Evaluating response of western corn rootworm to transgenic Bt corn: Single event, stacked event, and blended refuge

The western corn rootworm Diabrotica virgifera virgifera is a major pest of corn in the United States. Bt corn that targets corn rootworm has been adopted rapidly by growers, and this places selective pressure on the western corn rootworm to evolve resistance. Important factors affecting the development of resistance are the frequency of resistance alleles and the pattern of adult emergence from Bt and refuge plants. The goals of our research were to measure the frequency of non-recessive resistance alleles by conducting an F1 screen, to measure patterns of adult emergence from Bt and non-Bt corn, and to assess fitness of adults surviving as larvae on Bt corn in the field. A total of four treatments were evaluated: 1) a single-trait event with Cry3Bb1 (YieldGard VT Triple), 2) a stacked event with Cry3Bb1 and Cry34Ab1/35Ab1 (SmartStax), 3) a near isogenic hybrid that did not contain rootworm active Bt and 4) a stacked event with a blended non-Bt refuge. For each of these treatments, we conducted an F1 screen by collecting males that emerged from our four treatments, mating them to virgin non-diapausing females, and assaying survival of their larval progeny on the stacked event (Cry34Ab1/35Ab1 + Cry3Bb1), the single event (Cry3Bb1), and the non-Bt near isogenic hybrid. Additionally, for each treatment, we collected data on the abundance of adult male and female western corn rootworm emerging over the course of the summer, and the fitness of these individuals, as indexed by width of their head capsule.