Nutrition affects insect susceptibility to Bt toxins

Turmeric shortens lifespan in houseflies

Climate change poses a significant threat to food security and global public health with the increasing likelihood of insect pest outbreaks. Alternative ways to control insect populations, preferably using environmental-friendly compounds, are needed. Turmeric has been suggested as a natural insecticide with toxicity properties in some insect groups. However, empirical evidence of the effects of turmeric - and their interaction with other ecological factors such as diet - on insect survival has been limited. Here, we tested the effects of turmeric and its interactions with diets differing in protein source in the common housefly, Musca domestica. We found that turmeric shortened lifespan independent of diet and sex. Females in turmeric diets were heavier at death, which was likely driven by a combination of relatively lower rates of body mass loss during their lifetime and a higher percentage of water content at death. Each sex responded differently to the protein source in the diet, and the magnitude of the difference in lifespan between sexes were greatest in diets in which protein source was hydrolysed yeast; individuals from both sexes lived longest in sucrose-milk diets and shortest in diets with hydrolysed yeast. There was no evidence of an interaction between turmeric and diet, suggesting that the toxicity effects are independent of protein source in the diet. Given the seemingly opposing effects of turmeric in insects and mammals being uncovered in the literature, our findings provide further evidence in support of turmeric as a potential natural insecticide.

Increased environmental microbial diversity reduces the disease risk of a mosquitocidal pathogen

IMPORTANCE

The host-specific microbiotas of animals can both reduce and increase disease risks from pathogens. In contrast, how environmental microbial communities affect pathogens is largely unexplored. Aquatic habitats are of interest because water enables environmental microbes to readily interact with animal pathogens. Here, we focused on mosquitoes, which are important disease vectors as terrestrial adults but are strictly aquatic as larvae. We identified a pathogen of mosquito larvae from the field as a strain of Chromobacterium haemolyticum. Comparative genomic analyses and functional assays indicate this strain and other Chromobacterium are mosquitocidal but are also opportunistic pathogens of other animals. We also identify a critical role for diversity of the environmental microbiota in disease risk. Our study characterizes both the virulence mechanisms of a pathogen and the role of the environmental microbiota in disease risk to an aquatic animal of significant importance to human health.

The importance of time in nutrient regulation: a case study with spotted-wing Drosophila (Drosophila suzukii)

Introduction

The ability of living organisms to acquire the nutrients needed to carry out required physiological functions has important consequences for fitness. However, an organism must not simply meet the requirements for individual nutrients, but must ingest an optimal balance of multiple nutrients. Despite this, animals rarely consume truly balanced resources, and instead commonly feed selectively across multiple unbalanced resources to reach an optimal balance, i.e., intake target. Nutritional research has predominantly focused on the behavioral strategies employed during nutrient regulation, as well as the fitness consequence of failing to meet intake targets, but little work has been done on the temporal aspects of this process. For instance, within what timeframe must organisms reach their intake target before a fitness cost is incurred? Hours, days, weeks?

Methods

In this study, we investigated how nutrient regulation interval impacts consumption and performance in adult female spotted-wing Drosophila (Drosophila suzukii). Females were constrained to either a protein- orcarbohydrate-biased diet over different time intervals and at different schedules, while control flies were constrained to one diet for the entire feeding period.

Results

Regulation interval had a significant impact on feeding behavior and consumption. Total consumption was highest on the shorter interval treatments, where diets were alternated more frequently, and declined as the interval period increased. The relative consumption of both diets was statistically-different across intervals and was higher for the carbohydrate-biased diet. Consumption of the protein-biased diet was more variable across intervals and was more strongly impacted by the daily timing of diet switches. Performance data showed that shorter regulation intervals led to longer fly lifespans, a result commonly observed in studies exploring the impacts of diet macronutrient ratio variability on performance.

Discussion

These results show that the temporal aspects of nutrition, such as feeding intervals and the timing of resource availability, can have strong impacts on feeding behavior, nutrient regulation, and fitness. These results provide an insight into how consumers may deal with changes in host phenology, the availability of hosts, and changes in nutrient availability within hosts. Understanding these mechanisms will be important for predicting responses to changes in nutrient cycling and resource availability mediated by natural and anthropogenic habitat modifications, such as global climate change.

Biocontrol of Xyleborus affinis (Curculionidae: Scolitinae) Females and Progeny by Beauveria bassiana (Hypocreales: Cordycipitaceae) in a Sawdust Artificial Diet Model

The ambrosia beetle Xyleborus affinis, recently reported affecting avocado trees in Mexico, represents one of the most widespread insects worldwide. Previous reports have shown that Xyleborus genera members are susceptible to Beauveria bassiana and other entomopathogenic fungus strains. However, their effect on borer beetles' progeny has not been fully investigated. The aim of the present study was to determine the insecticidal activity of B. bassiana on X. affinis adult females and their progeny in an artificial sawdust diet bioassay model. The B. bassiana strains CHE-CNRCB 44, 171, 431, and 485 were individually tested on females at concentrations ranging from 2 × 10⁶ to 1 × 10⁹ conidia mL^-1. After 10 d of incubation, diet was evaluated to count laid eggs, larvae, and adults. Insect conidia loss after exposure was determined by attached conidia to each insect after 12 h of exposure. The results showed that females' mortality ranged between 3.4% and 50.3% in a concentration-response manner. Furthermore, we did not observe statistical differences among strains at the highest concentration. CHE-CNRCB 44 showed the highest mortality at the lowest concentration and reduced larvae and laid eggs at the highest concentration (p < 0.01). Strains CHE-CNRCB 44, 431, and 485 significantly decreased larvae, as compared with the untreated control. After 12 h, up to 70% of conidia was removed by the effect of the artificial diet. In conclusion, B. bassiana has the potential to control X. affinis adult females and progeny.

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.

Negative but antagonistic effects of neonicotinoid insecticides and ectoparasitic mites Varroa destructor on Apis mellifera honey bee food glands

Collaborative brood care by workers is essential for the functionality of eusocial Apis mellifera honey bee colonies. The hypopharyngeal food glands of workers play a crucial role in this context. Even though there is consensus that ubiquitous ectoparasitic mites Varroa destructor and widespread insecticides, such as neonicotinoids, are major stressors for honey bee health, their impact alone and in combination on the feeding glands of workers is poorly understood. Here, we show that combined exposure to V. destructor and neonicotinoids antagonistically interacted on hypopharyngeal gland size, yet they did not interact on emergence body mass or survival. While the observed effects of the antagonistic interaction were less negative than expected based on the sum of the individual effects, hypopharyngeal gland size was still significantly reduced. Alone, V. destructor parasitism negatively affected emergence body mass, survival, and hypopharyngeal gland size, whereas neonicotinoid exposure reduced hypopharyngeal gland size only. Since size is associated with hypopharyngeal gland functionality, a reduction could result in inadequate brood care. As cooperative brood care is a cornerstone of eusociality, smaller glands could have adverse down-stream effects on inclusive fitness of honey bee colonies. Therefore, our findings highlight the need to further study how ubiquitous stressors like V. destructor and neonicotinoids interact to affect honey bees.

Unveiling gene expression regulation of the Bacillus thuringiensis Cry3Aa toxin receptor ADAM10 by the potato dietary miR171c in Colorado potato beetle

BACKGROUND

The Colorado potato beetle (CPB) is a worldwide devastating pest of potato plants and other Solanaceae characterized by its remarkable ability to evolve resistance to insecticides. Bacillus thuringiensis (Bt) Cry3Aa toxin represents an environmentally safe alternative for CPB control but larvae susceptibility to this toxin has been reported to vary depending on the host plant on which larvae feed. To gain more insight into how nutrition mediates Bt tolerance through effects on gene expression, here we explored the post-transcriptional regulation by microRNAs (miRNAs) of the CPB-ADAM10 gene encoding the Cry3Aa toxin functional receptor ADAM10.

RESULTS

The lower CPB-ADAM10 gene expression in CPB larvae fed on potato plants cv. Vivaldi than those fed on potato cv. Monalisa or tomato plants was inversely related to Cry3Aa toxicity. By high-throughput sequencing we identified seven CPB miRNAs and one potato miRNA predicted to base pair with the CPB-ADAM10 messenger RNA. No differential expression of the endogenous lde-miR1175-5p was found in larvae feeding on any of the two potato plant varieties. However, statistically significant increased amounts of potato stu-miR171c-5p were detected in CPB larvae fed on potato cv. Vivaldi compared to larvae fed on potato cv. Monalisa.

CONCLUSION

Our results support a role for dietary miRNAs in Bt toxicity by regulating the CPB-ADAM10 gene encoding the Cry3Aa toxin receptor ADAM10 in CPB larvae and opening up the possibility of exploiting plant natural variation in miRNAs to provide more sustainable potato crop protection against CPB. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Host-Specific larval lepidopteran mortality to pathogenic Serratia mediated by poor diet

Insect guts often harbor an abundance of bacteria. Many of these members are commensal, but some may emerge as opportunistic pathogens when the host is under stress. In this study, we evaluated how dietary nutritional concentration mediates a shift from commensal to pathogenic, and if host species influences those interactions. We used the lepidopterans (Noctuidae) fall armyworm (Spodoptera frugiperda), beet armyworm (Spodoptera exigua), and corn earworm (Helicoverpa zea) as hosts and a Serratia strain initially isolated from healthy fall armyworm. Diet concentration was altered by bulk reduction in nutritional content with dilution using cellulose. Our experiments revealed that low nutrient diet increased mortality from Serratia for beet armyworm and corn earworm. However, for fall armyworm, little mortality was observed in any of the diet combinations. Dietary nutrition and oral inoculation with Serratia did not change the expression of two antimicrobial peptides in fall and beet armyworm, suggesting that other mechanisms that mediate mortality were involved. Our results have implications for how pathogens may persist as commensals in the digestive tract of insects. These findings also suggest that diet plays a very important role in the switch from commensal to pathogen. Finally, our data indicate that the host response to changing conditions is critical in determining if a pathogen may overtake its host and that these three lepidopteran species have different responses to opportunistic enteric pathogens.

Buffered fitness components: Antagonism between malnutrition and an insecticide in bumble bees

Global insect biodiversity declines due to reduced fitness are linked to interactions between environmental stressors. In social insects, inclusive fitness depends on successful mating of reproductives, i.e. males and queens, and efficient collaborative brood care by workers. Therefore, interactive effects between malnutrition and environmental pollution on sperm and feeding glands (hypopharyngeal glands (HPGs)) would provide mechanisms for population declines, unless buffered against due to their fitness relevance. However, while negative effects for bumble bee colony fitness are known, the effects of malnutrition and insecticide exposure singly and in combination on individuals are poorly understood. Here we show, in a fully-crossed laboratory experiment, that malnutrition and insecticide exposure result in neutral or antagonistic interactions for spermatozoa and HPGs of bumble bees, Bombus terrestris, suggesting strong selection to buffer key colony fitness components. No significant effects were observed for mortality and consumption, but significant negative effects were revealed for spermatozoa traits and HPGs. The combined effects on these parameters were not higher than the individual stressor effects, which indicates an antagonistic interaction between both. Despite the clear potential for additive effects, due to the individual stressors impairing muscle quality and neurological control, simultaneous malnutrition and insecticide exposure surprisingly did not reveal an increased impact compared to individual stressors, probably due to key fitness traits being resilient. Our data support that stressor interactions require empirical tests on a case-by-case basis and need to be regarded in context to understand underlying mechanisms and so adequately mitigate the ongoing decline of the entomofauna.

Quantity versus quality: Effects of diet protein-carbohydrate ratios and amounts on insect herbivore gene expression

Dietary protein and digestible carbohydrates are two key macronutrients for insect herbivores, but the amounts and ratios of these two macronutrients in plant vegetative tissues can be highly variable. Typically, insect herbivores regulate their protein-carbohydrate intake by feeding selectively on nutritionally complementary plant tissues, but this may not always be possible. Interestingly, lab experiments consistently demonstrate that performance - especially growth and survival - does not vary greatly when caterpillars and nymphal grasshoppers are reared on diets that differ in their protein-carbohydrate content. This suggests insect herbivores employ post-ingestive physiological mechanisms to compensate for variation in diet protein-carbohydrate profile. However, the molecular mechanisms that underlie this compensation are not well understood. Here we explore, for the first time in an insect herbivore, the transcriptional effects of two dietary factors: protein-to-carbohydrate ratio (p:c) and total macronutrient (p + c) content. Specifically, we reared Helicoverpa zea caterpillars on three diets that varied in diet p:c ratio and one diet that varied in total p + c concentration, all within an ecologically-relevant range. We observed two key findings. Caterpillars reared on diets with elevated total p + c content showed large differences in gene expression. In contrast, only small differences in gene expression were observed when caterpillars were reared on diets with different p:c ratios (spanning from protein-biased to carbohydrate-biased). The invariable expression of many metabolic genes across these variable diets suggests that H. zea caterpillars employ a strategy of constitutive expression to deal with protein-carbohydrate imbalances rather than diet-specific changes. This is further supported by two findings. First, few genes were uniquely associated with feeding on a protein- and carbohydrate-biased diet. Second, many differentially-expressed genes were shared across protein-biased, carbohydrate-biased, and concentrated diet treatments. Our study provides insights into the post-ingestive physiological mechanisms insect herbivores employ to regulate protein-carbohydrate intake. Most notably, it suggests that H. zea, and perhaps other generalist species, use similar post-ingestive mechanisms to deal with protein-carbohydrate imbalances - regardless of the direction of the imbalance.

Evidence, causes, and consequences of declining nitrogen availability in terrestrial ecosystems

The productivity of ecosystems and their capacity to support life depends on access to reactive nitrogen (N). Over the past century, humans have more than doubled the global supply of reactive N through industrial and agricultural activities. However, long-term records demonstrate that N availability is declining in many regions of the world. Reactive N inputs are not evenly distributed, and global changes-including elevated atmospheric carbon dioxide (CO₂) levels and rising temperatures-are affecting ecosystem N supply relative to demand. Declining N availability is constraining primary productivity, contributing to lower leaf N concentrations, and reducing the quality of herbivore diets in many ecosystems. We outline the current state of knowledge about declining N availability and propose actions aimed at characterizing and responding to this emerging challenge.

The Protein Paradox: Elucidating the Complex Nutritional Ecology of the Invasive Berry Pest, Spotted-Wing Drosophila (Diptera: Drosophila suzukii)

Spotted-wing drosophila (SWD), Drosophila suzukii, has become one of the most widely studied insect species over the last decade, largely due to its recent invasion and rapid expansion across the Americas and Europe. Unlike other drosophilid species, which colonize rotting fruit, SWD females possess a serrated ovipositor that allows them to lay eggs in intact ripening fruit, causing significant economic problems for fruit/berry producers worldwide. Though an impressive amount of research has been conducted on SWD's ecology and physiology, aspects of their nutritional ecology remain ambiguous. This review synthesizes the research to date to provide a more comprehensive view of SWD's nutritional relationship with its fruit hosts and associated microbes. Overall, data suggest that SWD's ability to utilize novel resources is likely due to changes in their ecological, rather than physiological, niche that are largely mediated by microbial associations. Studies show that SWD's nutrient intake is comparable to other drosophilid species, indicating limited adaptation to feeding on lower-protein resources. Instead, data show that fruit protein content is a reliable predictor of host suitability and that fruit-microbe dynamics have a strong impact on protein availability. In particularly, fruit protein increases after infestation with SWD-associated microbes, suggesting that initially-suboptimal intact fruits can become protein-rich on a timeframe that is relevant for larval nutrition. This body of work suggests that microbial associations between flies and their fruit hosts can compensate for the nutritional differences between intact and rotting fruit, and that these relationships are likely responsible for SWD's expanded nutritional niche.

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.

Pollen nutrition fosters honeybee tolerance to pesticides

A reduction in floral resource abundance and diversity is generally observed in agro-ecosystems, along with widespread exposure to pesticides. Therefore, a better understanding on how the availability and quality of pollen diets can modulate honeybee sensitivity to pesticides is required. For that purpose, we evaluated the toxicity of acute exposure and chronic exposures to field realistic and higher concentrations of azoxystrobin (fungicide) and sulfoxaflor (insecticide) in honeybees provided with pollen diets of differing qualities (named S and BQ pollens). We found that pollen intake reduced the toxicity of the acute doses of pesticides. Contrary to azoxystrobin, chronic exposures to sulfoxaflor increased by 1.5- to 12-fold bee mortality, which was reduced by pollen intake. Most importantly, the risk of death upon exposure to a high concentration of sulfoxaflor was significantly lower for the S pollen diet when compared with the BQ pollen diet. This reduced pesticide toxicity was associated with a higher gene expression of vitellogenin, a glycoprotein that promotes bee longevity, a faster sulfoxaflor metabolization and a lower concentration of the phytochemical p-coumaric acid, known to upregulate detoxification enzymes. Thus, our study revealed that pollen quality can influence the ability of bees to metabolize pesticides and withstand their detrimental effects, providing another strong argument for the restoration of suitable foraging habitat.

Host resistance to Bacillus thuringiensis is linked to altered bacterial community within a specialist insect herbivore

Evolution of resistance to transgenic crops producing toxins from Bacillus thuringiensis (Bt) threatens the sustainability of the technology. Examination of resistance mechanisms has largely focused on characterization of mutations in proteins serving as Bt toxin binding sites. However, insect microbial communities have the potential to provide host resistance to pesticides in a myriad of ways. Previous findings suggest the killing mechanism of Bt relies on enteric bacteria becoming pathogenic in the disrupted gut environment of the insect following Bt intoxication. Thus, here we hypothesized that resistance to Bt would alter the microbiome composition of the insect. Previous studies have manipulated the microbiome of susceptible insects and monitored their response to Bt. In our study, we characterized the associated bacterial communities of Bt‐resistant and ‐susceptible western corn rootworms, a widespread pest of maize in the United States. We found resistant insects harbor a bacterial community that is less rich and distinct from susceptible insects. After feeding on Bt‐expressing maize, susceptible insects exhibited dysbiosis of the associated bacterial community, whereas the community within resistant insects remained relatively unchanged. These results suggest resistance to Bt produces alterations in the microbiome of the western corn rootworm that may contribute to resistance. We further demonstrated that by itself, feeding on Bt toxin‐expressing seedlings caused a shift in the microbiota. This work provides a broader picture of the effect stressors have on microbiome composition, and the potential heritable changes induced as a result of intense selection.

The molecular mechanisms that determine different degrees of polyphagy in the Bemisia tabaci species complex

The whitefly Bemisia tabaci is a closely related group of >35 cryptic species that feed on the phloem sap of a broad range of host plants. Species in the complex differ in their host-range breadth, but the mechanisms involved remain poorly understood. We investigated, therefore, how six different B. tabaci species cope with the environmental unpredictability presented by a set of four common and novel host plants. Behavioral studies indicated large differences in performances on the four hosts and putative specialization of one of the species to cassava plants. Transcriptomic analyses revealed two main insights. First, a large set of genes involved in metabolism (>85%) showed differences in expression between the six species, and each species could be characterized by its own unique expression pattern of metabolic genes. However, within species, these genes were constitutively expressed, with a low level of environmental responsiveness (i.e., to host change). Second, within each species, sets of genes mainly associated with the super-pathways "environmental information processing" and "organismal systems" responded to the host switching events. These included genes encoding for proteins involved in sugar homeostasis, signal transduction, membrane transport, and immune, endocrine, sensory and digestive responses. Our findings suggested that the six B. tabaci species can be divided into four performance/transcriptomic "Types" and that polyphagy can be achieved in multiple ways. However, polyphagy level is determined by the specific identity of the metabolic genes/pathways that are enriched and overexpressed in each species (the species' individual metabolic "tool kit").

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

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

Protein-carbohydrate regulation and nutritionally mediated responses to Bt are affected by caterpillar population history

BACKGROUND

The widespread adoption of genetically modified crops, including Bacillius thuringensis (Bt) crops that target chewing insects, has transformed agricultural pest management. This increased use of Bt has raised concerns about the onset of resistance amongst target pests. Recent studies have shown that for some caterpillars, nutritional foraging (e.g. the ratio of proteins and carbohydrates consumed) can affect the insect susceptibility to the Bt toxin Cry1Ac. However, studies on both nutritional foraging and Bt susceptibility tend to rely on laboratory colonies without specifically addressing physiological differences that may occur between populations of the same species. Here, we used choice assays, no choice assays and dose response assays to address two overarching questions: Do populations of Spodoptera frugiperda (J.E. Smith) vary in their protein-carbohydrate foraging behavior? and Does protein-carbohydrate intake impact S. frugiperda's susceptibility to the Bt toxin Cry1F?

RESULTS

All three of our S. frugiperda populations actively regulated their protein-carbohydrate intake, but we observed significant differences between populations with respect to their self-selected protein-carbohydrate intake. We also found that feeding at the protein-carbohydrate intake target slightly increased Cry1F susceptibility for one S. frugiperda population, but had no effect on the other two populations.

CONCLUSIONS

Our findings indicate that inherent differences exist in the nutritional physiology of three S. frugiperda populations, possibly related to the time spent in culture. This suggests that population-level differences are an important consideration when drawing parallels between field-collected and laboratory-reared insects.

Development of Tenebrio molitor (Coleoptera: Tenebrionidae) on Poultry Litter-Based Diets: Effect on Chemical Composition of Larvae

In order to investigate a low-cost and sustainable food source, the present study evaluated the use of poultry litter for rearing Tenebrio molitor Linnaeus, 1758 (Coleoptera:  Tenebrionidae). The experiment was performed with five diets containing increasing levels of poultry litter (0, 25, 50, 75, and 100%) replacing the control diet and five replicates with 50 larvae per sample unit. Larval growth and development were evaluated and the chemical compositions of diet and T. molitor larvae were determined. Larval development and reproduction efficiency of T. molitor were similar in all treatments. The sole use of poultry litter to feed T. molitor reduced the crude protein of flour by only 8%. Including 50% or more poultry litter in the standard diet is the best-suited formulation for larvae production and incorporation of minerals in the larvae. Mealworm can be grown successfully on diets composed by poultry litter, the diet did not affect survival, growth, and development; however, studies spanning several insect generations should be performed to determine the effects of diet composition on adult fecundity. The knowledge acquired using poultry litter to feed T. molitor will be useful to carry out new research, in addition to evidencing the possibility of low-cost mass rearing of these larvae.

Gut bacteria of the silkworm Bombyx mori facilitate host resistance against the toxic effects of organophosphate insecticides

Organophosphate insecticides that are heavily used in agriculture for pest control have caused growing environmental problems and public health concerns worldwide. Ironically, insecticide resistance develops quickly in major lepidopteran pests, partially via their microbial symbionts. To investigate the possible mechanisms by which the microbiota confers insecticide resistance to Lepidoptera, the model organism silkworm Bombyx mori (Lepidoptera: Bombycidae) was fed different antibiotics to induce gut dysbiosis (microbiota imbalance). Larvae treated with polymyxin showed a significantly lower survival rate when exposed to chlorpyrifos. Through high-throughput sequencing, we found that the abundances of Stenotrophomonas and Enterococcus spp. changed substantially after treatment. To assess the roles played by these two groups of bacteria in chlorpyrifos resistance, a germ-free (GF) silkworm rearing protocol was established to avoid the influence of natural microbiota and antibiotics. Monoassociation of GF silkworms with Stenotrophomonas enhanced host resistance to chlorpyrifos, but not in Enterococcus-fed larvae, consistent with larval detoxification activity. GC-μECD detection of chlorpyrifos residues in feces indicated that neither Stenotrophomonas nor Enterococcus degraded chlorpyrifos directly in the gut. However, gut metabolomics analysis revealed a highly species-specific pattern, with higher levels of essential amino acid produced in the gut of silkworm larvae monoassociated with Stenotrophomonas. This critical nutrient provisioning significantly increased host fitness and thereby allowed larvae to circumvent the deleterious effects of these toxic chemicals more efficiently. Altogether, our study not only suggests a new mechanism for insecticide resistance in notorious lepidopteran pests but also provides a useful template for investigating the interplay between host and gut bacteria in complex environmental systems.

Selection of Beauveria bassiana (Hypocreales: Cordycipitaceae) strains to control Xyleborus affinis (Curculionidae: Scolytinae) females

BACKGROUND

Xyleborus affinis Eichhoff (Coleoptera: Curculionidae) is an ambrosia beetle reported to affect avocado trees (Persea americana Mill.). The use of the entomopathogenic fungus (EPF) Beauveria bassiana (Bals.-Criv.) Vuill. for ambrosia beetle control represents an alternative to insecticides.

METHODS

This study was designed in two stages to select B. bassiana strains with potential to control X. affinis females. In the first stage, 19 B. bassiana Mexican strains from EPF collection, isolated from Coleoptera (CHE-CNRCB, http://www.gob.mx/senasica/documentos/coleccion-de-hongos-entomopatogenos), were tested. Analyses included radial growth rate, conidial yield, spore germination, and germ tube length. Results were analysed by Principal Component Analysis (PCA) to identify clusters within favourable growth phenotypes. For the second stage, 10 selected strains were re-analysed for virulence-related metabolic characteristic, including cell wall-bound cuticle-degrading enzymes-Pr1-like proteases and β-N-acetyl glucosaminidases (NAGase) chitinases, conidial hydrophobicity and monopolar germination parameters. A second PCA analysis was run for those virulence parameters analysed, and upon results strains CHE-CNRCB 44, 171, 431 and 485 were selected and tested against X. affinis females. Females were treated with a 1 × 108 conidia mL-1 suspension (recommended rate), using a Potter Tower.

RESULTS

All strains showed insecticidal activity, inducing up to 58% mortality; about 30% dead beetles developed aerial mycelia (CHE-CNRCB 485) and the fastest mortality rate was t0 = 1.95 (CHE-CNRCB 44).

CONCLUSION

Since all selected strains showed virulence against X. affinis females, results indicated the possibility of selecting B. bassiana strains based on multiple metabolic attributes, as a preliminary test to perform bioassays against order-related target insects.

A Semi-Synthetic Diet and the Potential Important Chemicals for Mythimna separata (Lepidoptera: Noctuidae)

Armyworm feeding in large, destructive groups is hugely difficult to control and the oriental armyworm, Mythimna separata (Walk), is one such pest. In this study, we reported a semisynthetic artificial diet for the oriental armyworm. This diet is based on Ritter's diet, a formula developed for Heliothis zea. The survival of M. separata was extremely low and only around 2% insects can reach the adult stage on Ritter's diet. But, it can reach up to 100% if corn leaf powder (CLP) was mixed, and insects grew faster and gained more mass. After testing a set of mixtures of Ritter's diet and CLP, we found that 14.3% was the optimal proportion of CLP for making the artificial diet. We then used chloroform to extract CLP. Insect performance was still much better on Ch-extracted CLP diets than that on Ritter's diet, but it was poorer than that on the diets containing unprocessed CLP, suggesting that the essential factor(s) was only partially extracted from corn leaf. We then used methanol and dichloromethane, two solvents differing in their polarity, to process the extractions and analyzed the extracted chemicals using gas chromatography-mass spectrometry (GC-MS). Insects had a better performance on dichloromethane-extracted CLP diet in comparison to methanol-extracted one, indicating that the important factor(s) is more prone to methanol extraction. The reported recipe here is useful for the research on M. separata and possibly other grain-crop eating armyworms. The functions of the chemicals extracted from corn leaf tissue can be investigated in the future studies.

Development, survival, and feeding behavior of Helicoverpa zea (Lepidoptera: Noctuidae) relative to Bt protein concentrations in corn ear tissues

The corn earworm, Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae), preferentially oviposits and feeds on ears of corn (Zea mays L.) and can be managed using transgenic hybrids that produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt). Concentrations of Bt proteins can vary spatially and temporally in plant tissues, creating a heterogeneous environment that can increase the risk of resistance development. We planted small-plot trials of nine Bt and non-Bt corn hybrids in South Carolina in 2016 and 2017 and investigated the development, survival, feeding injury, and feeding behavior in corn ear tissues. ELISA was used to quantify the concentrations of Cry1F and Cry2Ab2 in young silk, old silk, maternal tip tissue, kernels, and husk. Cry1F and Cry2Ab2 significantly varied with silk age and both proteins were generally highest in the silk and tip tissue. Hybrids with pyramided proteins significantly reduced feeding injury to the silk, tip, and kernel ear tissues, which was less apparent with single Bt protein hybrids. The pyramided hybrid expressing Vip3A incurred no injury to either the ear tip or kernels, and only eight 1st instar larvae were collected in the silk of 520 sampled ears. Age of larvae significantly varied among ear tissues but not between hybrids. Depending on hybrid family, mean larval instar in the silk, tip, and kernels was 1st or 2nd, 3rd, and 5th, respectively. Instar-specific feeding penetrance into corn ears increased with age but did not differ between hybrids. We characterized the instar- and tissue-specific feeding behavior of H. zea larvae but did not detect differences in feeding behavior between Bt and non-Bt hybrids. Implications for resistance management strategies such as seed mixtures are discussed.

Helicoverpa zea (Lepidoptera: Noctuidae) Preference for Plant Structures, and Their Location, Within Bt Cotton Under Different Nitrogen and Irrigation Regimes

Helicoverpa zea Boddie is a common economic pest of cotton (Gossypium hirsutum L.), including transgenic cotton varieties that express Bacillus thuringiensis (Bt). Helicoverpa zea oviposition is similar in Bt and non-Bt cotton, but behavior of H. zea larvae can be different in the presence of Bt, with neonates moving away from terminals faster in single-toxin Bt than non-Bt cotton or avoiding Bt-treated diet in the lab. We quantified H. zea oviposition and larval distribution on structures within cotton plants in small plot experiments of Cry1Ac + Cry1F cotton for 2 yr under different irrigation and nitrogen treatments. More eggs were oviposited on plants receiving nitrogen application during 2016 and on leaves in the top section of irrigated plants during 2017, but other treatment effects on eggs or larvae were minimal. Helicoverpa zea eggs were most common on leaves in the top third of plants at position zero and middle section of cotton plants throughout the season, but some oviposition occurred on fruiting structures as well. First and second instars were more common on squares in the top section of plants during 2016 and bolls in the middle and lower sections during 2017 due to oviposition lower in the canopy during 2017. During both years, third through fifth instars were more common on bolls in the middle and lower section of plants closer to the main stem. These findings have resistance management implications as extended larval feeding on bolls could optimize nutrition, decrease Bt susceptibility, and potentially influence behavioral resistance.

Susceptibility of Corn Earworm (Lepidoptera: Noctuidae) to Cry1A.105 and Cry2Ab2 in North and South Carolina

The corn earworm, Helicoverpa zea (Boddie), is managed in corn and cotton in the United States primarily using transgenic cultivars that produce insecticidal proteins from the bacterium Bacillus thuringiensis (Bt). However, increasing reports of resistance to one or more Bt proteins threaten the continued efficacy of Bt traits. To better understand the development of resistance of H. zea to Bt corn and cotton in the southeastern United States, we monitored for resistance to Cry1A.105 and Cry2Ab2 among 22 field populations of H. zea collected in non-Bt and Bt corn expressing Cry1A.105 + Cry2Ab2 during 2017 and 2018. Colonies were established in the laboratory and progeny were screened in diet-overlay bioassays to purified Cry1A.105 and Cry2Ab2 proteins. Compared with two susceptible laboratory colonies, all 14 field colonies tested with Cry1A.105 were highly resistant, with resistance ratios (RRs) ranging from 13.5 to >4,000. For Cry2Ab2, 19 colonies were tested and RRs ranged from 0.26 to 33.7. Field populations were significantly more susceptible to Cry2Ab2 than Cry1A.105. We documented variability in F0 and F1 pupal weight and developmental rates of natural populations of H. zea, but observed no significant correlation with susceptibility to either Cry1A.105 or Cry2Ab2. Our results expand on the recent reports of H. zea resistance to Cry1A and Cry2A proteins and will aid in the design and deployment of future pyramided crops in the United States.

First evidence of protein-carbohydrate regulation in a plant bug (Lygus hesperus)

Lygus bugs are highly polyphagous piercing/sucking insects found throughout North America. Collectively, they have been reported to feed on over 330 plant species (one of the broadest host range ever documented for a group of insects); they also feed on many economically important crops. Despite its prevalence across North America and status as a common pest in many agroecosystems, very little is known about how Lygus bugs regulate their intake of nutrients. In reality, little is known about nutrient regulation for most hemipterans, specifically non-phloem feeding species in the suborder Heteroptera. This likely reflects difficulties in developing adequate artificial diets for insects with piercing/sucking mouthparts. There is, however, an artificial diet for L. hersperus, and in this study we modified it and performed choice and no-choice experiments to determine how L. hesperus regulates its intake of two macronutrients - protein (p) and carbohydrates (c) - that are tightly linked to survival and performance in other insect herbivores. In choice experiments L. hesperus was allowed to select between two foods with different protein:carbohydrate ratios. We documented strong regulation for protein and carbohydrates, with late instar nymphs selecting a slightly protein-biased intake target (protein-carbohydrate ratio = 1.5:1). We also performed no-choice experiments, where nymphs were restricted to a single food. Here, the protein-carbohydrate ratio of their food had a strong impact on survival, which was highest for nymphs reared on the treatment with a protein-carbohydrate ratio closest to the self-selected intake target (determined by the choice experiments), but no significant impact on developmental time or mass gain. Our data are the first of their kind for a non-phloem feeding hemipteran and provide a starting point for more broadly understanding and further investigating the nutritional ecology/physiology of Lygus bugs. Our study also provides a framework for exploring nutrient regulation in other hemipterans and for optimizing artificial diets for piercing/sucking insects, especially heteropterans.

Comparative Susceptibility of Western Corn Rootworm (Coleoptera: Chrysomelidae) Neonates to Selected Insecticides and Bt Proteins in the Presence and Absence of Feeding Stimulants

The susceptibility of western corn rootworm, Diabrotica virgifera virgifera LeConte, larvae to nine insecticides from five different classes and to Bt proteins eCry3.1Ab and mCry3A in the presence or absence of feeding stimulants, was estimated in filter paper and diet toxicity assays, respectively. The use of a synthetic feeding stimulant blend of the sugars glucose, sucrose, and fructose plus linoleic acid at a ratio of 30:4:4:0.3 mg/ml of distilled water was evaluated to determine whether they increase the efficacy of insecticides and Bt proteins. The efficacy of thiamethoxam diluted in solutions with feeding stimulants was significantly increased when compared to thiamethoxam dilutions in water (>60-fold). Differences in the efficacy of the other insecticide classes when diluted in feeding stimulant solutions were no greater than fivefold when compared to the insecticides diluted in water. The presence of corn root juice as a natural feeding stimulant diminished toxicity of the insecticides, except for thiamethoxam, even though larval fresh weight was higher when fed on root juice compared to feeding stimulant or water. The use of feeding stimulants in diet toxicity assays did not enhance efficacy of eCry3.1Ab nor mCry3A proteins. Feeding stimulants can be recommended in combination with thiamethoxam to increase larval mortality. These results are discussed in terms of applicability of feeding stimulants to improve susceptibility of western corn rootworm larvae to pesticides in general.

Developmental and hemocytological effects of ingesting Fukushima's radiocesium on the cabbage white butterfly Pieris rapae

High morphological abnormality and mortality rates have been reported in the pale grass blue butterfly, Zizeeria maha, since the Fukushima nuclear accident. However, it remains uncertain if these effects are restricted to this butterfly. Here, we evaluated the effects of ingesting cabbage leaves grown with contaminated soils from Fukushima on the development and hemocytes of the cabbage white butterfly, Pieris rapae. Contaminated cabbage leaves containing various low levels of anthropogenic ¹³⁴Cs and ¹³⁷Cs radioactivity (less than natural ⁴⁰K radioactivity) were fed to larvae from Okinawa, the least contaminated locality in Japan. Negative developmental and morphological effects were detected in the experimental groups. The cesium (but not potassium) radioactivity concentration was negatively correlated with the granulocyte percentage in hemolymph, and the granulocyte percentage was positively correlated with the pupal eclosion rate, the adult achievement rate, and the total normality rate. These results demonstrated that ingesting low-level radiocesium contaminants in Fukushima (but not natural radiopotassium) imposed biologically negative effects on the cabbage white butterfly, as in the pale grass blue butterfly, at both cellular and organismal levels.

Comparison of Six Artificial Diets for Western Corn Rootworm Bioassays and Rearing

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is considered the most important maize (Zea mays L.) pest in the U.S. Corn Belt. Bioassays testing susceptibility to Bacillus thuringiensis Berliner (Bt) and other toxins of corn rootworm larvae often rely on artificial diet formulations. Successful bioassays on artificial diet for corn rootworm have sometimes been challenging because of microbial contamination. Toward the long-term goal of developing a universal artificial diet for western corn rootworm larvae, we compared larval survival, dry weight, and percentage of molt in 10-d bioassays from six current diets of which we were aware. In addition, as part of longer term rearing efforts, we recorded molting over an extended period of development (60 d). Six different artificial diets, including four proprietary industry diets (A, B, C, and D), the first published artificial diet for western corn rootworm (Pleau), and a new diet (WCRMO-1) were evaluated. Western corn rootworm larval survival was above 90% and contamination was 0% on all diets for 10 d. Diet D resulted in the greatest dry weight and percentage molting when compared with the other diets. Although fourth-instar western corn rootworm larvae have not been documented previously (only three instars have been previously documented), as many as 10% of the larvae from Diet B molted into a fourth instar prior to pupating. Overall, significant differences were found among artificial diets currently used to screen western corn rootworm. In order for data from differing toxins to be compared, a single, reliable and high-quality western corn rootworm artificial diet should eventually be chosen by industry, academia, and the public as a standard for bioassays.

Quantifying Plant Soluble Protein and Digestible Carbohydrate Content, Using Corn (Zea mays) As an Exemplar

Elemental data are commonly used to infer plant quality as a resource to herbivores. However, the ubiquity of carbon in biomolecules, the presence of nitrogen-containing plant defensive compounds, and variation in species-specific correlations between nitrogen and plant protein content all limit the accuracy of these inferences. Additionally, research focused on plant and/or herbivore physiology require a level of accuracy that is not achieved using generalized correlations. The methods presented here offer researchers a clear and rapid protocol for directly measuring plant soluble proteins and digestible carbohydrates, the two plant macronutrients most closely tied to animal physiological performance. The protocols combine well characterized colorimetric assays with optimized plant-specific digestion steps to provide precise and reproducible results. Our analyses of different sweet corn tissues show that these assays have the sensitivity to detect variation in plant soluble protein and digestible carbohydrate content across multiple spatial scales. These include between-plant differences across growing regions and plant species or varieties, as well as within-plant differences in tissue type and even positional differences within the same tissue. Combining soluble protein and digestible carbohydrate content with elemental data also has the potential to provide new opportunities in plant biology to connect plant mineral nutrition with plant physiological processes. These analyses also help generate the soluble protein and digestible carbohydrate data needed to study nutritional ecology, plant-herbivore interactions and food-web dynamics, which will in turn enhance physiology and ecological research.

Long-Term Empirical and Observational Evidence of Practical Helicoverpa zea Resistance to Cotton With Pyramided Bt Toxins

Evidence of practical resistance of Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) to Bt cotton in the United States is debatable, supported with occasional reports of boll damage in the field. Our objective was to provide both empirical and long-term observational evidence of practical resistance by linking both in-season and end-of-season measurements of H. zea damage to pyramided Bt cotton bolls and to provide Cry1Ac diet-based bioassay data in support of these damage estimates. In-season boll damage from H. zea was highly correlated to end-of-season damaged bolls. Across North Carolina, Bt cotton fields with end-of-season bolls damaged by H. zea increased during 2016 compared to previous years. Elevated damage was coupled with an increase in field sprays targeting H. zea during 2016, but not related to an increase in H. zea abundance. Bioassay data indicated that there was a range of Cry1Ac susceptibility across the southeastern United States. Given the range of susceptibility to Cry1Ac across the southeastern United States, it is probable that resistant populations are common. Since H. zea is resistant to cotton expressing pyramided Cry toxins, the adoption of new cotton varieties expressing Vip3Aa will be rapid. Efforts should be made to delay resistance of H. zea to the Vip3Aa toxin to avoid foliar insecticide use.

Carotenoids moderate the effectiveness of a Bt gene against the European corn borer, Ostrinia nubilalis

We assessed the effectiveness of a biofortified maize line (4BtxHC) which accumulates high levels of antioxidant carotenoids that also expressed the insecticidal Cry1Ac Bacillus thuringiensis (Bt) gene against the European corn borer Ostrinia nubilalis. This line had been previously engineered to accumulate carotenoids specifically in the seed endosperm, whereas the Bt gene was expressed constitutively. The concentrations of Bt toxin (Cry 1Ac) in the leaves of the 4Bt and 4BtxHC lines were not significantly different at 47±6 μg/g of fresh weight (FW); neither were they in the kernels of both lines (35±3 μg/g FW). The kernels and leaves were toxic to the larvae of O. nubilalis. However, the insecticidal activity was substantially lower (ca. 20%) than that of lines that expressed only Bt in spite that the two lines showed a quantity of toxin not significantly different in kernels or in leaves. Although the reduced effectiveness of Cry1Ac in kernels may not be entirely surprising, the observation of the same phenomenon in vegetative tissues was unexpected. When semi-artificial diets containing kernels from 4Bt supplemented with different levels of β-carotene were used in insect bioassays, the β-carotene moderated the effectiveness of the Bt similarly to the plant material with carotenoid enrichment. To elucidate the biochemical basis of the reduced effectiveness of Bt toxin in the carotenoid-enriched plants, we measured the activity of three enzymes known to be implicated in the detoxification defence, namely, catalase, superoxide dismutase and glutathione S-transferase. Whereas Cry1Ac expression significantly increased SOD and CAT enzymatic activity in the absence of carotenoids, carotenoids, either in 4BtxHC or in artificial diets enriched with β-carotene, significantly lowered CAT activity. Carotenoids can therefore moderate the susceptibility of the maize borer O. nubilalis to Cry1Ac, and we hypothesize that their role as antioxidants could explain this phenomenon via their scavenging of reactive oxygen species produced during Cry1Ac detoxification in the larvae. The involvement of this mechanism in the decreased mortality caused by Cry1Ac when carotenoids are present in the diet is discussed.

Herbivore-Induced Defenses in Tomato Plants Enhance the Lethality of the Entomopathogenic Bacterium, Bacillus thuringiensis var. kurstaki

Plants can influence the effectiveness of microbial insecticides through numerous mechanisms. One of these mechanisms is the oxidation of plant phenolics by plant enzymes, such as polyphenol oxidases (PPO) and peroxidases (POD). These reactions generate a variety of products and intermediates that play important roles in resistance against herbivores. Oxidation of the catecholic phenolic compound chlorogenic acid by PPO enhances the lethality of the insect-killing bacterial pathogen, Bacillus thuringiensis var. kurstaki (Bt) to the polyphagous caterpillar, Helicoverpa zea. Since herbivore feeding damage often triggers the induction of higher activities of oxidative enzymes in plant tissues, here we hypothesized that the induction of plant defenses would enhance the lethality of Bt on those plants. We found that the lethality of a commercial formulation of Bt (Dipel® PRO DF) on tomato plants was higher if it was applied to plants that were induced by H. zea feeding or induced by the phytohormone jasmonic acid. Higher proportions of H. zea larvae killed by Bt were strongly correlated with higher levels of PPO activity in the leaflet tissue. Higher POD activity was only weakly associated with higher levels of Bt-induced mortality. While plant-mediated variation in entomopathogen lethality is well known, our findings demonstrate that plants can induce defensive responses that work in concert with a microbial insecticide/entomopathogen to protect against insect herbivores.

Protein-carbohydrate regulation in Helicoverpa amigera and H. punctigera and how diet protein-carbohydrate content affects insect susceptibility to Bt toxins

Many animals, including insects, demonstrate a remarkable ability to regulate their intake of key macronutrients (e.g., soluble protein and digestible carbohydrates), which allows them to optimize fitness and performance. Additionally, regulating the intake of these two macronutrients enhances an animal's ability to defend itself against pathogens, mitigate the effects of secondary plant metabolites, and decrease susceptibility to toxins. In this study, we first compared how Bt-resistant and -susceptible lines of Helicoverpa armigera and Helicoverpa punctigera regulate their intake of protein (p) and digestible carbohydrates (c). We found that there was no difference in the self-selected protein-carbohydrate intake target between resistant and susceptible genotypes of either species. We then explored the extent to which food protein-carbohydrate content altered the susceptibility of these species to three Bt toxins: Cry1Ac, Cry2Ab, and Vip3Aa. We found that H. armigera on diets that had protein-carbohydrate profiles that matched their self-selected protein-carbohydrate intake target were significantly less susceptible to Cry1Ac. In contrast, diet protein-carbohydrate content did not affect H. punctigera susceptibility to Cry1Ac. For both H. armigera and H. punctigera, susceptibility to Cry2Ab and Vip3Aa toxins did not change as a function of diet protein-carbohydrate profile. These results, when combined with earlier work on H. zea, suggest food protein-carbohydrate content can modify susceptibility to some Bt toxins, but not others. An increased understanding of how the nutritional environment can modify susceptibility to different Bt toxins could help improve pest management and resistance management practices.

A new artificial diet for western corn rootworm larvae is compatible with and detects resistance to all current Bt toxins

Insect resistance to transgenic crops is a growing concern for farmers, regulatory agencies, the seed industry, and researchers. Since 2009, instances of field-evolved Bt resistance or cross resistance have been documented for each of the four Bt proteins available for western corn rootworm (WCR), a major insect pest. To characterize resistance, WCR populations causing unexpected damage to Bt maize are evaluated in plant and/or diet toxicity assays. Currently, it is not possible to make direct comparisons of data from different Bt proteins due to differing proprietary artificial diets. Our group has developed a new, publicly available diet (WCRMO-1) with improved nutrition for WCR larvae. For the current manuscript, we tested the compatibility of all Bt proteins currently marketed for WCR on the WCRMO-1 diet and specific proprietary diets corresponding to each toxin using a susceptible colony of WCR. We also tested WCR colonies selected for resistance to each protein to assess the ability of the diet toxicity assay to detect Bt resistance. The WCRMO-1 diet is compatible with each of the proteins and can differentiate resistant colonies from susceptible colonies for each protein. Our diet allows researchers to monitor resistance without the confounding nutritional differences present between diets.