Efficacy of Thiamethoxam to Suppress Soybean Aphid Populations in Minnesota Soybean

Soybean aphid poses a serious threat to soybean production in the United States and Canada by reducing plant height, pod number, and yield. Since its introduction, foliar insecticides have been the most common method to control soybean aphid. Treatment of soybean seed with a systemic insecticide may provide producers with an alternative to foliar applied insecticides. Therefore, our objective was to evaluate the efficacy of a seed treatment to suppress soybean aphid populations in soybean. In 2003, 2004, and 2005 we conducted field studies and supportive laboratory and in‐field bioassays (2003 and 2004 only for bioassays) to assess mortality under controlled conditions. In the field, soybean aphid populations were assessed weekly by counting the total number of aphids per plant. In an excised‐leaf bioassay, aphid mortality persisted 23 to 35 days after planting. In an in‐field bioassay (2004 only), intact plants showed longer persistence of thiomethoxam and aphid mortality persisted 49 days after planting and mortality was significantly higher in the older leaves than in newly‐expanded leaves. In all years and under various aphid densities, thiamethoxam significantly reduced season‐long aphid pressure by 45.0 to 66.7%. However, thiamethoxam treatment did not significantly increase yield in three of four location‐years, which coincided with low aphid density in untreated control plots.