Monitoring for Neonicotinoid Resistance in Soybean Aphid

The use of neonicotinoid seed-treated soybean is widespread throughout the North Central U.S. This imposes high selective pressures to consecutive generations of soybean aphid (A. glycines Matsumura), the most economically damaging soybean insect pest, thereby increasing the probability for resistance to develop to this class of insecticides. Currently, thiamethoxam is the most common neonicotinoid used for soybean seed treatments. The susceptibility of various field and laboratory populations of soybean aphid to thiamethoxam was determined through glass-vial and systemic detached-leaf bioassays. Dead and living aphids were recorded after 24 hours in the vial bioassay, and after 7 days in the systemic bioassay. In addition we imposed selection with thiamethoxam at a concentration approximating the LC₉₀ using whole-plant systemic exposure. After 7 days of exposure, living aphids were tested with the vial bioassay and also used to calculate the instantaneous rate of increase (ri). For both parameters, comparisons were made between the thiamethoxam selected, pooled field populations and a susceptible laboratory strain. Thiamethoxam was highly toxic and inhibited population growth at sub-lethal concentrations. The tolerance ratios (TR) were relatively small and unlikely to indicate significant levels of resistance. The results of this study provide a baseline for future assessment and contribute to the validation of standardized bioassay techniques that are critical for resistance monitoring of soybean aphids. The instantaneous rate of increase associated with concentration-response bioassays provided valuable information about the risks of resistance to develop in wild soybean aphid populations.