Title: Selecting for Resistance to the Cry3Bb1 Protein in a Genetically Diverse Population of Non-diapausing Western Corn Rootworm Authors
Submitted to: University Annual Newsletter
Publication Type: Other
Publication Acceptance Date: March 18, 2008
Publication Date: March 22, 2008
Citation: French, B.W., Bagley, M., Nielson, C.N. 2008. Selecting for Resistance to the Cry3Bb1 Protein in a Genetically Diverse Population of Non-diapausing Western Corn Rootworm. Resistant Pest Mangement Newsletter, 17(2):54. Technical Abstract: The western corn rootworm (WCR, Diabrotica virgifera virgifera) is an economically important pest of maize in North America. Larvae inflict the most damage by feeding on maize roots. Feeding damage can disrupt nutrient flow, thereby reducing seed production, and may also cause plant lodging that can disrupt harvesting. In 2003, the U. S. Environmental Protection Agency approved the commercial use of Monsanto’s Bt maize expressing the Cry3Bb1 protein. When ingested, this protein is toxic to these beetles. However, given the previous adaptability of western corn rootworm beetles to control measures, evolution of resistance to the toxin is a concern. To understand how resistance might evolve to the Cry3Bb1 we created a genetically diverse base population (NDBP) of non-diapausing WCR. Genetic variation from four geographically distinct diapausing western corn rootworm populations was introduced into a non-diapausing colony through assigned and random matings. From this genetically diverse population we created three experimental and two control lines. Our goal was to achieve 80% larval mortality for each generation of selection. Therefore, prior to selecting for Cry3Bb1 resistance we established the optimum Bt exposure duration to achieve ~80% mortality due to Bt selection while minimizing mortality from other factors. After several preliminary trials comparing survivorship for 1st, 2nd, and 3rd larval instars on MON863, MON863 isoline, and a no maize control (i.e. starved), we began our selection experiments with 24 h old neonate larvae as the initial exposure stage. The neonate larvae from each experimental line were exposed to Bt maize seedlings in a Petri dish. The time of exposure to the Bt maize seedlings increased each generation for several generations. Initial exposure time was 24 h, which increased in 12 h increments for five generations, and then larvae were exposed in 24 h increments for another five generations. Control lines were treated identically to experimental lines except for the Bt exposure. Beginning with the fourth generation and alternating every other generation, we reared larvae from each line entirely on Bt maize to assess the degree of resistance. From the NDBP we currently selected three experimental colonies (EC1, EC2, EC3) for resistance to the Cry3Bb1 protein for nine generations, increasing exposure time with each generation. We have also maintained two control lines (CC1, CC2) on MON863 isoline. There was little difference in emergence for the EC and CC lines on isoline corn, indicating there may be little cost to selection for Cry3Bb1 resistance. We also selected for rapid Cry3Bb1 resistance from both the EC and CC lines starting at the fourth generation. Intense selection pressure to survive produced completely resistant CC lines, which were never exposed to Cry3Bb1, as quickly as EC lines. This exemplifies the adaptability of WCR to plant incorporated protectants and the importance of resistance management plans for WCR and other corn rootworm species. We will continue to investigate the genes involved in Cry3Bb1 resistance and potential fitness costs associated with resistance. Understanding how resistance to a transgenic crop can evolve could help in insect resistance management guidelines.