial hybrids. Therefore, a study was conducted in northeast Nebraska from 2020-2022 to reevaluate current corn rootworm management options in continuous maize (consecutive planting for ≥2 years). Results from on-farm experiments to evaluate a standard soil-applied insecticide (Aztec® 4.67G) in combination with non-rootworm Bt or rootworm-active Bt pyramided maize (Cry3Bb1 + Gpp34Ab1/Tpp35Ab1) are reported within the context of WCR Bt resistance levels present. Corrected survival from Bt pyramid single-plant bioassays (<0.3, 0.3-0.49, >0.5) was used to place populations into 3 resistance categories. Variables evaluated included root injury, adult emergence, proportion lodged maize, and grain yield. Key results: A composite analysis of all populations across resistance levels indicated that addition of soil insecticide to Bt pyramid significantly reduced adult emergence and lodging but did not significantly increase root protection or yield. Within and among resistance category analyses of root injury revealed that the Bt pyramid remained highly efficacious at any non-rootworm Bt root injury level when resistance was absent or low. When corrected survival was >0.3, mean Bt pyramid root injury tracked more closely in a positive linear fashion with mean non-rootworm Bt root injury (rootworm density x level of resistance interaction). Similar trends were obtained for adult emergence but not yield. Mean Bt pyramid root injury rating was <0.75 in most populations with Bt resistance, which contributed to no significant yield differences among categories. Results are discussed within the context of IPM:IRM tradeoffs and the need to reduce WCR densities in this system to decrease the impact of the density x resistance interaction to bridge use of current pyramids with new technologies introduced over the next decade.

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.

The first extensive analysis of species composition and abundance of stink bugs (Hemiptera: Pentatomidae) on soybean crops in Brazil.

BACKGROUND: Soybean is the third-greatest global commodity crop with respect to grain production, Brazil is the largest soybean producer in the world. We performed the first extensive survey including all the five main soybean cultivation regions in Brazil over three seasons (2018/2019, 2019/2020, and 2020/2021). A total of 2386 localities were sampled, corresponding to 145 municipalities in 11 states. Sampling was carried out between the R1 and R8 soybean growth stages, using a beating sheet. RESULTS: Fifteen species were recorded, with five species accounting for more than 99% of the sampled insects. The Neotropical brown stink bug, Euschistus heros (F.), was the most abundant species (82.4% of the adults and 84.1% of the nymphs overall), with differences in the mean abundance between soybean macroregions. The melacanthus green belly stink bug, Diceraeus melacanthus Dallas was the second most abundant species overall, followed by the brown winged stink bug, Edessa meditabunda (F.), the furcatus green belly stink bug, Diceraeus furcatus (F.) and the red-banded green stink bug, Piezodorus guildinii (Westwood). The relative abundance of each species differed between soybean macroregions. The mean abundance of nymphs and adults of Euschistus heros at different soybean reproductive stages showed an increase from early reproductive stages to the beginning of the late reproductive stages (R5 or R6). CONCLUSION: This large-scale assessment of stink bugs provides a basis for outlining integrated pest management programs and drives the development of monitoring and control strategies, as well as future studies investigating population dynamics over time and space in soybean fields. © 2024 Society of Chemical Industry.

Genetic basis of resistance to Cry1Ac in Rachiplusia nu (Lepidoptera: Noctuidae): inheritance mode, cross-resistance patterns and fitness cost.

BACKGROUND: Rachiplusia nu (Guenée) was historically a secondary soybean pest in Brazil, but a key soybean pest in Argentina. From 2021 onwards, injury caused by R. nu has been reported in soybean that expresses the Cry1Ac toxin from Bacillus thuringiensis (Berliner) in both countries. In this study, we selected resistant and susceptible strains of R. nu to Cry1Ac using Cry1Ac-containing leaf tissue and characterized the inheritance of resistance, cross-resistance patterns and fitness cost. RESULTS: Neonates of the Cry1Ac-resistant strain of R. nu were able to develop on Cry1Ac soybean leaves and emerge as fertile adults, while neonates from the susceptible and heterozygous strains did not survive beyond 10 days. The resistance ratio to Cry1Ac estimated in diet-overlay bioassays in the resistant strain was > 736.92-fold. The inheritance pattern of Cry1Ac resistance in R. nu was characterized as autosomal recessive and monogenic. The Cry1Ac-resistant strain of R. nu also exhibited high resistance to Cry1A.105 (resistance ratio > 159.87-fold), but negligible resistance to Cry2Ab2 (resistance ratio = 1.25-fold). Life history data showed that the resistance to Cry1Ac in R. nu is not associated with a substantial fitness cost. CONCLUSIONS: The inheritance pattern of Cry1Ac resistance in R. nu is autosomal recessive, monogenic and not associated with obvious fitness costs. Cross-resistance occurred between Cry1Ac and Cry1A.105 in R. nu but not between Cry1Ac and Cry2Ab2, indicating that Cry1A.105/Cry2Ab2/Cry1Ac soybean is a valuable tool to manage Cry1Ac resistance in R. nu. This is the first study reporting the genetic basis of Cry1Ac resistance in R. nu. © 2024 Society of Chemical Industry. Published by John Wiley & Sons Ltd.

Field-evolved resistance: assessing the problem and ways to move forward.

"Field-evolved resistance" is defined as a "genetically based decrease in susceptibility of a population to a toxin caused by exposure to the toxin in the field." The key component of "field-evolved" resistance is that it does confer decreased susceptibility to an insecticide in the field. Another key component is that the decrease in susceptibility to the insecticide is because of previous exposure of the target insect to the toxin in the field. Several studies have reported field-evolved resistance to crops engineered to express proteins from the bacterium, Bacillus thuringiensis (Bt). However, there has not been a consistent standard in the application of the definition of field-evolved resistance for Bt crops. The inconsistency in applying the definition arises from differences in the methods used to detect resistance, the ecology of the interaction between the pest and the Bt crop, and the effective dose the pest encounters while feeding on the Bt crop. Using case studies of reported resistance to Bt crops, it is demonstrated resistance does not come in a single form, and that in most cases, resistance can still be managed.

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.

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.

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.