Do aphid resistant soybeans need insecticide for optimal yield?

Soybean growers in the North Central US now have access to soybean aphid resistant varieties that contain the Rag1 gene. Previously, growers were limited to insecticides for managing outbreaks of soybean aphid that exceeded established economic thresholds (ET). Field tests of the Rag1 gene within the North Central US have shown that soybeans containing this gene are not always aphid‐free and reported populations have increased to densities well above published EILs for susceptible soybean. As aphid‐resistant cultivars become available, the ET established for susceptible soybean may not be appropriate for managing aphids infesting these varieties. In particular, an underlying assumption in the threshold and injury level calculations is a population doubling time of 7 d. For resistant lines, doubling times range from 10-14 d. The objectives of this study were to 1) develop yield-loss relationships using naturally-infested field plots and 2) calculate ETs and EILs appropriate for Rag1 bearing varieties. This study was conducted in multiple states within the North Central US including IA, IN, KS, MI, MN, NE, ND, OH, SD, and WI. We measured plant exposure to soybean aphids by calculating the cumulative aphid‐days (CAD) throughout each growing season. For each plot we averaged across replications to make a decision when to apply a foliar insecticide for our target CAD densities of 8,000, 16,000 and 32,000. Soybean aphid densities in replicated field plots were relatively low in many of the participating states. Results from the 2009 and 2010 field studies will be discussed.