Ambient Moisture Causes Methomyl Residues on Corn Plants to Rapidly Lose Toxicity to the Pest Slug, Arion subfuscus, Müller (Gastropoda, Stylommatophora).

Abstract

Abstract The carbamate insecticide methomyl is sometimes used to control slugs in field corn and soybean by foliar applications, but control outcomes in research trials and commercial operations have been mixed. In this study, laboratory bioassays were conducted on dusky slug, Arion subfuscus Muller, a common pest of corn and soybean in the Mid-Atlantic United States, to evaluate residual toxicity of Lannate LV (methomyl) at low and high concentrations corresponding to label recommended field rates, and if toxicity may be affected by ambient moisture or repellency to treated plants. Without wetting events, methomyl residues on corn plants caused 90 to 100% mortality of A. subfuscus for two days and 70 to 90% mortality for six days. When corn plants were briefly misted with ca. 0.3 cm of water 6 hours after methomyl application, residues from low and high methomyl rates caused 36% mortality 12 hours after treatment, and 0 and 5% mortality 24 hours after treatment for low and high rates, respectively. Repellency of A. subfuscus to corn plants treated with the high rate of methomyl was narrowly significant (P = 0.04) and low rate was not significant. These results suggests that high ambient moisture needed to elicit slug activity also abates toxicity of methomyl residues, making it unlikely to achieve control in the field.