Trichoderma virens and Pseudomonas chlororaphis Differentially Regulate Maize Resistance to Anthracnose Leaf Blight and Insect Herbivores When Grown in Sterile versus Non-Sterile Soils

Soil-borne Trichoderma spp. have been extensively studied for their biocontrol activities against pathogens and growth promotion ability in plants. However, the beneficial effect of Trichoderma on inducing resistance against insect herbivores has been underexplored. Among diverse Trichoderma species, consistent with previous reports, we showed that root colonization by T. virens triggered induced systemic resistance (ISR) to the leaf-infecting hemibiotrophic fungal pathogens Colletotrichum graminicola. Whether T. virens induces ISR to insect pests has not been tested before. In this study, we investigated whether T. virens affects jasmonic acid (JA) biosynthesis and defense against fall armyworm (FAW) and western corn rootworm (WCR). Unexpectedly, the results showed that T. virens colonization of maize seedlings grown in autoclaved soil suppressed wound-induced production of JA, resulting in reduced resistance to FAW. Similarly, the bacterial endophyte Pseudomonas chlororaphis 30-84 was found to suppress systemic resistance to FAW due to reduced JA. Further comparative analyses of the systemic effects of these endophytes when applied in sterile or non-sterile field soil showed that both T. virens and P. chlororaphis 30-84 triggered ISR against C. graminicola in both soil conditions, but only suppressed JA production and resistance to FAW in sterile soil, while no significant impact was observed when applied in non-sterile soil. In contrast to the effect on FAW defense, T. virens colonization of maize roots suppressed WCR larvae survival and weight gain. This is the first report suggesting the potential role of T. virens as a biocontrol agent against WCR.

Movement Ecology of Adult Western Corn Rootworm: Implications for Management

Movement of adult western corn rootworm, Diabrotica virgifera virgifera LeConte, is of fundamental importance to this species' population dynamics, ecology, evolution, and interactions with its environment, including cultivated cornfields. Realistic parameterization of dispersal components of models is needed to predict rates of range expansion, development, and spread of resistance to control measures and improve pest and resistance management strategies. However, a coherent understanding of western corn rootworm movement ecology has remained elusive because of conflicting evidence for both short- and long-distance lifetime dispersal, a type of dilemma observed in many species called Reid's paradox. Attempts to resolve this paradox using population genetic strategies to estimate rates of gene flow over space likewise imply greater dispersal distances than direct observations of short-range movement suggest, a dilemma called Slatkin's paradox. Based on the wide-array of available evidence, we present a conceptual model of adult western corn rootworm movement ecology under the premise it is a partially migratory species. We propose that rootworm populations consist of two behavioral phenotypes, resident and migrant. Both engage in local, appetitive flights, but only the migrant phenotype also makes non-appetitive migratory flights, resulting in observed patterns of bimodal dispersal distances and resolution of Reid's and Slatkin's paradoxes.

Bt corn hybrids expressing mCry3A and eCry3.1Ab Proteins protect corn roots against western corn rootworm injury

BACKGROUND

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is one of the most serious pests of corn (Zea mays L.) In 2017 and 2018, studies were conducted in fields with and without known unexpected root injury to Cry3Bb1, to determine root protection by Bt corn hybrids expressing both mCry3A and eCry3.1Ab insecticidal crystal proteins, and hybrids expressing either mCry3A or eCry3.1Ab only against the WCR root injury. Node injury was evaluated using the Iowa State University 0-3 node-injury scale (NIS), and the consistency of root protection was also determined.

RESULTS

In 2017, with medium to high larval feeding pressure, the Bt corn hybrids expressing both mCry3A and eCry3.1Ab in the breeding stack, molecular stack, and Bt corn hybrid expressing eCry3.1Ab only, sustained low node injury compared with Bt corn hybrid expressing mCry3A only, and the non-Bt corn. In 2018, with low larval feeding pressure in most of the locations, node injury was not different for the Bt and Non-Bt corn hybrids. Across all locations in both years, the Bt corn hybrids expressing both mCry3A and eCry3.1Ab provided better and consistent node injury protection.

CONCLUSION

Bt corn hybrids expressing both mCry3A and eCry3.1Ab proteins provided better root protection and consistency than the Bt corn hybrid expressing mCry3A only, and non-Bt. Therefore, stacking of Bt traits will be the best option for managing insect resistance. © 2023 Society of Chemical Industry.

Enzyme kinetics of deoxyuridine triphosphatase from Western corn rootworm

OBJECTIVE

The Western corn rootworm (WCR), Diabrotica virgifera virgifera, is a highly adaptable insect pest that has evolved resistance to a variety of control strategies, including insecticides. Therefore, it is interesting to examine how housekeeping proteins in WCR have been changed under WCR-controlling strategies. In this study, we focused on one of such proteins in WCR, a ubiquitous enzyme 5'-triphosphate nucleotidohydrolase (dUTPase). In the thymidine synthetic pathway, dUTPase hydrolyzes deoxyuridine triphosphate (dUTP) and supplies the substrate, deoxyuridine monophosphate, for the thymidylate synthase (TS). It decreases the cellular content of uracil, reducing uracil misincorporation into DNA. Suppressing the dUTPase activity, therefore, contributes to thymineless death. In this study, we investigated the enzymatic properties of dUTPase.

RESULTS

The WCR dUTPase gene (DUT) was synthesized with the addition of His-tag corresponding DNA sequence and then cloned and expressed in Escherichia coli, and the protein product was purified. The product of WCR DUT hydrolyzed dUTP and was designated as dUTPase. WCR dUTPase did not hydrolyze dATP, dTTP, dCTP, or dGTP. WCR dUTPase was analyzed via size-exclusion chromatography and exhibited a molecular weight corresponding to that of trimer. The present format can be interpreted as nuclear trimer type. Possible isomers will be examined once transcriptome analyses are conducted.

Maize OPR2 and LOX10 Mediate Defense against Fall Armyworm and Western Corn Rootworm by Tissue-Specific Regulation of Jasmonic Acid and Ketol Metabolism

Foliage-feeding fall armyworm (FAW; Spodoptera frugiperda) and root-feeding western corn rootworm (WCR; Diabrotica virgifera virgifera) are maize (Zea mays L.) pests that cause significant yield losses. Jasmonic acid (JA) plays a pivotal defense role against insects. 12-oxo-phytodienoic acid (12-OPDA) is converted into JA by peroxisome-localized OPDA reductases (OPR). However, little is known about the physiological functions of cytoplasmic OPRs. Here, we show that disruption of ZmOPR2 reduced wound-induced JA production and defense against FAW while accumulating more JA catabolites. Overexpression of ZmOPR2 in Arabidopsis enhanced JA production and defense against beet armyworm (BAW; Spodoptera exigua). In addition, lox10opr2 double mutants were more susceptible than either single mutant, suggesting that ZmOPR2 and ZmLOX10 uniquely and additively contributed to defense. In contrast to the defensive roles of ZmOPR2 and ZmLOX10 in leaves, single mutants did not display any alteration in root herbivory defense against WCR. Feeding on lox10opr2 double mutants resulted in increased WCR mortality associated with greater herbivory-induced production of insecticidal death acids and ketols. Thus, ZmOPR2 and ZmLOX10 cooperatively inhibit the synthesis of these metabolites during herbivory by WCR. We conclude that ZmOPR2 and ZmLOX10 regulate JA-mediated resistance in leaves against FAW while suppressing insecticidal oxylipin synthesis in roots during WCR infestation.

IPD072Aa from Pseudomonas chlororaphis Targets Midgut Epithelial Cells in Killing Western Corn Rootworm (Diabrotica virgifera virgifera)

IPD072Aa from Pseudomonas chlororaphis is a new insecticidal protein that has been shown to have high activity against western corn rootworm (WCR). IPD072 has no sequence signatures or predicted structural motifs with any known protein revealing little insight into its mode of action using bioinformatic tools. As many bacterially derived insecticidal proteins are known to act through mechanisms that lead to death of midgut cells, we evaluated whether IPD072Aa also acts by targeting the cells of WCR midgut. IPD072Aa exhibits specific binding to brush border membrane vesicles (BBMVs) prepared from WCR guts. The binding was found to occur at binding sites that are different than those recognized by Cry3A or Cry34Ab1/Cry35Ab1, proteins expressed by current maize traits that target WCR. Using fluorescence confocal microscopy, immuno-detection of IPD072Aa in longitudinal sections from whole WCR larvae that were fed IPD072Aa revealed the association of the protein with the cells that line the gut. High-resolution scanning electron microscopy of similar whole larval sections revealed the disruption of the gut lining resulting from cell death caused by IPD072Aa exposure. These data show that the insecticidal activity of IPD072Aa results from specific targeting and killing of rootworm midgut cells. IMPORTANCE Transgenic traits targeting WCR based on insecticidal proteins from Bacillus thuringiensis have proven effective in protecting maize yield in North America. High adoption has led to WCR populations that are resistant to the trait proteins. Four proteins have been developed into commercial traits, but they represent only two modes of action due to cross-resistance among three. New proteins suited for trait development are needed. IPD072Aa, identified from the bacterium Pseudomonas chlororaphis, was shown to be effective in protecting transgenic maize against WCR. To be useful, IPD072Aa must work through binding to different receptors than those utilized by current traits to reduce risk of cross-resistance and understanding its mechanism of toxicity could aid in countering resistance development. Our results show that IPD072Aa binds to receptors in WCR gut that are different than those utilized by current commercial traits and its targeted killing of midgut cells results in larval death.

Pest categorisation of cowpea mosaic virus

The EFSA Panel on Plant Health conducted a pest categorisation of cowpea mosaic virus (CPMV) for the EU territory. The identity of CPMV, a member of the genus Comovirus (family Secoviridae), is established and detection and identification methods are available. The pathogen is not included in the Commission Implementing Regulation (EU) 2019/2072. It has been reported from the Americas, and several countries in Africa and Asia and it is not known to be present in the EU in natural conditions. CPMV is considered a major pathogen of cowpea on which it causes symptoms ranging from mild to severe mosaic, chlorosis and necrosis. The virus has been reported sporadically on some other cultivated species of the family Fabaceae, including soybean and some common bean varieties. CPMV is transmitted by cowpea seeds, with uncertainty on the transmission rate. There is uncertainty on seed transmission by other Fabaceae host species due to lack of information. CPMV is also transmitted by several beetle species, one of which, Diabrotica virgifera virgifera, is present in the EU. Seeds for sowing of cowpea are identified as the major entry pathway. The cultivated area and production of cowpea in the EU territory are mainly limited to local varieties cultivated at a small scale in Mediterranean EU Member States. Should the pest establish in the EU, an impact is expected on cowpea crops at local scale. There is high uncertainty on the potential impact that CPMV would cause on other natural hosts cultivated in the EU due to the lack of information from the areas of CPMV's current distribution. Despite the uncertainty concerning the potential impact on bean and soybean crops in the EU, CPMV satisfies the criteria that are within the remit of EFSA to assess for it to be regarded as a potential Union quarantine pest.

Root volatile profiles and herbivore preference are mediated by maize domestication, geographic spread, and modern breeding

Domestication affected the abundances and diversity of maize root volatiles more than northward spread and modern breeding, and herbivore preference for roots was correlated with volatile diversity and herbivore resistance. Studies show that herbivore defenses in crops are mediated by domestication, spread, and breeding, among other human-driven processes. They also show that those processes affected chemical communication between crop plants and herbivores. We hypothesized that (i) preference of the herbivore (Diabrotica virgifera virgifera) larvae for embryonic roots of maize (Zea mays mays) would increase and (ii) root volatile diversity would decrease with the crop's domestication, northward spread to present-day USA, and modern breeding. We used Balsas teosinte (Zea mays parviglumis), Mexican and USA landrace maizes, and US inbred maize lines to test these hypotheses. We found that herbivore preference and volatile diversity increased with maize domestication and northward spread but decreased with modern breeding. Additionally, we found that the abundances of single volatiles did not consistently increase or decrease with maize domestication, spread, and breeding; rather, volatiles grouped per their abundances were differentially affected by those processes, and domestication had the greatest effects. Altogether, our results suggested that: the herbivore's preference for maize roots is correlated with volatile diversity and herbivore resistance; changes in abundances of individual volatiles are evident at the level of volatile groups; and maize domestication, but not spread and breeding, affected the abundances of some green leaf volatiles and sesquiterpenes/sesquiterpenoids. In part, we discussed our results in the context of herbivore defense evolution when resources for plant growth and defense vary across environments. We suggested that variability in relative abundance of volatiles may be associated with their local, functional relevance across wild and agricultural environments.

Patterns of Microbiome Composition Vary Across Spatial Scales in a Specialist Insect

Microbial communities associated with animals vary based on both intrinsic and extrinsic factors. Of many possible determinants affecting microbiome composition, host phylogeny, host diet, and local environment are the most important. How these factors interact across spatial scales is not well understood. Here, we seek to identify the main influences on microbiome composition in a specialist insect, the western corn rootworm (WCR; Diabrotica virgifera virgifera), by analyzing the bacterial communities of adults collected from their obligate host plant, corn (Zea mays), across several geographic locations and comparing the patterns in communities to its congeneric species, the northern corn rootworm (NCR; Diabrotica barberi). We found that bacterial communities of WCR and NCR shared a portion of their bacterial communities even when collected from disparate locations. However, within each species, the location of collection significantly influenced the composition of their microbiome. Correlations of geographic distance between sites with WCR bacterial community composition revealed different patterns at different spatial scales. Community similarity decreased with increased geographic distance at smaller spatial scales (~25 km between the nearest sites). At broad spatial scales (>200 km), community composition was not correlated with distances between sites, but instead reflected the historical invasion path of WCR across the United States. These results suggest bacterial communities are structured directly by dispersal dynamics at small, regional spatial scales, while landscape-level genetic or environmental differences may drive community composition across broad spatial scales in this specialist insect.

Evidence of western corn rootworm (Diabrotica virgifera virgifera LeConte) field-evolved resistance to Cry3Bb1 + Cry34/35Ab1 maize in Nebraska

BACKGROUND

Western corn rootworm (WCR; Diabrotica virgifera virgifera) field-evolved resistance to transgenic maize expressing the Cry3Bb1 protein derived from Bacillus thuringiensis (Bt) has been confirmed across the United States Corn Belt. Although use of pyramided hybrids expressing Cry3Bb1 + Cry34/35Ab1 has increased in recent years to mitigate existing WCR Bt resistance, susceptibility of Nebraska WCR populations to this rootworm-Bt pyramid has not been assessed. Plant-based bioassays were used to characterize the susceptibility of WCR populations to Cry3Bb1 and Cry3Bb1 + Cry34/35Ab1 maize. Populations were collected from areas of northeastern Nebraska with a history of planting Bt maize that expressed Cry3Bb1 and Cry34/35Ab1.

RESULTS

Significant differences in mean corrected survival among populations within Bt hybrids indicated a mosaic of WCR susceptibility to Cry3Bb1 + Cry34/35Ab1 and Cry3Bb1 maize occurred in the landscape. All field populations exhibited some level of resistance to one or both Bt hybrids when compared to susceptible laboratory control populations in bioassays. Most WCR populations exhibited incomplete resistance to Cry3Bb1 + Cry34/35Ab1 maize (92%) and complete resistance to Cry3Bb1 maize (79%).

CONCLUSION

The present study confirms the first cases of field-evolved resistance to Cry3Bb1 + Cry34/35Ab1 maize in Nebraska and documents a landscape-wide WCR Cry3Bb1 resistance pattern in areas characterized by long-term continuous maize production and associated planting of Cry3Bb1 hybrids. Use of a multi-tactic integrated pest management approach is needed in areas of continuous maize production to slow or mitigate resistance evolution to Bt maize. © 2021 Society of Chemical Industry.

RNAi for Western Corn Rootworm Management: Lessons Learned, Challenges, and Future Directions

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is considered one of the most economically important pests of maize (Zea mays L.) in the United States (U.S.) Corn Belt with costs of management and yield losses exceeding USD ~1-2 billion annually. WCR management has proven challenging given the ability of this insect to evolve resistance to multiple management strategies including synthetic insecticides, cultural practices, and plant-incorporated protectants, generating a constant need to develop new management tools. One of the most recent developments is maize expressing double-stranded hairpin RNA structures targeting housekeeping genes, which triggers an RNA interference (RNAi) response and eventually leads to insect death. Following the first description of in planta RNAi in 2007, traits targeting multiple genes have been explored. In June 2017, the U.S. Environmental Protection Agency approved the first in planta RNAi product against insects for commercial use. This product expresses a dsRNA targeting the WCR snf7 gene in combination with Bt proteins (Cry3Bb1 and Cry34Ab1/Cry35Ab1) to improve trait durability and will be introduced for commercial use in 2022.

A Random-Screening Approach to Identify RNAi Targets for the Control of Western Corn Rootworm (Diabrotica. virgifera virgifera Le Conte)

Identification of active target genes in bioassay screening is the first important step for application of RNA interference (RNAi) for pest control. Here, we describe the methodology for performing high-throughput RNAi target screening against important agriculture pest, Western corn rootworm in 96-well microplate. Two approaches are presented to identify active targets from random-cDNA library or testing a certain group of specific targets via in silico sequence analysis. Methods of PCR primer design, DNA template preparation, and dsRNA production described here can be applied for other pests.

Restoration of susceptibility following removal of selection for Cry34/35Ab1 resistance documents fitness costs in resistant population of western corn rootworm, Diabrotica virgifera virgifera

BACKGROUND

Management of the corn pest, western corn rootworm (WCR), Diabrotica virgifera virgifera (LeConte) (Coleoptera: Chrysomelidae), relies heavily on the planting of transgenic corn expressing toxins produced by the bacterium Bacillus thuringiensis (Bt). This has resulted in the evolution of resistance to all of the four commercially available Bt toxins targeting coleopteran insects. In this study, we evaluated the susceptibility of a Cry34/35Ab1-resistant WCR colony in seedling and diet toxicity assays after removal from selection for six and nine generations. In addition, female fecundity, egg fertility, adult lifespan, larval development, and adult emergence were evaluated in two Cry34/35Ab1-resistant and two susceptible WCR colonies to assess fitness costs.

RESULTS

Susceptibility to Cry34/35Ab1 was restored in a colony removed from selection after six and nine generations based on diet toxicity assays and comparisons of relative survival, head capsule width, and dry weight in plant assays. Thus, pronounced fitness costs associated with resistance to Cry34/35Ab1 were documented by susceptibility being restored within six generations. In separate studies evaluating specific fitness costs, larval fitness when reared on isoline corn did not differ between resistant and susceptible colonies. However, beetles from susceptible colonies lived longer than resistant beetles which resulted in females from susceptible colonies producing significantly more eggs than resistant colonies, with no differences in egg fertility.

CONCLUSIONS

The presence of a fitness cost that may contribute to the restoration of susceptibility to Bt has not been documented in other Cry3-resistant WCR populations and could have significant impact on the deployment of resistance management practices. Published 2021. This article is a U.S. Government work and is in the public domain in the USA.

Molecular characterization of western corn rootworm pyrethroid resistance

BACKGROUND

Western corn rootworm (WCR) pyrethroid resistance has been confirmed in the western US Corn Belt. Toxicological and biochemical studies indicated that multiple mechanisms of resistance might be involved in the resistance trait, such as enhanced metabolism and/or kdr target-site mutation(s) in the voltage-gated sodium channels. To characterize the mechanisms of WCR pyrethroid resistance at the molecular level, pairwise comparisons were made between RNA-Seq data collected from pyrethroid-resistant and -susceptible WCR populations. Gene expression levels and sodium channel sequences were evaluated.

RESULTS

Seven transcripts exhibited significantly different expression (q ≤ 0.05) when comparing field-collected pyrethroid-resistant (R-Field) and -susceptible (S-Field) WCR populations. Three of the differentially expressed transcripts were P450s overexpressed in R-Field (9.2-26.2-fold). A higher number (99) of differentially expressed transcripts was found when comparing laboratory-derived pyrethroid-resistant (R-Lab) and -susceptible (S-Lab) WCR populations. Eight of the significant transcripts were P450s overexpressed in R-Lab (2.7-39.8-fold). This study did not detect kdr mutations in pyrethroid-resistant WCR populations. Other differentially expressed transcripts that may play a role in WCR pyrethroid resistance are discussed.

CONCLUSION

This study revealed that P450-mediated metabolism is likely to be a major mechanism of WCR pyrethroid resistance, which could affect the efficacy of other insecticides sharing similar metabolic pathways. Additionally, results suggested that although laboratory selection of a pyrethroid-resistant WCR population may help to characterize resistance mechanisms, a field-selected population provided rare and perhaps major variants corresponding to the resistance trait.

Characterization, expression patterns, and transcriptional responses of three core RNA interference pathway genes from Ostrinia nubilalis

RNA interference (RNAi) is commonly used in the laboratory to analyze gene function, and RNAi-based pest management strategies are now being employed. Unfortunately, RNAi is hindered by inefficient and highly-variable results when different insects are targeted, especially lepidopterans, such as the European corn borer (ECB), Ostrinia nubilalis (Lepidoptera: Crambidae). Previous efforts to achieve RNAi-mediated gene suppression in ECB revealed low RNAi efficiency with both double-stranded RNA (dsRNA) injection and ingestion. One mechanism that can affect RNAi efficiency in insects is the expression and function of core RNAi pathway genes, such as those encoding Argonaut 2 (Ago2), Dicer 2 (Dcr2), and a dsRNA binding protein (R2D2). To determine if deficiencies in these core RNAi pathway genes contribute to low RNAi efficiency in ECB, full-length complementary DNAs encoding OnAgo2, OnDcr2, and OnR2D2 were cloned, sequenced, and characterized. A comparison of domain architecture suggested that all three predicted proteins contained the necessary domains to function. However, a comparison of evolutionary distances revealed potentially important variations in the first RNase III domain of OnDcr2, the double-stranded RNA binding domains of OnR2D2, and both the PAZ and PIWI domains of OnAgo2, which may indicate functional differences in enzymatic activity between species. Expression analysis indicated that transcripts for all three genes were expressed in all developmental stages and tissues investigated. Interestingly, the introduction of non-target dsRNA into ECB second-instar larvae via microinjection did not affect OnAgo2, OnDcr2, or OnR2D2 expression. In contrast, ingestion of the same dsRNAs resulted in upregulation of OnDcr2 but downregulation of OnR2D2. The unexpected transcriptional responses of the core machinery and the divergence in amino-acid sequence between specific domains in each core RNAi protein may possibly contribute to low RNAi efficiency in ECB. Understanding the contributions of different RNAi pathway components is critical to adapting this technology for use in controlling lepidopteran pests that exhibit low RNAi efficiency.

The Use of Insecticides to Manage the Western Corn Rootworm, Diabrotica virgifera virgifera, LeConte: History, Field-Evolved Resistance, and Associated Mechanisms

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Dvv) is a significant insect pest of maize in the United States (U.S.). This paper reviews the history of insecticide use in Dvv management programs, Dvv adaptation to insecticides, i.e., field-evolved resistance and associated mechanisms of resistance, plus the current role of insecticides in the transgenic era. In the western U.S. Corn Belt where continuous maize is commonly grown in large irrigated monocultures, broadcast-applied soil or foliar insecticides have been extensively used over time to manage annual densities of Dvv and other secondary insect pests. This has contributed to the sequential occurrence of Dvv resistance evolution to cyclodiene, organophosphate, carbamate, and pyrethroid insecticides since the 1950s. Mechanisms of resistance are complex, but both oxidative and hydrolytic metabolism contribute to organophosphate, carbamate, and pyrethroid resistance facilitating cross-resistance between insecticide classes. History shows that Dvv insecticide resistance can evolve quickly and may persist in field populations even in the absence of selection. This suggests minimal fitness costs associated with Dvv resistance. In the transgenic era, insecticides function primarily as complementary tools with other Dvv management tactics to manage annual Dvv densities/crop injury and resistance over time.

Models of Diabrotica Populations: Demography, Population Genetics, Geographic Spread, and Management

Simple Summary

Two beetles that are serious pests of maize, Diabrotica virgifera virgifera and Diabrotica barberi, have caused problems for farmers in the USA and Europe for many years. Because both species have developed resistance to several management tactics, including insecticides and crop rotation, mathematical modeling has been used to evaluate their life cycles for weaknesses and new tactics for value. This review highlights lessons learned from the past 35 years. Some models have focused on the probability of the beetles spreading across regions. Other models have been developed to estimate the risk of the evolution of resistance. These models are thoroughly reviewed with respect to the biological attributes incorporated in these models and the impact of those attributes on the evolution of resistance.

Abstract

Both Diabrotica virgifera virgifera LeConte and D. barberi Smith and Lawrence are among the most damaging insects impacting corn in North America. D. virgifera virgifera has also invaded Europe and has become an important pest in that region. Computer models have become an important tool for understanding the impact and spread of these important pests. Over the past 30 years, over 40 models have been published related to these pests. The focus of these models range from occupancy models (particularly for Europe), impact of climate change, range expansion, economics of pest management, phenology, to the evolution of resistance to toxins and crop rotation. All of these models share characteristics. We elaborate on the methods in which modelers have incorporated the biology of these pests, including density-dependence, movement, fecundity and overwintering mortality. We discuss the utility of both spatially-explicit, complex models and spatially-implicit, generational models and where each might be appropriate. We review resistance models that either explain past evolution to crop rotation, insecticides or insecticidal traits or attempt to predict the consequences of resistance management strategies.

Stability of double-stranded RNA in gut contents and hemolymph of Ostrinia nubilalis larvae

RNA interference (RNAi) is a revolutionary technique for silencing gene expression, but the success of this technique is dependent upon the stability of double-stranded RNA (dsRNA) molecules. In many insects, especially lepidopteran species, RNAi efficiency is limited by high instability of dsRNA in the gut and/or hemolymph, preventing the development of RNAi-based strategies for many serious pests. Previous attempts to perform RNAi on Ostrinia nubilalis (ECB, Lepidoptera: Crambidae) indicate low RNAi efficiency with both dsRNA injection and feeding. To investigate the contribution of dsRNA instability to low RNAi efficiency in ECB, a serious of ex vivo incubation experiments were performed where dsRNA integrity was assessed following incubation in larval gut continents and hemolymph using gel electrophoresis or RT-qPCR. DsRNA was less stable in the gut contents from ECB than in gut contents from Diabrotica virgifera virgifera, a coleopteran exhibiting high RNAi efficiency. Furthermore, characterization of dsRNA stability in ECB gut contents and hemolymph revealed that dsRNA was rapidly degraded under physiologically relevant conditions as a result of enzymatic activity that was neither size- nor sequence-dependent. These findings suggest that instability of dsRNA in ECB tissues is a contributing factor to the poor efficiency of RNAi in this pest. This work advances our understanding of mechanisms impacting RNAi efficiency in ECB and related lepidopteran insects for which novel pest management strategies are needed, and may facilitate the development of strategies for enhancing dsRNA stability in ECB tissues.

Enhanced metabolism and selection of pyrethroid-resistant western corn rootworms (Diabrotica virgifera virgifera LeConte)

Western corn rootworm (WCR) pyrethroid resistance has been previously reported in the United States (US) western Corn Belt, and cross-resistance and synergism studies suggested that both target site insensitivity and enhanced metabolism may be conferring WCR resistance to pyrethroids. The present study aimed to investigate the potential mechanisms of WCR pyrethroid resistance and to estimate the heritability of the resistance trait. Biochemical assays using model substrates and spectrophotometry revealed 2-4-fold higher activity of P450s and esterases in pyrethroid-resistant WCR populations, whereas the biological activity of glutathione S-transferase was similar between populations tested. No mutation in the voltage-gated sodium channel was detected in pyrethroid-resistant WCR individuals by sequencing PCR products containing the para-homologous L1014, T929, and M918 amino acid positions that are commonly associated with target site mutations in other pyrethroid-resistant insects. A pilot estimation of pyrethroid resistance heritability obtained during laboratory selection of a WCR population suggested a major genetic component of the resistance trait and predicted a 10-fold increase in WCR bifenthrin resistance within ~7 generations of insecticide lethal exposure. Results support earlier indirect evidence that enhanced metabolism may be contributing to WCR resistance to pyrethroids and illustrates the potential of WCR pyrethroid resistance evolution.

Field efficacy of soil insecticides on pyrethroid-resistant western corn rootworms (Diabrotica virgifera virgifera LeConte)

BACKGROUND

Field-evolved pyrethroid resistance has been confirmed in western corn rootworm (WCR) populations collected from the United States (US) western Corn Belt. Resistance levels of WCR adults estimated in lab bioassays were confirmed to significantly reduce the efficacy of foliar-applied bifenthrin. The objective of the present study was to investigate the impact of WCR pyrethroid resistance levels on the performance of common soil-applied insecticide formulations (23.4% tefluthrin, 17.15% bifenthrin, and 0.1% cyfluthrin + 2.0% tebupirimphos). Field trials were conducted in 2016 and 2017 in three Nebraska, US, counties (Saunders, Clay, and Keith) where distinct levels of WCR susceptibility to pyrethroids (susceptible, moderately resistant, and highly resistant) had been previously reported in adult and larval bioassays.

RESULTS

All soil insecticide treatments effectively protected maize roots from a pyrethroid-susceptible WCR population at Saunders. In contrast, the efficacy of bifenthrin and tefluthrin soil insecticides was significantly reduced at Clay and Keith, where pyrethroid-resistant WCR populations were reported. At Keith, where an additional failure of the cyfluthrin + tebupirimphos soil insecticide was observed, WCR laboratory dose-response bioassays showed a consistent ∼5-fold resistance level to the active ingredients bifenthrin, tefluthrin, and cyfluthrin.

CONCLUSION

The efficacy of common soil insecticides used in the US for WCR management was significantly reduced in populations exhibiting relatively low levels of WCR pyrethroid resistance. Using a multitactical approach to manage WCR within an integrated pest management framework may mitigate resistance evolution and prolong the usefulness of WCR insecticides within the system. © 2019 Society of Chemical Industry.

Resistance and Tolerance to Root Herbivory in Maize Were Mediated by Domestication, Spread, and Breeding

Plants may defend against herbivory and disease through various means. Plant defensive strategies against herbivores include resistance and tolerance, which may have metabolic costs that affect plant growth and reproduction. Thus, expression of these strategies may be mediated by a variety of factors, such as resource availability, herbivory pressure, and plant genetic variation, among others. Additionally, artificial selection by farmers and systematic breeding by scientists may mediate the expression of resistance and tolerance in crop plants. In this study, we tested whether maize defense against Western corn rootworm (WCR) was mediated by the crop's domestication, spread, and modern breeding. We expected to find a trend of decreasing resistance to WCR with maize domestication, spread, and breeding, and a trend of increasing tolerance with decreasing resistance. To test our expectations, we compared resistance and tolerance among four Zea plants spanning those processes: Balsas teosinte, Mexican landrace maize, US landrace maize, and US inbred maize. We measured the performance of WCR larvae as a proxy for plant resistance, and plant growth as affected by WCR feeding as a proxy for plant tolerance. Our results showed that domestication and spread decreased maize resistance to WCR, as expected, whereas breeding increased maize resistance to WCR, contrary to expected. Our results also showed that maize resistance and tolerance to WCR are negatively correlated, as expected. We discussed our findings in relation to ecological-evolutionary hypotheses seeking to explain defense strategy evolution in the contexts of plant resistance-productivity trade-offs, plant tolerance-resistance trade-offs, and varying resource availability vis-à-vis plant physiological stress and herbivory pressure. Finally, we suggested that defense strategy evolution in maize, from domestication to the present, is predicted by those ecological-evolutionary hypotheses.

Responses of two ladybird beetle species (Coleoptera: Coccinellidae) to dietary RNAi

BACKGROUND

One concern with the adoption of RNAi-based genetically engineered (GE) crops is the potential harm to valued non-target organisms. Species of Coccinellidae (Coleoptera) are important natural enemies and might be exposed to the insecticidal dsRNA produced by the plant. To assess their susceptibility to dietary RNAi, we fed Adalia bipunctata and Coccinella septempunctata with a dsRNA designed to target the vATPase A of the western corn rootworm, Diabrotica virgifera virgifera (Dvv dsRNA). Specific dsRNAs designed to target the vATPase A of the two ladybird beetle species served as positive controls.

RESULTS

Our results revealed that both species were sensitive to dietary RNAi when ingesting their own dsRNAs, with C. septempunctata being more sensitive than A. bipunctata. Dvv dsRNA also adversely affected the two ladybird beetles as indicated by a significantly (but marginally) prolonged developmental time for A. bipunctata and a significantly reduced survival rate for C. septempunctata. These results, however, were obtained at Dvv dsRNA concentrations that were orders of magnitude higher than expected to occur in the field. Gene expression analyses confirmed the bioactivity of the dsRNA treatments and the results from the feeding bioassays. These results are consistent with the bioinformatics analyses, which revealed a higher number of 21-nucleotide-long matches, a requirement for effective RNAi, of the Dvv dsRNA with the vATPase A of C. septempunctata (34 matches) than with that of A. bipunctata (six matches).

CONCLUSION

Feeding bioassays revealed that two ladybird species are responsive to dietary RNAi. The two species, however, differed in their sensitivity. © 2019 Society of Chemical Industry.

Modeling effects of ecological factors on evolution of polygenic pesticide resistance

Widespread use of pesticides has resulted in the evolution of resistance in many insect pests worldwide limiting their use in pest control. Effective pest and resistance management practices require understanding of the genetics of resistance and of the life history of the pest. Most models for pesticide resistance assume that resistance is monogenic, conferred by a single gene. However, resistance could evolve as a polygenic quantitative trait resulting from the action of several genes, especially when pesticide dose is low. Further, fitness of the pest could be density-dependent and might depend upon abiotic factors such as temperature. It is not known how these factors affect the evolution of polygenic resistance or pest population dynamics when resistance evolves. We use the western corn rootworm, Diabrotica virgifera virgifera, as a case study and use data on density-dependent survival, heritability and survival rates on the transgenic Cry3Bb1 toxin and corresponding LC₅₀ values, to model polygenic resistance to Cry3Bb1. We found that LC₅₀ increased rapidly even at doses that produced a mortality of less than 99.9%. However, survival reached 100% only when mortality was as high as 99.9%. Fast response to high selection pressure produced cyclical larval densities while lower selection pressures produced equilibrium densities. Interestingly we found that a relatively low density observed in a population may not be evidence for a low survival to the pesticide. As a consequence we found that larger refuges might not necessarily help in reducing pest densities especially when pesticide mortality is low. This effect, arising from the tradeoff between response to selection and density dependence, calls for careful assessment of the evolution of resistance based on change in survival as well as on pest densities. When selection pressure is low a lower initial density resulted in a larger response to selection. Finally, we showed that populations with shorter developmental times developed resistance faster initially irrespective of selection pressure. However, when selection pressure is low survival eventually became higher in populations with longer developmental times. Since developmental time depends on degree days spatio-temporal variation in temperature could be an important factor in polygenic resistance evolution.

Potential global distribution of Diabrotica species and the risks for agricultural production

BACKGROUND

Despite efforts in the last few decades to prevent biological invasions, agricultural pests continue to spread as a result of globalization and international trade. This study was conducted to identify suitable areas for the occurrence of four Diabrotica species and to assess the potential impact of these species in a scenario of invasion followed by spread throughout the estimated suitable regions.

RESULTS

Our findings reveal that a large proportion of the suitable areas for Diabrotica species overlap with cultivated areas. Niche analyses also demonstrated that these species occupy a small proportion of the suitable habitats available to them, indicating that, if new areas are invaded, there is a risk of spread throughout adjacent regions.

CONCLUSION

Most of the suitable areas for Diabrotica species overlap with highly productive agricultural areas, suggesting that a potential spread of these species may cause economic loss. Our study provides a valuable contribution to the development of tools aiming to predict the potential spread of these species throughout the world. © 2018 Society of Chemical Industry.

Transgenic Bt Corn, Soil Insecticide, and Insecticidal Seed Treatment Effects on Corn Rootworm (Coleoptera: Chrysomelidae) Beetle Emergence, Larval Feeding Injury, and Corn Yield in North Dakota

Northern, Diabrotica barberi Smith & Lawrence (Coleoptera: Chrysomelidae), and western, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), corn rootworms are economic pests of corn, Zea mays L. in North America. We measured the impacts of corn hybrids incorporated with Cry3Bb1, Cry34/35Ab1, and pyramided (Cry3Bb1 + Cry34/35Ab1) Bacillus thuringiensis Berliner (Bt) proteins, tefluthrin soil insecticide, and clothianidin insecticidal seed treatment on beetle emergence, larval feeding injury, and corn yield at five locations from 2013 to 2015 in eastern North Dakota. In most cases, emergence was significantly lower in Bt-protected corn than in non-Bt corn hybrids. Exceptions included Wyndmere, ND (2013), where D. barberi emergence from Cry34/35Ab1 plots was not different from that in the non-Bt hybrid, and Arthur, ND (2013), where D. v. virgifera emergence from Cry3Bb1 plots did not differ from that in the non-Bt hybrid. Bt hybrids generally produced increased grain yield compared with non-Bt corn where rootworm densities were high, and larval root-feeding injury was consistently lower in Bt-protected plots than in non-Bt corn. The lowest overall feeding injury and emergence levels occurred in plots planted with the Cry3Bb1 + Cry34/35Ab1 hybrid. Time to 50% cumulative emergence of both species was 5-7 d later in Bt-protected than in non-Bt hybrids. Tefluthrin and clothianidin were mostly inconsequential in relation to beetle emergence and larval root injury. Our findings could suggest that some North Dakota populations could be in early stages of increased tolerance to some Bt toxins; however, Bt corn hybrids currently provide effective protection against rootworm injury in eastern North Dakota.

Quantitative Trait Loci Mapping of Western Corn Rootworm (Coleoptera: Chrysomelidae) Host Plant Resistance in Two Populations of Doubled Haploid Lines in Maize (Zea mays L.)

Over the last 70 yr, more than 12,000 maize accessions have been screened for their level of resistance to western corn rootworm, Diabrotica virgifera virgifera (LeConte; Coleoptera: Chrysomelidae), larval feeding. Less than 1% of this germplasm was selected for initiating recurrent selection or other breeding programs. Selected genotypes were mostly characterized by large root systems and superior root regrowth after root damage caused by western corn rootworm larvae. However, no hybrids claiming native (i.e., host plant) resistance to western corn rootworm larval feeding are currently commercially available. We investigated the genetic basis of western corn rootworm resistance in maize materials with improved levels of resistance using linkage disequilibrium mapping approaches. Two populations of topcrossed doubled haploid maize lines (DHLs) derived from crosses between resistant and susceptible maize lines were evaluated for their level of resistance in three to four different environments. For each DHL topcross an average root damage score was estimated and used for quantitative trait loci (QTL) analysis. We found genomic regions contributing to western corn rootworm resistance on all maize chromosomes, except for chromosome 4. Models fitting all QTL simultaneously explained about 30 to 50% of the genotypic variance for root damage scores in both mapping populations. Our findings confirm the complex genetic structure of host plant resistance against western corn rootworm larval feeding in maize. Interestingly, three of these QTL regions also carry genes involved in ascorbate biosynthesis, a key compound we hypothesize is involved in the expression of western corn rootworm resistance.

Sequestration and activation of plant toxins protect the western corn rootworm from enemies at multiple trophic levels

Highly adapted herbivores can phenocopy two-component systems by stabilizing, sequestering and reactivating plant toxins. However, whether these traits protect herbivores against their enemies is poorly understood. We demonstrate that the western corn rootworm Diabrotica virgifera virgifera, the most damaging maize pest on the planet, specifically accumulates the root-derived benzoxazinoid glucosides HDMBOA-Glc and MBOA-Glc. MBOA-Glc is produced by D. virgifera through stabilization of the benzoxazinoid breakdown product MBOA by N-glycosylation. The larvae can hydrolyze HDMBOA-Glc, but not MBOA-Glc, to produce toxic MBOA upon predator attack. Accumulation of benzoxazinoids renders D. virgifera highly resistant to nematodes which inject and feed on entomopathogenic symbiotic bacteria. While HDMBOA-Glc and MBOA reduce the growth and infectivity of both the nematodes and the bacteria, MBOA-Glc repels infective juvenile nematodes. Our results illustrate how herbivores combine stabilized and reactivated plant toxins to defend themselves against a deadly symbiosis between the third and the fourth trophic level enemies.

The western corn rootworm is the most damaging pest of maize plants. Out of sight, the larvae of this beetle feed on maize roots, and cause billions of dollars worth of losses each year. One of the reasons why this pest remains such a problem is it can adapt and resist many crop protection strategies.

Biological control refers to combating a pest using its own natural enemies – for example, its predators. Biological control of the western corn rootworm has been attempted using nematode worms. Normally, the nematodes locate and enter an insect larvae, release bacteria that kill it, and then feed and multiply within the dead larvae. Yet, the western corn rootworm seems at least partly able to resist these nematodes, and the success of biological control in the field has been variable.

Several insect herbivores are known to accumulate, or sequester, plant toxins in their own body for self-defense. Previously, in 2012, researchers reported that the western corn rootworm is resistant and attracted to the major toxins in maize roots, the benzoxazinoids. The blood-like fluid of the western corn rootworm also repels many predators. Could the western corn rootworm be sequestering maize benzoxazinoids to resist the biological control of nematodes and their bacterial partners?

Plants store benzoxazinoids in a non-toxic form. If herbivores damage the plant, these molecules quickly break down into compounds that are toxic to most insects. Now Robert et al. – who include two of the researchers involved in the 2012 study – show that the western corn rootworm uses a similar defense system to protect itself against biological control nematodes and their bacterial partners. First, the larvae convert a benzoxazinoid breakdown product by adding a glucose molecule. They then release large amounts of this modified molecule to repel young nematodes. Second, via an unknown mechanism, the larvae stabilize a second plant-derived benzoxazinoid, sequester its non-toxic form in their bodies, and activate it upon nematode attack. The resulting toxins can kill both nematodes and their bacterial partners. By combining different chemical strategies to stabilize and activate plant toxins, the western corn rootworm is able to resist the nematodes used for biological control.

These findings can help to explain why biological control has had limited success against the western corn rootworm. In the long run, they may lead to more effective biological control programs, for instance by stopping the western corn rootworm from sequestering benzoxazinoids or by using natural enemies that are resistant to the insect’s toxins.

A Maize Inbred Exhibits Resistance Against Western Corn Rootwoorm, Diabrotica virgifera virgifera

Insect resistance against root herbivores like the western corn rootworm (WCR, Diabrotica virgifera virgifera) is not well understood in non-transgenic maize. We studied the responses of two American maize inbreds, Mp708 and Tx601, to WCR infestation using biomechanical, molecular, biochemical analyses, and laser ablation tomography. Previous studies performed on several inbreds indicated that these two maize genotypes differed in resistance to pests including fall armyworm (Spodoptera frugiperda) and WCR. Our data confirmed that Mp708 shows resistance against WCR, and demonstrates that the resistance mechanism is based in a multi-trait phenotype that includes increased resistance to cutting in nodal roots, stable root growth during insect infestation, constitutive and induced expression of known herbivore-defense genes, including ribosomal inhibitor protein 2 (rip2), terpene synthase 23 (tps23) and maize insect resistance cysteine protease-1 (mir1), as well high constitutive levels of jasmonic acid and production of (E)-β-caryophyllene. In contrast, Tx601 is susceptible to WCR. These findings will facilitate the use of Mp708 as a model to explore the wide variety of mechanisms and traits involved in plant defense responses and resistance to herbivory by insects with several different feeding habits.

Assessment of Inheritance and Fitness Costs Associated with Field-Evolved Resistance to Cry3Bb1 Maize by Western Corn Rootworm

The western corn rootworm, Diabrotica virgifera virgifera LeConte, is among the most serious insect pests of maize in North America. One strategy used to manage this pest is transgenic maize that produces one or more crystalline (Cry) toxins derived from the bacterium Bacillus thuringiensis (Bt). To delay Bt resistance by insect pests, refuges of non-Bt maize are grown in conjunction with Bt maize. Two factors influencing the success of the refuge strategy to delay resistance are the inheritance of resistance and fitness costs, with greater delays in resistance expected when inheritance of resistance is recessive and fitness costs are present. We measured inheritance and fitness costs of resistance for two strains of western corn rootworm with field-evolved resistance to Cry3Bb1 maize. Plant-based and diet-based bioassays revealed that the inheritance of resistance was non-recessive. In a greenhouse experiment, in which larvae were reared on whole maize plants in field soil, no fitness costs of resistance were detected. In a laboratory experiment, in which larvae experienced intraspecific and interspecific competition for food, a fitness cost of delayed larval development was identified, however, no other fitness costs were found. These findings of non-recessive inheritance of resistance and minimal fitness costs, highlight the potential for the rapid evolution of resistance to Cry3Bb1 maize by western corn rootworm, and may help to improve resistance management strategies for this pest.

Histopathological Effects of Bt and TcdA Insecticidal Proteins on the Midgut Epithelium of Western Corn Rootworm Larvae (Diabrotica virgifera virgifera)

Western corn rootworm (WCR, Diabrotica virgifera virgifera LeConte) is a major corn pest in the United States, causing annual losses of over $1 billion. One approach to protect against crop loss by this insect is the use of transgenic corn hybrids expressing one or more crystal (Cry) proteins derived from Bacillus thuringiensis. Cry34Ab1 and Cry35Ab1 together comprise a binary insecticidal toxin with specific activity against WCR. These proteins have been developed as insect resistance traits in commercialized corn hybrids resistant to WCR feeding damage. Cry34/35Ab1 is a pore forming toxin, but the specific effects of Cry34/35Ab1 on WCR cells and tissues have not been well characterized microscopically, and the overall histopathology is poorly understood. Using high-resolution resin-based histopathology methods, the effects of Cry34/35Ab1 as well as Cry3Aa1, Cry6Aa1, and the Photorhabdus toxin complex protein TcdA have been directly visualized and documented. Clear symptoms of intoxication were observed for all insecticidal proteins tested, including swelling and sloughing of enterocytes, constriction of midgut circular muscles, stem cell activation, and obstruction of the midgut lumen. These data demonstrate the effects of these insecticidal proteins on WCR midgut cells, and the collective response of the midgut to intoxication. Taken together, these results advance our understanding of the insect cell biology and pathology of these insecticidal proteins, which should further the field of insect resistance traits and corn rootworm management.

Effects of Bt Corn and Egg Density on Western Corn Rootworm (Coleoptera: Chrysomelidae) Adult Emergence and Estimation of Effective Bt Dose

Since 2003, rootworm-protected transgenic corn has been commercially deployed in the United States as a principal method of control of western corn rootworm, Diabrotica virgifera virgifera LeConte. Durability of this technology depends partly on larval mortality ("dose") exerted by the traits, but density-dependent mortality can confound calculations of dose. Research reported here examined the effects of density-dependent mortality on adult emergence and estimates of trait dose. At sites in Illinois and Indiana, western corn rootworm eggs were infested at four densities on non-Bt corn and at a single density on corn hybrids with transgenic events MON 88017 (VT Triple PRO), DAS-59122-7 (Herculex Insect Protection), and MON 88017 × DAS-59122-7 (SmartStax corn). Beetles were collected weekly in large emergence cages. Density-dependent mortality and the effect of Bt traits were examined using percent survival from egg to adult, sex ratio, and beetle mass. Beetle emergence from Bt treatments was very low, and percent survival from non-Bt treatments was greatest at the lowest egg density (410 eggs per row-meter). Therefore, emergence from the lowest infestation density on non-Bt corn was used to estimate the effective dose of the Bt treatments. Sex ratio and beetle mass were unaffected by density-dependent effects and were not consistently affected by Bt traits. Dose was estimated at 97.4-99.3% for MON 88017, 98.8-99.9% for DAS-59122-7, and 99.7-100.0% for MON 88017 × DAS-59122-7. This study confirms the need to account for density-dependent mortality when estimating dose of corn rootworm protection events even at relatively low egg infestation densities.

Patterns of Gene Expression in Western Corn Rootworm (Diabrotica virgifera virgifera) Neonates, Challenged with Cry34Ab1, Cry35Ab1 and Cry34/35Ab1, Based on Next-Generation Sequencing

With Next Generation Sequencing technologies, high-throughput RNA sequencing (RNAseq) was conducted to examine gene expression in neonates of Diabrotica virgifera virgifera (LeConte) (Western Corn Rootworm, WCR) challenged with individual proteins of the binary Bacillus thuringiensis insecticidal proteins, Cry34Ab1 and Cry35Ab1, and the combination of Cry34/Cry35Ab1, which together are active against rootworm larvae. Integrated results of three different statistical comparisons identified 114 and 1300 differentially expressed transcripts (DETs) in the Cry34Ab1 and Cry34/35Ab1 treatment, respectively, as compared to the control. No DETs were identified in the Cry35Ab1 treatment. Putative Bt binding receptors previously identified in other insect species were not identified in DETs in this study. The majority of DETs (75% with Cry34Ab1 and 68.3% with Cry34/35Ab1 treatments) had no significant hits in the NCBI nr database. In addition, 92 DETs were shared between Cry34Ab1 and Cry34/35Ab1 treatments. Further analysis revealed that the most abundant DETs in both Cry34Ab1 and Cry34/35Ab1 treatments were associated with binding and catalytic activity. Results from this study confirmed the nature of these binary toxins against WCR larvae and provide a fundamental profile of expression pattern of genes in response to challenge of the Cry34/35Ab1 toxin, which may provide insight into potential resistance mechanisms.

Knockdown of genes in the Toll pathway reveals new lethal RNA interference targets for insect pest control

RNA interference (RNAi) is a promising alternative strategy for ecologically friendly pest management. However, the identification of RNAi candidate genes is challenging owing to the absence of laboratory strains and the seasonality of most pest species. Tribolium castaneum is a well-established model, with a strong and robust RNAi response, which can be used as a high-throughput screening platform to identify potential RNAi target genes. Recently, the cactus gene was identified as a sensitive RNAi target for pest control. To explore whether the spectrum of promising RNAi targets can be expanded beyond those found by random large-scale screening, to encompass others identified using targeted knowledge-based approaches, we constructed a Cactus interaction network. We tested nine genes in this network and found that the delivery of double-stranded RNA corresponding to fusilli and cactin showed lethal effects. The silencing of cactin resulted in 100% lethality at every developmental stage from the larva to the adult. The knockdown of pelle, Dorsal-related immunity factor and short gastrulation reduced or even prevented egg hatching in the next generation. The combination of such targets with lethal and parental RNAi effects can now be tested against different pest species in field studies.

Host Recognition Responses of Western (Family: Chrysomelidae) Corn Rootworm Larvae to RNA Interference and Bt Corn

Western corn rootworm Diabrotica virgifera virgifera LeConte is an important pest of corn whose larvae exhibit particular quantifiable patterns of locomotion after exposure to, and removal from, host roots and nonhost roots. Using EthoVision software, the behavior and locomotion of the western corn rootworm larvae was analyzed to determine the level of host recognition to germinated roots of differing corn hybrids containing either rootworm targeted Bt genes, RNA interference (RNAi) technology, the stack of both Bt and RNAi, or the isoline of these. The behavior of the rootworm larvae indicated a significant host preference response to all corn hybrids (with or without insecticidal traits) compared to the filter paper and oat roots. A weaker host response to the RNAi corn roots was observed in the susceptible larvae when compared to the resistant larvae, but not for the Bt + RNAi vector stack. Additionally, the resistant larvae demonstrated a weaker host response to the isoline corn roots when compared to the susceptible larvae. Although weaker, these host responses were significantly different from those observed in the negative controls, indicating that all hybrids tested do contain the contact cues necessary to elicit a host preference response by both Cry3Bb1-resistant and Cry3Bb1-susceptible larvae that would work to hinder resistance development in refuge in a bag fields.

Dietary Risk Assessment of v-ATPase A dsRNAs on Monarch Butterfly Larvae

By suppressing the expression of genes with essential biological functions, in planta RNAi can negatively affect the development and survival of target pests. As a part of a concerted effort to assess the risks of RNAi transgenic crops on non-target organisms, we developed an in vivo toxicity assay to examine the impacts of ingested dsRNAs incurred to the monarch butterfly, Danaus plexippus (L.), an iconic eco-indicator in North America. To create the worst case scenario, the full-length v-ATPase A cDNAs from the target pest, western corn rootworm, Diabrotica virgifera virgifera, and the non-target D. plexippus were respectively cloned. A 400 bp fragment with the highest sequence similarity between the two species was used as the template to synthesize dsRNAs for the subsequent dietary RNAi toxicity assay. Specifically, newly hatched neonates were provisioned with leaf disks surface-coated with v-ATPase A dsRNAs synthesized from D. v. virgifera and D. plexippus, respectively, a control dsRNA, β-glucoruronidase, from plants, and H₂O. The endpoint measurements included gene expressions and life history traits. The 2283 bp D. plexippus v-ATPase A cDNA contains a 99 bp 5'-untranslated region, a 330 bp 3'-untranslated region, and an 1851 bp ORF encoding 617 amino acids. The temporal RNAi study did not detect any impact to D. plexippus v-ATPase A expression by the assay days and treatments. This was reflected in the phenotypic impacts of dietary RNAi, in which both survival rate and development time were not affected by the uptake of ingested dsRNAs. These combined results suggest that D. plexippus larvae are not susceptible to dietary RNAi, therefore, the impact of transgenic RNAi plants on this non-target organism is, likely, negligible.

Development of a CO2 -releasing coformulation based on starch, Saccharomyces cerevisiae and Beauveria bassiana attractive towards western corn rootworm larvae

BACKGROUND

CO₂ is known as an attractant for many soil-dwelling pests. To implement an attract-and-kill strategy for soil pest control, CO₂ -emitting formulations need to be developed. The aim of the present work was to develop a slow-release bead system in order to bridge the gap between application and hatching of western corn rootworm larvae.

RESULTS

We compared different Ca-alginate beads containing Saccharomyces cerevisiae for their potential to release CO₂ over a period of several weeks. The addition of starch improved CO₂ release, resulting in significantly higher CO₂ concentrations in soil for at least 4 weeks. The missing amylase activity was compensated for either by microorganisms present in the soil or by coencapsulation of Beauveria bassiana. Formulations containing S. cerevisiae, starch and B. bassiana were attractive for western corn rootworm larvae within the first 4 h following exposure; however, when considering the whole testing period, the maize root systems remained more attractive for the larvae.

CONCLUSION

Coencapsulation of S. cerevisiae, starch and B. bassiana is a promising approach for the development of attractive formulations for soil applications. For biological control strategies, the attractiveness needs to be increased by phagostimuli to extend contact between larvae and the entomopathogenic fungus growing out of these formulations. © 2016 Society of Chemical Industry.

Effects of Field History on Corn Root Injury and Adult Abundance of Northern and Western Corn Rootworm (Coleoptera: Chrysomelidae)

Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), and northern corn rootworm, Diabrotica barberi Smith & Lawrence, are major pests of corn (Zea mays L.). Corn producing Bacillus thuringiensis (Bt) toxins are widely used to manage Diabrotica spp.; however, Bt resistance by D. v. virgifera has led to high levels of feeding injury in the field. We tested whether field history affected root injury and abundance of adult Diabrotica spp. In 2013 and 2014, four types of cornfields were sampled: 1) recently rotated fields, 2) continuous cornfields, 3) fields with a history of injury to Bt corn (past problem fields), and 4) fields with greater than one node of injury to Bt corn at the time of sampling (current problem fields). Data were collected on field history, root injury, and the abundance of adult Diabrotica spp. from each field. Root injury and the abundance of D. v. virgifera were significantly greater in current problem fields compared to the other field types, while D. barberi were significantly more abundant in recently rotated fields. Root injury and the abundance of D. v. virgifera did not differ among recently rotated fields, continuous cornfields, and past problem fields. Analysis of field history showed that recently rotated fields were characterized by significantly less Bt corn, soil-applied insecticides, and years planted to corn continuously. These results suggest that greater cropping practice diversity can reduce management inputs for Diabrotica spp.; however, its effects on resistance evolution remain undetermined.

RNAi induced knockdown of a cadherin-like protein (EF531715) does not affect toxicity of Cry34/35Ab1 or Cry3Aa to Diabrotica virgifera virgifera larvae (Coleoptera: Chrysomelidae)

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, is an important maize pest throughout most of the U.S. Corn Belt. Bacillus thuringiensis (Bt) insecticidal proteins including modified Cry3Aa and Cry34/35Ab1 have been expressed in transgenic maize to protect against WCR feeding damage. To date, there is limited information regarding the WCR midgut target sites for these proteins. In this study, we examined whether a cadherin-like gene from Diabrotica virgifera virgifera (DvvCad; GenBank accession # EF531715) associated with WCR larval midgut tissue is necessary for Cry3Aa or Cry34/35Ab1 toxicity. Experiments were designed to examine the sensitivity of WCR to trypsin activated Cry3Aa and Cry34/35Ab1 after oral feeding of the DvvCad dsRNA to knockdown gene expression. Quantitative real-time PCR confirmed that DvvCad mRNA transcript levels were reduced in larvae treated with cadherin dsRNA. Relative cadherin expression by immunoblot analysis and nano-liquid chromatography - mass spectrometry (nanoLC-MS) of WCR neonate brush border membrane vesicle (BBMV) preparations exposed to DvvCad dsRNA confirmed reduced cadherin expression when compared to BBMV from untreated larvae. However, the larval mortality and growth inhibition of WCR neonates exposed to cadherin dsRNA for two days followed by feeding exposure to either Cry3Aa or Cry34/35Ab1 for four days was not significantly different to that observed in insects exposed to either Cry3Aa or Cry34/35Ab1 alone. In combination, these results suggest that cadherin is unlikely to be involved in the toxicity of Cry3Aa or Cry34/35Ab1 to WCR.

Western Corn Rootworm (Coleoptera: Chrysomelidae) Larval Movement in eCry3.1Ab+mCry3A Seed Blend Scenarios

Corn fields planted with plant-incorporated Bacillus thuringiensis (Bt) proteins must have a portion of the field planted with non-Bt, isoline, plants that serve as a refuge for susceptible insects. In the Corn Belt, refuge seeds are now blended in the bag with Bt seeds for corn hybrids containing two or more toxins targeted toward the western corn rootworm, Diabrotica virgifera virgifera LeConte. Syngenta's corn hybrid, Agrisure Duracade, containing the eCry3.1Ab (event 5307) and mCry3a (event MIR604) rootworm-targeted toxins were registered as a seed blend in 2014. Western corn rootworm larval movement between the refuge plants and the Duracade plants was assessed to determine western corn rootworm survival and amount of root damage on these plants when planted in all possible seed blend scenarios. In this study, western corn rootworm larvae moved between isoline and Bt plants and adult survival was greater on Bt plants if movement from a neighboring infested isoline plant had occurred. However, root damage to these Bt plants did not reach economic levels. The low numbers of western corn rootworm larvae that did move from an infested Bt plant to an isoline plant could potentially select for resistance if they survived to adulthood.

Methyl Anthranilate as a Repellent for Western Corn Rootworm Larvae (Coleoptera: Chrysomelidae)

Methyl anthranilate was identified as the active compound in extracts of maize (Zea mays L.) roots that were shown to be repellent to neonate western corn rootworm (Diabrotica virgifera virgifera LeConte) larvae. A bioassay-driven approach was used to isolate the active material from diethyl ether extracts of roots from germinating maize seeds. Separation of the extract on a Florisil column yielded an active fraction of 90:10 hexane:diethyl ether. Analysis with gas chromatography-mass spectrometry identified two compounds in the active fraction: indole (2,3-benzopyrrole) and methyl anthranilate (methyl 2-aminobenzoate). When tested in behavioral bioassays, methyl anthranilate elicited a significant (P < 0.05) repellent response at doses of 1, 10, and 100 µg. In subsequent single-choice bioassays, 1, 10, and 100 µg of methyl anthranilate prevented larvae from approaching 10 mmol/mol concentrations of carbon dioxide, which is normally highly attractive to the larvae. Indole, the other compound identified from the active fraction, did not elicit a behavioral response by the larvae. Methyl anthranilate has potential for development as a management tool for western corn rootworm larvae and may be best suited for use in a push-pull control strategy.

Evidence of Resistance to Cry34/35Ab1 Corn by Western Corn Rootworm (Coleoptera: Chrysomelidae): Root Injury in the Field and Larval Survival in Plant-Based Bioassays

Western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is a serious pest of corn in the United States, and recent management of western corn rootworm has included planting of Bt corn. Beginning in 2009, western corn rootworm populations with resistance to Cry3Bb1 corn and mCry3A corn were found in Iowa and elsewhere. To date, western corn rootworm populations have remained susceptible to corn producing Bt toxin Cry34/35Ab1. In this study, we used single-plant bioassays to test field populations of western corn rootworm for resistance to Cry34/35Ab1 corn, Cry3Bb1 corn, and mCry3A corn. Bioassays included nine rootworm populations collected from fields where severe injury to Bt corn had been observed and six control populations that had never been exposed to Bt corn. We found incomplete resistance to Cry34/35Ab1 corn among field populations collected from fields where severe injury to corn producing Cry34/35Ab1, either singly or as a pyramid, had been observed. Additionally, resistance to Cry3Bb1 corn and mCry3A corn was found among the majority of populations tested. These first cases of resistance to Cry34/35Ab1 corn, and the presence of resistance to multiple Bt toxins by western corn rootworm, highlight the potential vulnerability of Bt corn to the evolution of resistance by western corn rootworm. The use of more diversified management practices, in addition to insect resistance management, likely will be essential to sustain the viability of Bt corn for management of western corn rootworm.

Field-Based Assessment of Resistance to Bt Corn by Western Corn Rootworm (Coleoptera: Chrysomelidae)

Western corn rootworm, Diabrotica virgifera virgifera LeConte, is a serious pest of corn and is managed with corn that produces insecticidal toxins derived from the bacterium Bacillus thuringiensis (Bt). Beginning in 2009, resistance to Cry3Bb1 corn, and severe injury to Cry3Bb1 corn in the field, was observed in Iowa. However, few data exist on how Cry3Bb1-resistant western corn rootworm interact with various management practices in the field. Using a field experiment, we measured adult emergence and feeding injury to corn roots for both Cry3Bb1-resistant and Cry3Bb1-susceptible populations of western corn rootworm when tested against various Bt corn hybrids and a soil-applied insecticide. Between 2012 and 2013, we evaluated five fields that were associated with greater than one node of feeding injury to Cry3Bb1 corn by western corn rootworm (i.e., problem-field populations), and a laboratory strain that had never been exposed to Bt corn (i.e., control population). Adult emergence for western corn rootworm and root injury to corn were significantly higher in problem-field populations than control populations for both Cry3Bb1 corn and mCry3A corn. By contrast, corn with Cry34/35Ab1, either alone or pyramided with Cry3Bb1, significantly reduced adult emergence and root injury in both problem fields and control fields. In problem fields, application of the soil-applied insecticide to Cry3Bb1 corn significantly reduced root injury, but not adult emergence. Our results are discussed in terms of developing strategies for managing western corn rootworm with resistance to Cry3Bb1 and mCry3A, and delaying the additional evolution of Bt resistance by this pest.

Multiple Assays Indicate Varying Levels of Cross Resistance in Cry3Bb1-Selected Field Populations of the Western Corn Rootworm to mCry3A, eCry3.1Ab, and Cry34/35Ab1

Minnesota populations of Diabrotica virgifera virgifera LeConte, the western corn rootworm, surviving Cry3Bb1-expressing corn in the field and western corn rootworm populations assumed to be susceptible to all Bt proteins were evaluated for susceptibility to Cry3Bb1, mCry3A, eCry3.1Ab, and Cry34/35Ab1 in diet assays and three different plant-based assays. Rootworm populations originating from Cry3Bb1 fields and that consistently experienced greater than expected damage had increased survival and larval growth compared to control populations assayed on Cry3Bb1 as well as mCry3a and eCry3.1Ab. Cross resistance was documented between Cry3Bb1 and both mCry3A and eCry3.1Ab as single toxins. Despite very high resistance ratios in some comparisons, cross resistance was not complete and also varied with the population being evaluated, the trait measured, and the susceptible rootworm population used for comparison. Regardless of resistance and cross resistance, all proteins, even Cry3Bb1, retained some efficacy in terms of either reducing rootworm larval growth, protecting plants from damage, or both, for all rootworm populations evaluated. For one Cry3Bb1-selected population, a resistance ratio of 9.1-fold was found to Cry34/35Ab1 when evaluating EC 50 values relative to a susceptible control population; however, resistance to Cry34/35Ab1 was not evident in all assays in this population. The United States Environmental Protection Agency recently suggested eliminating diet assays as part of the Bt resistance monitoring process. However, given the variability of responses of western corn rootworm populations to different proteins in different assays, both plant and diet assays are needed as options for detecting and fully characterizing resistance.

Early Detection and Mitigation of Resistance to Bt Maize by Western Corn Rootworm (Coleoptera: Chrysomelidae)

Transgenic Bt maize that produces less than a high-dose has been widely adopted and presents considerable insect resistance management (IRM) challenges. Western corn rootworm, Diabrotica virgifera virgifera LeConte, has rapidly evolved resistance to Bt maize in the field, leading to local loss of efficacy for some corn rootworm Bt maize events. Documenting and responding to this resistance has been complicated by a lack of rapid diagnostic bioassays and by regulatory triggers that hinder timely and effective management responses. These failures are of great concern to the scientific and agricultural community. Specific challenges posed by western corn rootworm resistance to Bt maize, and more general concerns around Bt crops that produce less than a high-dose of Bt toxin, have caused uncertainty around current IRM protocols. More than 15 years of experience with IRM has shown that high-dose and refuge-based IRM is not applicable to Bt crops that produce less than a high-dose. Adaptive IRM approaches and pro-active, integrated IRM-pest management strategies are needed and should be in place before release of new technologies that produce less than a high-dose. We suggest changes in IRM strategies to preserve the utility of corn rootworm Bt maize by 1) targeting local resistance management earlier in the sequence of responses to resistance and 2) developing area-wide criteria to address widespread economic losses. We also favor consideration of policies and programs to counteract economic forces that are contributing to rapid resistance evolution.

Developing an in vivo toxicity assay for RNAi risk assessment in honey bees, Apis mellifera L

Maize plants expressing dsRNA for the management of Diabrotica virgifera virgifera are likely to be commercially available by the end of this decade. Honey bees, Apis mellifera, can potentially be exposed to pollen from transformed maize expressing dsRNA. Consequently, evaluation of the biological impacts of RNAi in honey bees is a fundamental component for ecological risk assessment. The insecticidal activity of a known lethal dsRNA target for D. v. virgifera, the vATPase subunit A, was evaluated in larval and adult honey bees. Activity of both D. v. virgifera (Dvv)- and A. mellifera (Am)-specific dsRNA was tested by dietary exposure to dsRNA. Larval development, survival, adult eclosion, adult life span and relative gene expression were evaluated. The results of these tests indicated that Dvv vATPase-A dsRNA has limited effects on larval and adult honey bee survival. Importantly, no effects were observed upon exposure of Am vATPase-A dsRNA suggesting that the lack of response involves factors other than sequence specificity. The results from this study provide guidance for future RNAi risk analyses and for the development of a risk assessment framework that incorporates similar hazard assessments.

Effect of Seed Blends and Soil-Insecticide on Western and Northern Corn Rootworm Emergence from mCry3A+eCry3.1Ab Bt Maize

Seed blends containing various ratios of transgenic Bt maize (Zea mays L.) expressing the mCry3A+eCry3.1Ab proteins and non-Bt maize (near-isoline maize) were deployed alone and in combination with a soil applied pyrethroid insecticide (Force CS) to evaluate the emergence of the western corn rootworm, Diabrotica virgifera virgifera LeConte, in a total of nine field environments across the Midwestern United States in 2010 and 2011. Northern corn rootworm, Diabrotica barberi Smith & Lawrence emergence was also evaluated in four of these environments. Both western and northern corn rootworm beetle emergence from all Bt treatments was significantly reduced when compared with beetle emergence from near-isoline treatments. Averaged across all environments, western corn rootworm beetle emergence from 95:5, 90:10, and 80:20 seed blend ratios of mCry3A+eCry3.1Ab: near-isoline were 2.6-, 4.2-, and 6.7-fold greater than that from the 100:0 ratio treatment. Northern corn rootworm emergence from the same seed blend treatments resulted in 2.8-, 3.2-, and 4.2-fold more beetles than from the 100:0 treatment. The addition of Force CS (tefluthrin) significantly reduced western corn rootworm beetle emergence for each of the three treatments to which it was applied. Force CS also significantly delayed the number of days to 50% beetle emergence in western corn rootworms. Time to 50% beetle emergence in the 100% mCry3A+eCry3.1Ab treatment with Force CS was delayed 13.7 d when compared with western corn rootworm beetle emergence on near-isoline corn. These data are discussed in terms of rootworm resistance management.

Susceptibility of Nebraska Western Corn Rootworm (Coleoptera: Chrysomelidae) Populations to Bt Corn Events

Transgenic plants have been widely adopted by growers to manage the western corn rootworm, Diabrotica virgifera virgifera LeConte, in field corn. Because of reduced efficacy in some Nebraska fields after repeated use of Cry3Bb1-expressing hybrids, single plant bioassays were conducted in 2012 and 2013 to characterize the susceptibility of western corn rootworm populations to the rootworm-active proteins Cry3Bb1, mCry3A, and Cry34/35Ab1. Results demonstrate that there are heritable differences in susceptibility of Nebraska western corn rootworm populations to rootworm-active Bt traits. Proportional survival and corrected survival data coupled with field histories collectively support the conclusion that a level of field resistance to Cry3Bb1 has evolved in some Nebraska populations in response to selection pressure and that cross-resistance exists between Cry3Bb1 and mCry3A. There was no apparent cross-resistance between Cry34/35Ab1 and either Cry3Bb1 or mCry3A. The potential implications of these results on current and future corn rootworm management strategies are discussed.

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.

Monogalactosyldiacylglycerols as Host Recognition Cues for Western Corn Rootworm Larvae (Coleoptera: Chrysomelidae)

Monogalactosyldiacylglycerol (MGDG) was identified as a host recognition cue for larvae of the western corn rootworm Diabrotica virgifera virgifera LeConte. An active glycolipid fraction obtained from an extract of germinating maize roots was isolated with thin layer chromatography using a bioassay-driven approach. When analyzed with LC-MS (positive ion scanning), the assay-active spot was found to contain four different MGDG species: 18:3-18:3 (1,2-dilinolenoyl), 18:2-18:3 (1-linoleoyl, 2-linolenoyl), 18:2-18:2 (1,2-dilinoleoyl), and 18:2-16:0 (1-linoleoyl, 2-palmitoyl). A polar fraction was also needed for activity. When combined with a polar fraction containing a blend of sugars (glucose:fructose:sucrose:myoinositol), the isolated MGDG elicited a unique tight-turning behavior by neonate western corn rootworm larvae that is indicative of host recognition. In behavioral bioassays where disks treated with the active blend were exposed to successive sets of rootworm larvae, the activity of MGDG increased over four exposures, suggesting that larvae may be responding to compounds produced after enzymatic breakdown of MGDG. In subsequent tests with synthetic blends composed of theoretical MGDG-breakdown products, larval responses to four synthetic blends were not significantly different (P<0.5) than the response to isolated MGDG. GC-MS analysis showed modest increases in the amounts of the 16:0, 18:0, and 18:3 free fatty acids released from MGDG after a 30-min exposure to rootworm larvae, which is consistent with the enzymatic breakdown hypothesis.

Emergence and Abundance of Western Corn Rootworm (Coleoptera: Chrysomelidae) in Bt Cornfields With Structured and Seed Blend Refuges

To slow evolution of western corn rootworm (Diabrotica virgifera virgifera LeConte) resistance to Bt (Bacillus thuringiensis Berliner) corn hybrids, non-Bt "refuges" must be planted within or adjacent to Bt cornfields, allowing susceptible insects to develop without exposure to Bt toxins. Bt-susceptible adults from refuges are expected to find and mate with resistant adults that have emerged from Bt corn, reducing the likelihood that Bt-resistant offspring are produced. The spatial and temporal distribution of adults in four refuge treatments (20, 5, and 0% structured refuges and 5% seed blend) and adjacent soybean fields was compared from 2010 to 2012. Adult emergence (adults/trap/day) from refuge corn in structured refuge treatments was greater than that from Bt corn, except during the post-pollination period of corn phenology when emergence from refuge and Bt plants was often the same. Abundance of free-moving adults was greatest in and near refuge rows in structured refuge treatments during vegetative and pollination periods. By post-pollination, adult abundance became evenly distributed. In contrast, adult abundance in 5% seed blends and 0% refuges was evenly distributed, or nearly so, across plots throughout the season. The persistent concentration of adults in refuge rows suggests that structured refuge configurations may not facilitate the expected mixing of adults from refuge and Bt corn. Seed blends produce uniform distributions of adults across the field that may facilitate mating between Bt and refuge adults and ultimately delay the evolution of Bt resistance.