Elevated Numbers of Spider Mites on Thiamethoxam Seed-Treated Cotton in a Greenhouse Study

Thiamethoxam is a neonicotinoid seed treatment for cotton used widely in the cotton industry. Thiamethoxam (Cruiser^® formulation) is highly efficacious against key pests of cotton such as aphids, thrips and whiteflies. Anecdotal reports of outbreaks of spider mites, a non-susceptible pest, on thiamethoxam seed-treated cotton, and published reports of widespread outbreaks of spider mites after applications of another neonicotinoid insecticide, imidacloprid, prompted our investigation. Greenhouse experiments were design to test incidence and severity of spider mite outbreaks on cotton. In two separate trials, we found consistently higher numbers of mites on cotton treated with Cruiser^®. Absence of natural enemies that could suffer intoxication by exposure to thiamethoxam though cotton or tainted prey suggests a plant-mediated or direct effect of the insecticide on the mites. Additional experiments examining the arthropod community in seed-treated cotton fields, impact of spider mites on cotton yield and the effects of thiamethoxam on cotton physiology are under way.