Project : USDA ARS

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Research Project: Ecologically Sustainable Approaches to Insect Resistance Management in Bt Cotton

Location: Southern Insect Management Research

2021 Annual Report

Objectives
1. Determine ecological characteristics and insect-plant interactions, such as susceptibility and fitness, to help identify components that can be manipulated to minimize the evolution of resistance to Bt toxins in Helicoverpa zea. 1.A. Determine the status of Bt resistance in H. zea and develop methods to measure the subsequent fitness costs associated with survival on Bt crops relative to other wild and cultivated hosts. 1.B. Evaluate the impact of Bt corn as a primary host on subsequent H. zea damage and fitness on Bt cotton. 2. Characterize genomic, transcriptomic, and population genetic components of H. zea relative to their contribution to evolution of resistance to Bt toxins and develop computational methods for identifying interactions between gene co-regulatory networks that modulate resistance loci. 2.A. Develop computational methods to identify any gene regulatory networks that interact in responding to intoxication with Bt toxins in H. zea. 2.B. Characterize genomic, transcriptomic, and population genetic components of H. zea relative to their contribution to evolution of resistance to Bt toxins. 3. Develop and optimize early detection methods for invasive polyphagous pests of cotton.

Approach
Recent failures of transgenic crops producing Bacillus thuringiensis (Bt) insecticidal toxins to control noctuid pests and reports of field-evolved resistance to Bt toxins indicate an increase in tolerance to certain Bt toxins in pest populations. To gain a better understanding of factors contributing to the evolution of resistance and to identify components that could be manipulated to minimize the development of resistance to Bt toxins, ecological characteristics of bollworm (BW), Helicoverpa zea, and its interactions with host plants will be investigated. The long-term objective of this project is to identify ecologically sustainable approaches and develop new strategies for the efficient management of BW resistance to Bt toxins. A large-scale Bt resistance survey will be conducted by collecting BW populations from a range of wild and crop hosts across the southern U.S. Progeny from these populations will be assayed using discriminating doses of Bt toxins that compare susceptibility of field insects with control insects from laboratory colonies. This survey will serve as a basis for quantifying the incidence of Bt resistant in BW in areas where Bt corn and cotton production coexists. The fitness parameters will examine the impacts of Bt crops on tolerant insects and fitness of their offspring. This information will assist in determining the status of susceptibility of BW to Bt crops in the southern US. Impacts of Bt corn as a primary host of BW on subsequent damage and fitness on Bt cotton will be assessed using correlations between Bt toxin levels in kernels and larval survival on Bt field corn. Toxin levels in kernels and larval survival on different Bt corn hybrids will facilitate the inference of selection pressure on BW by Bt corn, which will be vital for developing insect resistance management (IRM) strategies in Bt cotton. Contribution of genetic components to Bt toxin resistance evolution will be studied using empirical and computational methods. Genetic loci linked Bt resistance will be evaluated using computational methods such as weighted gene co-regulatory network analysis to predict interactions between gene co-regulatory networks that modulate resistance. Genetic loci predicted to have a high probability of participation in modulating mode of action of Bt toxins will be used in quantitative, comparative, and population genetic studies to evaluate their roles in to Bt toxin resistance. This approach is expected to identify novel genetic loci involved in the toxin mode of action and those contributing to resistance to Bt toxins. This project will also develop novel technologies or improve upon those currently available to facilitate rapid detection of invasive pests. Species-specific antibody-based lateral flow immune assays (LFIA) will be used for rapid identification of species. When LFIA is not possible due to lack of species-specific antigens (targets) in proteins to develop antibodies, isothermal recombinase polymerase amplification (RPA) technology that can amplify species-specific DNA tagged with artificial antigens will be used to detect invasive species.

Progress Report
This project replaced expired project 6066-22000-083-000D. Limited progress was made on laboratory and field research components in this project during COVID19 pandemic. Most effort was on maintaining and preserving resources such as insect lines with unique traits that are critical for research projects. Wild populations of Helicoverpa species from New York, Pennsylvania, Texas, and Virginia were collected during the 2020 growing season, but no DNA extractions were possible to conduct genotyping assays. Development of software modules to identify gene co-regulatory networks was completed and tested to identify genes responding to Bt toxin ingestion in tobacco budworm.

Accomplishments
1. Large-scale resistance survey of bollworm from multiple wild and cultivated hosts on crystalline and vegetatively produced toxins. The large-scale resistance survey of bollworm from multiple wild and cultivated hosts on crystalline and vegetatively produced toxins has been completed. ARS researchers in Stoneville, Mississippi, identified the status of resistance to crystalline Bt toxins in bollworm and indicated that some populations of bollworm in the southern United States are beginning to evolve resistance to vegetative Bt toxin Vip3Aa. Because Vip3Aa is the only Bt toxin produced by transgenic corn and cotton that remains highly effective against some populations of this pest, action is needed now to preserve its efficacy.

2. Bt crops have had a profound effect on population dynamics of bollworm. Populations that develop on Bt or non-Bt corn are a major source of insects that infest cotton later in the growing season. Although survival rates are high for most currently planted Bt corn hybrids, the actual fitness of surviving individuals are difficult to assess. ARS researchers in Stoneville, Mississippi, have collected populations of corn earworms from multiple locations across the MS Delta and are currently examining differences in susceptibilities to commonly used insecticides based on their development on Bt or non-Bt corn hybrids. We are additionally examining the impacts on fecundity of female adults collected on Bt and non-Bt cotton and potential impacts on larval development. These studies are important to our understanding of the impacts of Bt corn the development, survival, and potential insecticidal control on subsequent generations of corn earworm on Bt cotton.

3. Geographically distinct Helicoverpa zea populations. ARS researchers in Stoneville, Mississippi, collected geographically distinct Helicoverpa zea populations from select weedy host plants to establish colonies from across the Mississippi Delta region to address ecological knowledge gaps and in preparation for assays with non-traited, single, and stacked or pyramided Bt events. Research investigating the role maternal host-plant nutrition plays in the subsequent F1 larvae response when exposed to lethal and sub-lethal doses of Bt toxins is being examined. To further assess the effects of weedy or wild host plants (non-cultivated crops) on bollworm fitness, colonies have been established from wild collections across the Delta region. Colonies have been individually reared on the specific weed from which they were collected from with fresh host material being offered until the desired stage was reached or until pupation. Weedy hosts of ecological relevance occurring across the Delta region were chosen based on their seasonal abundance and historical importance in the literature. Research colonies have never been in contact with artificial diets at any time during development. Comparative fitness of these select weedy hosts are being examined using standard developmental measures of larval weight, time between instars, pupation, fecundity, and effect on diapause.

4. Successfully completed initial testing of software modules. ARS researchers in Stoneville, Mississippi, successfully completed initial testing of software modules developed for weighted gene coregulatory network analysis and identified several genes that are co-regulated with known Bt toxin receptors such as alkaline phosphatase and aminopeptidase.

U.S. DEPARTMENT OF AGRICULTURE

Southern Insect Management Research: Stoneville, MS