Responses of rotation-resistant and rotation-susceptible populations of the western corn rootworm to transgenic corn expressing cry toxins and validation of a damage function for corn rootworm larvae

The western corn rootworm, Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is a significant insect pest of corn, Zea mays L. Yield losses and control costs associated with the corn rootworm complex---which includes the northern (Diabrotica barberi Smith & Lawrence) and southern (Diabrotica undecimpunctata howardi Barber) corn rootworms as well---exceed an estimated ;The primary goal of the multi-year, multi-state field experiment described in Chapter 2 was to determine if rotation-resistant and rotation-susceptible western corn rootworm larvae differ in their ability to injure transgenic Bt corn roots and cause yield loss. A second goal was to determine if these two populations exhibit differences in emergence or fitness when exposed to Bt corn. A variety of response variables were analyzed, including root injury and yields, as well as cumulative beetle emergence, sex ratios, head capsule widths, and weights. Densities of western corn rootworm beetles were quite low at many of the sites during this experiment. As a result, consistent trends related to root injury and fitness measurements for beetles were difficult to detect. The lack of statistical separation among the various locations and treatments evaluated indicates that many of the variables analyzed are of limited usefulness when densities of western corn rootworm beetles are minimal.;The goal of the greenhouse experiment described in Chapter 3 was to determine if rotation-resistant and rotation-susceptible western corn rootworm larvae differ with respect to survival or development on corn expressing one or more Bt toxins---a single-plant bioassay was used. Corn plants were infested with 225 near-hatch eggs at the V5 growth stage (five leaf collar). Larvae were allowed to develop undisturbed on the root systems for 17 d, after which they were recovered using Berlese funnels. Larvae were counted to estimate mortality; head capsule widths were recorded to assess development. No difference between rotation-resistant and rotationsusceptible larvae with respect to mortality caused by exposure to Bt toxins was observed. Head capsule widths suggest that larvae from these two populations develop similarly when reared on Bt corn. Potential explanations for the observed results are discussed.;An analysis of readily available field trial data used to validate an existing damage function for corn rootworm larvae is reported in Chapter 4. A nested error component model with unbalanced panel data spanning 19 location-years was used to describe the relationship between yield loss and root injury. The model suggests that for each node of roots injured by larvae, a yield loss of approximately 15% is expected. Statistically significant sources of variation included location and experimental error. Variation in weather across sites was likely the principal factor contributing to the significant effect of location. The large experimental error highlights the limitations of using a multi-year, geographically diverse damage function for predicting yield loss on a small scale. Major factors contributing to the variance components estimated by this model are discussed, and techniques for improving future analyses of the damage function for corn rootworm larvae are suggested.