Study of the translocation of thiamethoxam/mefenoxam seed treatments to soybean flowers

Neonicotinoids are a group of insecticides that have dominated the pesticide market during the last 19 years. This group of insecticides is widely used by soybean growers in the North Central Region of the United States. Thiamethoxam, like other neonicotinoids, is generally more water soluble than other insecticides with a low molecular weight. These characteristics allow the movement of the insecticide through the plant vascular system and may allow the accumulation in vegetative and floral tissues. Neonicotinoids are often applied in combination with systemic fungicides. However, the research on ecological risks of neonicotinoids has been focused on the analysis of these compounds without considering their interaction with other agrochemicals. Neonicotinoid residues have been identified in flowers of different crops including canola, corn, sunflower and cucumber. Currently, there is no information available on insecticide/fungicide residues in soybean flowers. The objective of this study was to identify the concentration of thiamethoxam and mefenoxam in soybean flowers. A field experiment with four treatments: thiamethoxam-mefenoxam, thiamethoxam only, mefenoxam only, and untreated seeds, was developed. Flowers were collected at reproductive soybean stages R1-R2 in different locations in Nebraska. Quantification of the insecticides in floral tissue was developed using a modification of the QuEChErs extraction method and HPLC-MS/MS analysis. To correlate the possible effects of thiamethoxam and mefenoxam residues in soybean flowers on beneficial insects, oral toxicity of these compounds was evaluated on the honey bee Apis melifera. The results suggest no significant toxic effects of mefenoxam alone or in combination with thiamethoxam.