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 DT50 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 DT50 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 DT50 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.

The degradation rate of thiamethoxam in European field studies.

BACKGROUND: Thiamethoxam is a systemic and contact pesticidal active substance in the neonicotinoid class of insecticides used worldwide to control a range of insects. Recently, concerns have been expressed regarding possible effects of neonicotinoids on bees and other wildlife. The DT50 of thiamethoxam in soil may be crucial to assessing the potential long-term exposure of non-target organisms to thiamethoxam. There are currently no detailed publicly available data for the field soil degradation of thiamethoxam under European conditions. We give field soil DT50 values of thiamethoxam from studies conducted in several European locations, under a range of realistic agronomic conditions. RESULTS: Field soil DT50 values normalised to 20 °C ranged between 7.1 and 92.3 days (geomean = 31.2 days; n = 18). CONCLUSION: The degradation rate of thiamethoxam was not significantly affected by application type, cropped fields versus bare soil, soil pH, organic matter content or repeated annual applications. Soil photolysis and leaching were negligible; therefore, calculated DT50 values were taken to represent microbial degradation. The field degradation rates of thiamethoxam were faster than those previously reported from laboratory degradation studies. They demonstrate that thiamethoxam will degrade to concentrations that are <10% of the maximum within a year of application, and will not accumulate in soil after repeated applications.