Baseline susceptibility of the soybean aphid to the neonicotinoid insecticide thiamethoxam in the north central region of the United States

The soybean aphid, Aphis glycinesMatsumura (Hemiptera: Aphididae), is an introduced species, and is one of the most important pests of soybean in North America. The risk of selection for resistance is a major concern regarding the widespread use of the neonicotinoid insecticide thiamethoxam as a soybean seed treatment in the Midwest United States, with treated acres reaching approximately 80% in some regions. Ten soybean aphid colonies were obtained from field collections from different geographic locations including South Dakota, North Dakota, Minnesota, Wisconsin, and Nebraska. A susceptible strain was obtained from Bayer Crop Sciences, and populations of biotypes 1, 2 and 3 were obtained from USDA-ARS-MWA-URBANA. Two newly developed bioassay methods were conducted to evaluate the differences in susceptibility of the soybean aphids to different concentrations of thiamethoxam. Wingless aphids from all development stages were used in the bioassays. Dead and living aphids were recorded after 12 and 24 hours for the vial bioassay, and after 7 days for the systemic bioassay. Results from both bioassay methods indicated consistent variation and similar responses in thiamethoxam susceptibility among populations. Differences between the most susceptible and most tolerant populations were calculated for the 12, 24 hours vial bioassay, and for the systemic bioassay, respectively showing a 9.69, 9.83, and 4.61 fold “difference”. The data obtained here provide a baseline for future assessment and may be very important develop standardized bioassay methods for resistance monitoring activities, resistance detection, and for the development of an insecticide resistance management program.

Key Words: baseline susceptibility, thiamethoxam, bioassay, Aphis glycines, Glycine max