Effects of neonicotinoid seed treatments on soybean aphid and its natural enemies

Insecticidal seed treatments are increasingly being applied to soybeans in North America, and several recent studies question what they add to current pest management. Here, we examine the effects of two neonicotinoid insecticidal seed treatments on insect populations (pest and natural enemies) in SD soybeans over 2 years. Moreover, we conducted laboratory experiments to determine the duration that seed treatments remained effective against the soybean aphid (Aphis glycines, Hemiptera: Aphididae) and how thiamethoxam affected survival of 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 within 46 days after planting, prior to aphid populations damaging the crop. Bean leaf beetles, a sporadic pest in our area, were reduced by the seed treatments. But, there were no yield benefits of insecticidal seed treatments over the 2 years of the study at this location. Natural enemy communities were significantly reduced by thiamethoxam seed treatments relative to the untreated control, particularly populations of Nabis americoferus (Hemiptera: Nabidae). Chrysoperla (Neuroptera: Chrysopidae) adults were reduced in the imidacloprid-treated plots. 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 the Northern Great Plains and adds to the discussion by suggesting that generalist predators are adversely affected by the insecticides.