Ten Years of Bt Resistance Monitoring in the European Corn Borer: What We Know, What We Don’t Know, and What We Can Do Better

The susceptibility of European corn borer (ECB) to the Bt toxins expressed by transgenic corn has been determined annually since 1995 for populations established from across the U.S. Corn Belt as part of a coordinated resistance monitoring program. Initial assessments were conducted before commercial release of transgenic corn to establish a baseline of susceptibility. From this initial baseline, diagnostic concentrations were designated. Annual assessments of susceptibility have been conducted using both diagnostic bioassays and complete dose response assessments. Although current bioassay-based techniques for resistance detection may be limited in sensitivity, ECB populations appear to remain susceptible to the Cry1Ab toxin which has been available to growers in transgenic hybrids since 1996. However, a population that exhibited higher rates of survival than expected in diagnostic bioassays triggered additional tests to confirm heritable resistance and on-plant survival. These tests indicated that although a relatively high level of resistance to Cry1Ab was present, the resistant insects did not survive on vegetative stage plants and more importantly, there was no indication of increased levels of resistance in subsequent years. One of the biggest limitations of current monitoring efforts has resulted from changes in the source of toxin available for bioassays. As a consequence, we have been developing quantitative techniques to measure toxin concentration and purity. Results indicate considerable variability in toxin potency and in accuracy of various quantitative techniques. Standardized methods to quantify Cry toxins are essential to data integrity and critical to future monitoring efforts