Thiamethoxam and soybean phenology effects on soybean aphid (Aphis glycines) population dynamics

Soybean aphid, Aphis glycines Matsumura, is a major pest in soybean, Glycine max (L.) Merr. , and has significantly changed soybean production across the North Central Region of the US. In addition to foliar-applied insecticides, neonicotinoid seed treatments have been effective at reducing aphid populations in the field, but use of systemic insecticides like thiamethoxam has not always resulted in a yield benefit. In addition, soybean growth stage and leaf age along with other biotic and abiotic factors can significantly influence soybean aphid population dynamics. However, the interaction between soybean phenology and thiamethoxam activity and the potential impact on soybean aphid growth rates in the field is not known. Our objectives were to quantify differences in plant biometrics between thiamethoxam-treated and untreated soybean plants and to assess the interaction between planting date (i.e., early, mid, and late) and systemic insecticide use (i.e., treated vs. untreated seed) on soybean aphid population dynamics. We used a factorial experimental design with planting date and insecticide as the main effects. In 2007 and 2008, whole plant samples were used to enumerate total number of aphids per plant for each treatment combination and aphid populations were monitored weekly. In addition, we measure various plant biometrics (e.g., height, biomass, node number) during the growing season. We also used single-plant exclusion cages to control for effects of natural enemies and aphid migration events on soybean aphid population dynamics. Interaction between planting date and thiamethoxam use as well as the impact of thiamethoxam on soybean biometrics will be discussed.