The route and rate of thiamethoxam soil degradation in laboratory and outdoor incubated tests, and field studies following seed treatments or spray application

BACKGROUND: The route and rate of degradation of thiamethoxam in the laboratory and field was investigated. The effect of dark incubation versus light/dark cycles, seed treatment versus spray, and watering‐in for spray application was explored in side‐by‐side trials. RESULTS: Geometric mean DT(50) values were 75.4 days in OECD307 studies, and 18.3 (spray) and 16.5 (seed treatment) days in the field. In laboratory soil core studies DT(50) values were 24.9 to 43.5 days, with the lowest value from the light/dark incubated soil core. Mean clothianidin formation was 19.7% applied thiamethoxam [mol/mol] in OECD307 studies and 17.5 (spray) and 3.4% (seed) in field trials. CONCLUSION: Soil DT(50) values decreased with increasingly realistic tests (laboratory OECD307 to soil cores to soil cores with a light/dark cycle to field trials). The majority of the differences were associated with the soil treatment in OECD307 studies which destroys soil structure and retards the degradation rate; and from the impact on soil pore water movement in light/dark conditions. Degradation rates in the field were comparable between spray application and seed treatments. Maximum clothianidin concentrations were four‐fold lower for seed treatments than for spray application in field studies. © 2018 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.