Monitoring for soybean aphid resistance to foliar insecticides in Minnesota

Soybean aphid is the most significant insect pest of soybean in the upper Midwest, and foliar insecticides such as pyrethroids and organophosphates have primarily been used to control it since it was first found in 2000. The heavy reliance on insecticides may increase the likelihood of insecticide resistance. Resistance has not been documented or monitored in Minnesota, so the goal of this project was to begin monitoring insecticide susceptibility to a pyrethroid (i.e., lambda-cyhalothrin) across the state.  During the summers of 2013 to 2015, aphids were collected from up to seven locations per year across Minnesota. A laboratory colony of biotype 1 aphids, previously unexposed to insecticides since collection in 2000, was also used to represent baseline susceptibility. Within 24 hours of collection from each location, aphids were transferred to soybean leaf discs previously submerged in one of eight different concentrations of lambda-cyhalothrin or water as a control. Mortality and nymph production were assessed over 48 hours. Adult survivorship and nymph production per surviving aphid decreased with higher concentrations.  Aphids from Lamberton in 2013 and Becker in 2015 were significantly less susceptible (LC50: 4.3 and 1.6 ppm, respectively) to lambda-cyhalothrin than the biotype 1 population (LC50: 0.5 ppm). No aphids were less susceptible than biotype 1 aphids in 2014. Our findings show spatial and temporal variability in soybean aphid susceptibility to lambda-cyhalothrin, but we did not detect a clear trend towards resistance that would represent a significant concern for growers at this time at field rates of 149 ppm.