Location: Integrated Cropping Systems ResearchTitle: Effects of neonicitinoid seed treatments on soybean aphid and its natural enemies Author
Submitted to: Journal of Pest Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2011
Publication Date: 2/25/2012
Citation: Seagraves, M.P., Lundgren, J.G. 2012. Effects of neonicitinoid seed treatments on soybean aphid and its natural enemies. Journal of Pest Science. 85(1):125-132. Interpretive Summary: In the U.S., seed and chemical companies are advocating the use of insecticidal seed treatments involving systemic insecticides to maximize yields. This practice has come under criticism by the entomological community, since the insecticides are largely absent before the key pest of soybeans, the soybean aphid, reaches damaging levels, and aphids do not require perennial treatment. Over two years, we tested the effects of thiamethoxam and imidacloprid (two neonicotinoid insecticides currently available to soybean producers) and an untreated control on insect communities (pests and beneficials) and soybean yields. We found no differences in the abundances of soybean aphids or other pests (thrips, grasshoppers) in the three treatments. Bean leaf beetles were significantly less abundant in the insecticide-treated plots, but yields and grain qualities were equivalent in all treatments. A laboratory bioassay using field-collected plant material showed that all insecticidal bioactivity against soybean aphids had left the plant prior to aphid populations outbreaking. Perhaps more alarming is that generalist predators were reduced by 25% in the thiamethoxam-treated plots relative to the untreated control. A laboratory assay using the minute pirate bug (Orius insidiosus) confirmed that thiamethoxam-treated plants were directly toxic to this key predator of the soybean aphid. We conclude that insecticidal seed treatments unnecessarily cost soybean producers $10-15 per acre without providing any pest management or yield benefits. Moreover, this practice may hurt IPM in the long term by reducing the predator community of the target pests that are not being controlled by the insecticides.
Technical Abstract: Insecticidal seed treatments are becoming a pervasive presence on soybeans in North America, and several recent studies question their efficacy. Here, we examine the effects of two neonicotinoid insecticidal seed treatments on insect populations (pest and natural enemies) in SD soybeans over two years. Moreover, we conducted laboratory experiments to determine the duration that seed treatments remained effective against the soybean aphid, and how thiamethoxam affected survival one of the aphid’s predators, Orius insidiosus (Hemiptera: Anthocoridae) on soybean. Soybean aphids, thrips, and grasshopper populations were unaffected by the insecticidal seed treatments in the field. The laboratory trial revealed that all bioactivity of the seed treatments against soybean aphids was gone by June 30th, prior to aphid populations damaging the crop. Bean leaf beetles, a sporadic pest in our area, was reduced by the seed treatments. But there were no yield benefits of insecticidal seed treatments for producers. Natural enemy communities were significantly reduced by thiamethoxam seed treatments relative to the untreated control, particularly populations of Chrysoperla and Nabis americoferus. In the laboratory, rearing O. insidiosus on soybean plants treated with thiamethoxam resulted in higher mortality for both the nymphs and the adult stage. Offering the predator insect prey on the thiamethoxam-treated plants improved survival of the adult stage, but not the nymphal stage. This work confirms that insecticidal seed treatments offer little benefit to soybean producers of our region, and adds to the discussion by suggesting that generalist predators are adversely affected by the insecticides.