Residues of thiamethoxam and mefenoxam in vegetative and floral tissue of soybean at the early reproductive stage resulting from seed treatments

Thiamethoxam with mefenoxam is the most widely used neonicotinoid insecticide/fungicide mixture applied to soybean (Glycine max [L]) as seed treatments. Based on the systemic nature of thiamethoxam and mefenoxam, residues of this insecticide/fungicide mixture may be present in soybean vegetative and floral tissue and negatively impact beneficial insects. Although neonicotinoids are often applied in combination with systemic fungicides, the research on ecological risks of neonicotinoids has been focused on the analysis of these compounds without considering their interaction with other agrochemicals. The objective of this study was to identify the concentration of thiamethoxam and mefenoxam in soybean flowers and leaves during early reproductive soybean stages. A field experiment with three treatments: a thiamethoxam-mefenoxam combination, thiamethoxam only, and untreated seeds, was conducted. Flowers were collected during early reproductive soybean stages R1-R2 at different locations in Nebraska during two years. Quantification of the pesticides in floral tissue was conducted using a modification of the QuEChErs extraction method and HPLC-MS/MS analysis. Mean concentrations of thiamethoxam and mefenoxam in floral and vegetative tissue were exteremely low or not detected although trace levels of thiamethoxam were higher in floral tissues from seed treatments relative to the control. The only residue consistently detected above the method detection limit among plants grown from thiamethoxam treated seeds was the thiamethoxiam metabolite, clothianidin. Results from this study provide important baseline information to estimate the window of activity for this compound in soybean in the North Central Region of the United States.