Vertical distribution and tissue selection of Helicoverpa zea (Lepidoptera: Noctuidae) adult oviposition and neonates on soybean with an indeterminate or determinate growth habit.

Helicoverpa zea (Boddie) is a polyphagous pest that can cause serious damage to crops, including soybeans (Glycine max L.). In soybeans with both determinate and indeterminate growth habits, H. zea larvae are more commonly found on leaves compared to blooms, stems, and pods. Past research demonstrated that H. zea adults tend to oviposit near the upper part of the plant canopy in soybeans with a determinate growth habit. However, ovipositional selection on soybeans with an indeterminate growth habit is unknown. We hypothesized that H. zea would oviposit more uniformly throughout the canopy on indeterminate soybean growth habits due to more diffuse reproductive tissue. We planted field and greenhouse experiments with varieties sharing a similar relative maturity (2 maturity group [MG] 5.2 varieties and 2 MG 5.4/5.5 varieties) but with different growth habits. To test oviposition selection, adult H. zea females were allowed to oviposit under field and caged conditions. We counted the number of H. zea eggs and neonates on each plant tissue type from each of 3 equal parts in the plant canopy: upper, middle, and lower. In both experiments, eggs and neonates were most common on leaves at the top of the plant regardless of soybean growth habit. Consequently, ovipositional selection is likely independent of reproductive tissue availability, and patterns of oviposition through the canopy are similar in growth habits. An improved understanding of H. zea ecology in soybeans relative to indeterminate growth habits may improve recommendations for managing this significant pest of soybean.

Continental-scale migration patterns and origin of Helicoverpa zea (Lepidoptera: Noctuidae) based on a biogeochemical marker.

Insect migrations have ecological and economic impacts, particularly in agriculture. However, there is limited knowledge about the migratory movements of pests at the continental scale, which is an important factor influencing the spread of resistance genes. Understanding the migratory patterns of economic pests, like Helicoverpa zea (Boddie), is essential for improving Integrated Pest Management (IPM) and Insect Resistance Management (IRM) strategies. In this study, we used stable hydrogen isotopic ratios in wing tissue as a biogeochemical marker to examine migratory patterns and estimate the native origins of H. zea individuals collected across a wide latitudinal gradient in North America. Samples collected at higher latitudes (Ontario, Canada and Minnesota, USA) exhibited a greater proportion (60%-96%) of nonlocal individuals, with an increased probability of origin from the southeastern United States. Populations from mid-latitudes (Florida, North Carolina, and South Carolina) showed a blend of local and nonlocal (40%-60%) individuals. Finally, 15% of the southernmost population individuals (Puerto Rico) were classified as migratory, with some having a probability of origin at higher latitudes (>30°). Overall, our results provide evidence of a northward spring/summer migration of H. zea in North America and underscore the significance of the southeastern United States as a hub for genetic flow. In addition, based on stable hydrogen isotopic ratios, there is strong evidence of reverse (southward) migration of H. zea from the continental United States to Puerto Rico. Our study highlights the implications for IPM and IRM programs and the need for management strategies that account for both northward and southward migration patterns.

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.

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.

Moisture and soil type are primary drivers of Helicoverpa zea (Lepidoptera: Noctuidae) pupation.

Studies in the lab have demonstrated that evaluating the effect of soil moisture and other variables is essential for understanding the importance of environmental factors influencing the Heliothinae pupal stage, but simulated field studies are conducted infrequently. We compared the pupation of Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) under saturated and unsaturated conditions across 3 distinct soil types (coarse sand, high organic muck, and fine-textured clay) and observed adult emergence, as well as pupal depth and weight. The interaction between soil type and moisture had a significant effect on adult emergence. Fewer adults emerged from dry fine-textured clay soil than from dry coarse sand and high organic muck. However, there was no effect of soil type and no interaction between soil and moisture on pupal depth. Soil moisture was the primary driver of pupal depth, suggesting prepupae use moisture to mediate their position within the pupal chamber. In addition, pupal weight was unrelated to soil type, moisture, or their interaction. Our study demonstrates that soil moisture can be a greater driver of H. zea pupation than soil type, but additional observations are necessary to understand the mechanism by which moisture impacts pupation.

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.

Magnitude and Extent of Helicoverpa zea Resistance Levels to Cry1Ac and Cry2Ab2 across the Southeastern USA.

After resistance is first detected, continued resistance monitoring can inform decisions on how to effectively manage resistant populations. We monitored for resistance to Cry1Ac (2018 and 2019) and Cry2Ab2 (2019) from southeastern USA populations of Helicoverpa zea. We collected larvae from various plant hosts, sib-mated the adults, and tested neonates using diet-overlay bioassays and compared them to susceptible populations for resistance estimates. We also compared LC50 values with larval survival, weight and larval inhibition at the highest dose tested using regression, and found that LC50 values were negatively correlated with survival for both proteins. Finally, we compared resistance rations between Cry1Ac and Cry2Ab2 during 2019. Some populations were resistant to Cry1Ac, and most were resistant to CryAb2; Cry1Ac resistance ratios were lower than Cry2Ab2 during 2019. Survival was positively correlated with larval weight inhibition for Cry2Ab. This contrasts with other studies in both the mid-southern and southeastern USA, where resistance to Cry1Ac, Cry1A.105, and Cry2Ab2 increased over time and was found in a majority of populations. This indicates that cotton expressing Cry proteins in the southeastern USA was at variable risk for damage in this region.

Helicoverpa zea (Lepidoptera: Noctuidae) In-Season and Overwintering Pupation Response to Soil Type.

Heliothinae soil pupation is understudied despite the key role this life stage plays in their development. Many Heliothinae are important agricultural pests and understanding the interplay of environment and pupation is important to optimize pest management tactics oriented toward pupae. We studied the impact of three soil types (coarse sand, high organic muck, and fine-textured clay) on Helicoverpa zea (Boddie) in-season and overwintering pupal survivorship, diapause, depth, and weight in at two locations (North and South Carolina). We introduced wild-collected (in-season) and laboratory-reared (over the winter) infestations of H. zea larvae to each of the three soils and later excavated pupae. In-season and over the winter pupal survivorship was lower in fine-textured clay soils than in coarse sand or high organic muck. In addition, pupal depth and weight, in-season and over the winter, varied significantly by soil type. In general, depth was the shallowest, and pupae weight was lower when recovered from fine-textured clay soils. Finally, diapausing characteristics varied significantly by location and year, likely impacted by differing environmental conditions. Our results suggest that fine-textured clay soils negatively impact Heliothinae pupation and may be suppressing populations in areas with these soil types.

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.

A Special Collection: Spodoptera frugiperda (Fall Armyworm): Ecology and Management of its World-Scale Invasion Outside of the Americas.

The Special Collection 'Spodoptera frugiperda (fall armyworm): Ecology and Management of its World-scale Invasion Outside of the Americas' presents reviews and research that address topics of overarching interest and contributes to a better understanding of this pest and its management, now that it has spread outside the Americas. The collection is a combination of invited articles presenting new information published for the first time, invited review papers, and a selection of relevant high-quality articles previously published in Journal of Economic Entomology (JEE). Articles in the Collection, as well as selected citations of articles in other publications, reflect the increase in research on S. frugiperda (J.E. Smith) (Lepidoptera: Noctuidae), which became the most important pest of maize in the world during recent years. This Special Collection addresses a wide range of topics, including pest behavior, host strains, insecticide and Bt protein resistance, biological control, host plant resistance, and yield loss estimates. Topics are presented in context of research primarily conducted in regions outside of the Americas where S. frugiperda has invaded and disrupted crop production to varying degrees. In recognition of further spread, the threat of a S. frugiperda invasion into Europe and potential management options are also presented.

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.

Pest population dynamics are related to a continental overwintering gradient.

Overwintering success is an important determinant of arthropod populations that must be considered as climate change continues to influence the spatiotemporal population dynamics of agricultural pests. Using a long-term monitoring database and biologically relevant overwintering zones, we modeled the annual and seasonal population dynamics of a common pest, Helicoverpa zea (Boddie), based on three overwintering suitability zones throughout North America using four decades of soil temperatures: the southern range (able to persist through winter), transitional zone (uncertain overwintering survivorship), and northern limits (unable to survive winter). Our model indicates H. zea population dynamics are hierarchically structured with continental-level effects that are partitioned into three geographic zones. Seasonal populations were initially detected in the southern range, where they experienced multiple large population peaks. All three zones experienced a final peak between late July (southern range) and mid-August to mid-September (transitional zone and northern limits). The southern range expanded by 3% since 1981 and is projected to increase by twofold by 2099 but the areas of other zones are expected to decrease in the future. These changes suggest larger populations may persist at higher latitudes in the future due to reduced low-temperature lethal events during winter. Because H. zea is a highly migratory pest, predicting when populations accumulate in one region can inform synchronous or lagged population development in other regions. We show the value of combining long-term datasets, remotely sensed data, and laboratory findings to inform forecasting of insect pests.

Evaluating invasion risk and population dynamics of the brown marmorated stink bug across the contiguous United States.

BACKGROUND: Invasive species threaten the productivity and stability of natural and managed ecosystems. Predicting the spread of invaders, which can aid in early mitigation efforts, is a major challenge, especially in the face of climate change. While ecological niche models are effective tools to assess habitat suitability for invaders, such models have rarely been created for invasive pest species with rapidly expanding ranges. Here, we leveraged a national monitoring effort from 543 sites over 3 years to assess factors mediating the occurrence and abundance of brown marmorated stink bug (BMSB, Halyomorpha halys), an invasive insect pest that has readily established throughout much of the United States. RESULTS: We used maximum entropy models to estimate the suitable habitat of BMSB under several climate scenarios, and generalized boosted models to assess environmental factors that regulated BMSB abundance. Our models captured BMSB distribution and abundance with high accuracy, and predicted a 70% increase in suitable habitat under future climate scenarios. However, environmental factors that mediated the geographical distribution of BMSB were different from those driving abundance. While BMSB occurrence was most affected by winter precipitation and proximity to populated areas, BMSB abundance was influenced most strongly by evapotranspiration and solar photoperiod. CONCLUSION: Our results suggest that linking models of establishment (occurrence) and population dynamics (abundance) offers a more effective way to forecast the spread and impact of BMSB and other invasive species than simply occurrence-based models, allowing for targeted mitigation efforts. Implications of distribution shifts under climate change are discussed. © 2022 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Standardized Field Trials in Cotton and Bioassays to Evaluate Resistance of Tobacco Thrips (Thysanoptera: Thripidae) to Insecticides in the Southern United States.

Foliar-applied insecticide treatments may be necessary to manage thrips in cotton (Gossypium hirsutum L.) under severe infestations or when at-planting insecticide seed treatments do not provide satisfactory protection. The most common foliar-applied insecticide is acephate. Field observations in Tennessee suggest that the performance of acephate has declined. Thus, the first objective was to perform leaf-dip bioassays to assess if tobacco thrips, Frankliniella fusca (Hinds) (Thysanoptera: Thripidae), in cotton production regions have evolved resistance to foliar-applied insecticides. A second objective was to assess the performance of commonly applied foliar insecticides for managing thrips in standardized field trials in Arkansas, Tennessee, Mississippi, and Texas. For both objectives, several insecticides were evaluated including acephate, dicrotophos, dimethoate, lambda-cyhalothrin, imidacloprid, and spinetoram. Field trials and bioassays were completed from 2018 to 2021. Dose-response bioassays with acephate were performed on tobacco thrips field populations and a susceptible laboratory population. Bioassay results suggest that tobacco thrips have developed resistance to acephate and other organophosphate insecticides; however, this resistance seems to be most severe in Arkansas, Tennessee, and the Delta region of Mississippi. Resistance to other classes of insecticides were perhaps even more evident in these bioassays. The performance of these insecticides in field trials was variable, with tobacco thrips only showing consistent signs of resistance to lambda-cyhalothrin. However, it is evident that many populations of tobacco thrips are resistant to multiple classes of insecticides. Further research is needed to determine heritability and resistance mechanism(s).

Helicoverpa zea (Lepidoptera: Noctuidae) Thresholds and Yield Compensation Between Soybeans with Determinate and Indeterminate Growth Habits.

Soybean (Glycine max L.) is an important row crop in the United States and Helicoverpa zea (Boddie) is one of the most serious insect pests in this system. Economic thresholds for H. zea were developed from soybean varieties with determinate growth habits. However, southern USA farmers have recently planted more soybeans varieties with indeterminate growth habits. Trials were conducted with two determinate and two indeterminate varieties within the same relative maturity group. Levels were compared among groups with differing H. zea pressure (low, medium, high, naturally infested) and manipulated using insecticides. Our objectives were to evaluate yield compensation differences among determinate and indeterminate varieties at these different H. zea pressures and to see if the existing economic threshold should be adjusted between growth habits. Since H. zea larval populations varied across trials, we compared trials with low populations, high populations, and no population. Generally, larval counts did not differ among varieties. We found no yield differences among varieties or between growth habits, regardless of H. zea pressure. In the high population tests, yield was highest in the low population plots, but there was no compensation by the plant in yield components except in number of pods with one seed. In contrast, yield components varied widely across varieties, but these differences were independent of H. zea pressure. These results suggest the economic threshold can be used for determinate and indeterminate growth habits, but more research is needed to confirm this with a larger selection of varieties, planting dates, and maturity groups.

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.

Sampling Optimization and Crop Interface Effects on Lygus lineolaris Populations in Southeastern USA Cotton.

Tarnished plant bug, Lygus lineolaris (Hemiptera: Miridae), is an economically damaging pest in cotton production systems across the southern United States. We systematically scouted 120 commercial cotton fields across five southeastern states during susceptible growth stages in 2019 and 2020 to investigate sampling optimization and the effect of interface crop and landscape composition on L. lineolaris abundance. Variance component analysis determined field and within-field spatial scales, compared with agricultural district and state, accounted for more variation in L. lineolaris density using sweep net and drop cloth sampling. This result highlights the importance of field-level scouting efforts. Using within-field samples, a fixed-precision sampling plan determined 8 and 23 sampling units were needed to determine L. lineolaris population estimates with 0.25 precision for sweep net (100 sweeps per unit) and drop cloth (1.5 row-m per unit) sampling, respectively. A spatial Bayesian hierarchical model was developed to determine local landscape (<0.5 km from field edges) effects on L. lineolaris in cotton. The proportion of agricultural area and double-crop wheat and soybeans were positively associated with L. lineolaris density, and fields with more contiguous cotton areas negatively predicted L. lineolaris populations. These results will improve L. lineolaris monitoring programs and treatment management decisions in southeastern USA cotton.

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.

Effectiveness of the natural resistance management refuge for Bt-cotton is dominated by local abundance of soybean and maize.

Genetically engineered crops expressing Bacillus thuringiensis (Bt) Cry toxins have transformed insect management in maize and cotton, reducing insecticide use and associated off-target effects. To mitigate the risk that pests evolve resistance to Bt crops, the US Environmental Protection Agency requires resistance management measures. The approved resistance management plan for Bt maize in cotton production regions requires a structured refuge of non-Bt maize equal to 20% of the maize planted; that for Bt cotton relies on the presence of an unstructured natural refuge comprising both non-Bt crop and non-crop hosts. We examined how abundance of Bt crops (cotton and maize) and an important non-Bt crop (soybean) component of the natural refuge affect resistance to Bt Cry1Ac toxin in local populations of Helicoverpa zea, an important lepidopteran pest impacted by Bt cotton and maize. We show refuge effectiveness is responsive to local abundances of maize and cotton and non-Bt soybean, and maize, in its role as a source of H. zea infesting cotton and non-Bt hosts, influences refuge effectiveness. These findings have important implications for commercial and regulatory decisions regarding deployment of Bt toxins targeting H. zea in maize, cotton, and other crops and for assumptions regarding efficacy of natural refuges.

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.