Determination and Dissipation of Thiamethoxam and Its Six Major Metabolites in Spring Wheat and Soil

Thiamethoxam seed treatments are an important measure in controlling wheat aphids; however, the residue analysis and dissipation dynamics of thiamethoxam and its metabolites in wheat were not clear yet. In this study, we developed a rapid method for the analysis of thiamethoxam and its six major metabolites (clothianidin, desmethyl-thiamethoxam, desmethyl-clothianidin, desnitro-thiamethoxam, desnitro-clothianidin, and thiamethoxam-urea) in a wheat-growing system. Average relative recoveries of the method were in the range of 70.3–118.4%, with the relative standard deviations being lower than 15.0% and the limit of quantitation being 1.0–5.0 μg/kg. The developed method was further applied to explore the dissipation dynamics of thiamethoxam and its six metabolites in spring wheat. The field experiment results demonstrated that wheat leaves were the main part where thiamethoxam was accumulated and metabolized, and the concentration of thiamethoxam in wheat leaves declined by more than 93% during the sampling period. Five metabolites (except for desmethyl-thiamethoxam) were detected in wheat leaves and wheat ears. Terminal residue of thiamethoxam and its metabolites at harvest time were below the maximum residue limit, indicating that the wheat grains were safe with regard to consumption.