Dr. Brigitte Tenhumberg, University of Nebraska, Lincoln, NE
Evaluating integrated resistance management strategies in variable environments
Only six years after the commercialization of Bt corn researchers found the first signs of resistance evolution of western corn rootworm (WCR), Diabrotica virgifera virgifera, populations against one of the available Bt toxins (Cry3Bb1). We propose to use mathematical models to evaluate a range of integrated resistance management strategies to identify those that are likely to work best in delaying the onset of resistance evolution. This evaluation will build on a WCR population dynamics model that specifically considers the effect of environmental variation and the effect of underlying genetic assumptions on model predictions and empirical data derived from bioassay of specific conventional insecticides to monitor changes in susceptibility. Past models have ignored environmental variables such as temperature and precipitation despite ample evidence that they critically influence WCR vital rates (e.g. survival, developmental time). Furthermore, all past WCR models assumed a single major gene that confers resistance, even though we know that the predicted speed of resistance evolution of models assuming major gene resistance differs dramatically from models assuming resistance is a quantitative trait. This is important because some researchers suspect that WCR resistance to Bt toxins is a quantitative trait.
Dr. Marcé Lorenzen, North Carolina State University
Development of a germline transformation system for Diabrotica virgifera virgifera
The western corn rootworm (WCR), Diabrotica virgifera virgifera, is a major pest of maize and is notorious for developing resistance to management methods due to its adaptive behavior. Researchers have been stymied in their quest to discover the genetic‐basis of WCR adaptation. However, new approaches are possible with the advent of genome sequencing and functional genomics. For example, the WCR genome has been sequenced, and RNA interference (RNAi) works exceptionally well for determining gene function in WCR. However, despite this important advancement, the problem of efficiently identifying resistance genes remains. A functional genomic tool that requires no upfront knowledge of DNA sequence would be a perfect complement to sequence‐based technologies such as RNAi. Here we propose to generate such a tool by developing a germline transformation system for this economically important pest. Specifically, we will utilize piggyBac‐ and/or Minos transposons to create transgenic WCR. Establishing transgenic technologies for this beetle is the first step towards bringing a wide‐range of transformation–based tools to bear on understanding WCR biology.
Dr. Bryony C. Bonning, Iowa State University
Corn Rootworm Viruses and RNA Interference
The goal of this project is to examine viruses of western corn rootworm (CRW), Diabrotica virgifera virgifera, to Objective 1 identify viruses of the CRW; Objective 2 assess the potential for use of viruses for CRW biological control; Objective 3 develop a vector for virus-induced gene silencing (VIGS) in CRW. A VIGS vector developed from a CRW virus will allow for (i) functional analysis of CRW genes and (ii) optimization of RNA interference-based CRW management strategies through stabilization and efficient targeting of silencing RNAs. Having found evidence for the presence of eight CRW viruses, we will identify additional viral sequences from comprehensive transcriptome, small RNA and viral genomic analyses. We will amplify and assess the pathogenicity of candidate viruses for use as biological control agents. We propose to use a CRW dicistrovirus for production of a VIGS vector by constructing and engineering an infectious clone for delivery of silencing RNAs. At the conclusion of this project we expect to have identified a virus for use in biological control of CRW and developed a VIGS vector for fundamental investigation of CRW gene function and as a means for optimized delivery of dsRNA to CRW from transgenic corn. This project is relevant to three of the CRW Knowledge Grant priority areas relating to biological control of corn rootworm, rootworm physiological studies, and transgenic approaches to rootworm control.
Photographs kindly provided by Marlin E. Rice (CRW larvae) and Tom Sappington (CRW adult).
Aaron Gassmann, Iowa State University
Effects of Current Agricultural Practices on Western Corn Rootworm and Optimal Strategies for Management
I am proposing field and laboratory research aimed at understanding interactions between western corn rootworm and Bt corn, and how to best manage western corn rootworm under current production practices. In fields with greater than expected damage to Bt corn, measurements of pest abundance will be coupled with bioassays of pest susceptibility to determine the relative importance of these factors in contributing to root injury. Additionally, bioassays will be used to measure the extent to which reduced rootworm susceptibility to Bt corn is limited to Bt fields with greater than expected damage or is found more broadly within the landscape. Multiple regression analysis will be applied to fields with a range of cropping histories including corn-on-corn production and crop rotation to determine how current agricultural practices affect rootworm abundance, root injury and rootworm susceptibility to Bt corn. Economic thresholds will be calculated from on-farm research that tests how root injury and yield are affected by transgenic management techniques, non-transgenic management techniques and both approaches in combination.
Bruce Hibbard, USDA-ARS, Columbia, MO
Optimization of plant and diet assays for short-term needs and an artificial diet for rootworm rearing
The western corn rootworm (WCR) is the most important insect pest of corn in the United States. Given a history of developing resistance, a logical concern exists that WCR will develop resistance to newer management tactics. To slow resistance development of insect pests to Bt crops, the U.S. Environmental Protection Agency (EPA) has mandated that an insect resistance management (IRM) plan must be in place prior to registration of any Bt product. One component of current IRM plans are resistance monitoring plans. The EPA is currently considering replacing diet bioassays for all WCR toxins with on-plant assays. Unfortunately, there is little scientific or economic data supporting this compared to other options. For higher dose next generation products it may be likely that diet-toxicity assays will be the most appropriate assays. Precedence seems to be an important component of EPA decisions. Good science is needed prior to full implementation of a modified monitoring program being mandated by EPA.
Dr. Kenneth Ostlie, University of Minnesota
Corn Rootworm Emergence, Scouting and Contemporary Thresholds for Field-Specific Management
The expanding geography and intensification of Bt-RW performance problems with western corn rootworm are creating management uncertainties for growers. Even as corn rootworms figure more prominently in crop management decisions, most growers are operating blind with little field-specific information on corn rootworm populations. In the face of this uncertainty, risk-adverse growers are opting for an insurance approach by layering multiple tactics without regard to actual field realities or their economic, rootworm management, or resistance management implications. The objectives of this research are to re-tool scouting methods and re-calibrate action thresholds for trait and insecticide management options. Degree-day models for corn rootworm emergence will be updated using data from on-farm Bt-RW trait x soil insecticide studies conducted in problem and non-problem fields. The implications of Bt-trait effects on beetle emergence will be explored in lab mating studies. Corn rootworm populations will be assessed using whole-plant counts and yellow sticky traps. Root and yield protection will be evaluated for selected management tactics the following year. These consequences will be related to preceding population levels to determine how this data can best be used in management decisions. Grower / ag professional surveys and focus groups will be used to provide the managerial context, economic and logistic constraints for re-designing the decision process and calculating the applicable thresholds for management actions against corn rootworms. Decision accuracy and value of scouting will be evaluated in the second cycle of on-farm research.