An experiment was conducted in 2013 to determine the extent that soil microbes degrade neonicotinoid insecticides, commonly used as insecticide seed treatments, into secondary metabolites. Soil was collected from a field where efficacy problems against thrips (Thysanoptera: Thripidae) were observed in cotton during 2013. At the same time, soil was also collected from an area with no previous exposure to insecticides. Part of the soil from each location was sterilized by autoclaving. Both sterilized and unsterilized soil were treated with an identical dilution of either Gaucho 600 (imidacloprid) or Cruiser 5F (thiamethoxam). After 25 days, samples were tested to determine the concentrations of neonicotinoid insecticides, including metabolites. Thiamethoxam and two of its metabolites were detected in soil treated with the Cruiser dilution. Imidacloprid and three of its metabolites were detected in soil treated with Gaucho. Sterilizing the soil sample significantly reduced the concentrations of imidacloprid and thiamethoxam metabolites. These results suggested that soil microbes were present in the soil samples from both locations that can degrade insecticides. The levels of degradation to secondary metabolites were approximately 14% and 2% or less for imidacloprid and thiamethoxam, respectively. It is unlikely that these relatively low levels of microbial metabolism would substantially impact the efficacy of insecticide seed treatments, especially considering the primary metabolites found retain some insecticidal activity.
