Efficacy of neonicotinoid seed treatments to reduce soybean aphid populations under field and controlled conditions in Nebraska

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is native to Asia and was recently (2000) detected in North America. Since then, it has become a significant threat to U.S. soybean production. Although neonicotinoid insecticide seed treatments, imidacloprid and thiamethoxam, have been suggested as a method of control, the season-long efficacy is still uncertain. Therefore, the use of imidacloprid and thiamethoxam seed treatments to control soybean aphid in Nebraska were examined. Soybean aphid populations were monitored weekly in an irrigated field study planted during the later half of the typical Nebraska planting window during 2005 and 2006. Imidacloprid and thiamethoxam were quantified by leaves through time, and leaf specific bioassays were conducted. In 2005, aphid populations were very low; however, in 2006 aphid numbers were significantly higher than 2005, reaching ≈1,200 aphids per plant in the untreated plots. Aphid injury significantly reduced yield and individual seed size in 2006. Imidacloprid significantly reduced aphid densities in 2006 but not below the economic threshold. In 2006, thiamethoxam held aphid densities below the economic threshold. Leaf specific bioassays and leaf specific imidacloprid and thiamethoxam quantification indicated that thiamethoxam was present in the plant at higher concentrations and for a longer period.