Baseline susceptibility of soybean aphid (Aphis glycines Matsumura) to thiamethoxam in the north central region of the United States

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is an introduced pest, first reported in the United States in 2000. It is considered a severe pest of soybean in North America. The widespread use of the insecticide thiamethoxam as a soybean seed treatment in the Midwestern United States is reaching approximately 80% in some regions. As a result, the risk of selection for resistance has increased. Ten soybean aphid colonies were obtained from different geographic locations in South Dakota, North Dakota, Minneapolis, Wisconsin, and Nebraska. Two bioassay methods, a vial bioassay and a systemic bioassay, were performed to evaluate susceptibility of A. glycines to five different concentrations of thiamethoxam. Aphid mortality was recorded after 12 and 24 hours for the vial bioassay, and after seven days for the systemic bioassay. Soybean aphid response to thiamethoxam in both bioassays was similar, and both allowed susceptibility measurement with consistent variation among populations tested (respectively presenting 4.0, 2.5, and 3.0 fold difference). Some significant differences were observed based on non-overlapping confident intervals, but they are unlikely to indicate significant levels of resistance. The data obtained in this study provides a baseline for future assessments and may be important for resistance monitoring and detection, and for the development of an insecticide resistance management program for A. glycines in North America.