Soybean aphid infestation and crop yield in relation to cultivar, foliar insecticide, and insecticidal seed treatment in South Dakota

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), remains an important pest of soybean, Glycine max (L.) Merr., in areas of the Midwestern United States and southern Canada. Options for managing soybean aphid are currently limited to insecticides and aphid-resistant varieties. Farmers commonly employ a prophylactic treatment of soybean seed with a neonicotinoid insecticide such as thiamethoxam and often a remedial, late-season application of foliar insecticides such as the pyrethroid, λ-cyhalothrin. Environmental concerns about detrimental side effects of neonicotinoids and the substantiation of pyrethroid resistance in field populations of soybean aphid have strengthened the case for planting aphid-resistant soybean cultivars. We conducted a three-year field study in eastern South Dakota, United States, that evaluated commercially-available active substances and enhanced plant materials for soybean aphid management that included a thiamethoxam seed treatment, spray application of λ-cyhalothrin, and an aphid-resistant soybean cultivar containing a pyramid of the Rag1 and Rag2 aphid-resistance genes. Soybean aphid infestations had sustained, economically injurious levels in 2015, but not the other two years. The three management tactics independently reduced soybean aphid population densities. Thiamethoxam and the resistant cultivar suppressed cumulative aphid-days (CAD) in all three years, whereas the λ-cyhalothrin spray decreased CAD in 2015 and 2017. By far, the resistant cultivar had the greatest impact on soybean aphid populations, reducing CAD by 28- to 150-fold year−1, with mean numbers of aphids much less than 100 soybean aphids plant−1. Thiamethoxam seed treatment reduced CAD by 1.7- to 3.5-fold year−1, and λ-cyhalothrin reduced it from 2.0- to 5.6-fold year−1. Soybean yield was not affected by treatments in 2015. Yield was greater for the susceptible than for the resistant cultivar in 2016, and yield was greater for plots without foliar insecticide in 2017. Implications for the use of these management tactics against soybean aphid are discussed.