Selection for resistance of soybean aphid to the neonicotinoid insecticide thiamethoxam

Neonicotinoid seed-treated soybean is a widespread preventative practice, exposing several consecutive generations and all developmental stages of A. glycines Matsumura (Hemiptera: Aphididae), thereby increasing the chances for resistance to develop. Small levels of resistance to organophosphate insecticides have been reported with soybean aphids in China, which suggests that it is important to understand the rate at which resistance will develop to other insecticides with unique modes of action, such as thiamethoxam. Establishing laboratory selected strains provides a means to evaluate potential mechanisms of resistance as well as investigate its associated molecular, genetic, and biochemical basis. We imposed selection with technical grade thiamethoxam at concentration approximating the LC₉₀ using a whole-plant systemic bioassay. Soybean seedlings were allowed to take up the insecticide solutions for 48 hours followed by the transfer of 100 wingless aphids. After a period of 9 days, surviving aphids were collected, and used to perform the concentration-response experiments, and to calculate the instantaneous rate of increase (ri). The LC₅₀’s were obtained with mortality recorded after 24 hours in a glass-vial bioassay. The ri was calculated from age synchronized aphids after 7 days. For both parameters, comparisons were made between the thiamethoxam selected, non-selected and susceptible strains. This selection will be imposed to the rate at which resistance evolves under these conditions. The results of this study will provide valuable information about the risk of resistance in soybean aphids, as well as population level response to thiamethoxam.