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Ecological risks of the conventional insecticide/fungicide seed treatment mixture of thiamethoxam and mefenoxam in soybean on beneficial insects

Carolina Camargo Gil, University of Nebraska - Lincoln


The impact of neonicotinoid seed treatments on beneficial insects has been a controversial topic during the last years. While neonicotinoids are usually used as mixtures with systemic fungicides, few studies have examined the impact of the mixtures on beneficial insects. Pesticide mixtures can have synergistic, additive, or antagonistic effects on the toxicity of neonicotinoids on non-target species. Thiamethoxam with mefenoxam is the most used neonicotinoid insecticide/fungicide mixture applied to soybean. Based on the systemic nature of thiamethoxam and mefenoxam, residues of this insecticide/fungicide mixture can be present in soybean vegetative and floral tissue with potential impacts to beneficial insects. This study focuses on the interaction of these compounds, their environmental fate in plants, their toxic effects on honey bees, and lethal and sub-lethal effects on key predatory species in soybean. Concentrations of neonicotinoids in both floral and vegetative tissues were low or not detected, and the effects on target and non-target insects are more likely to be sub-lethal, if at all. There was a mild antagonist interaction with the fungicide, resulting in reduced honey bee mortality. In predatory species, there were no significant differences in the abundance of Orius insidiosus and Chrysoperla rufilabris in soybean treated with thiamethoxam alone or with the mixture. Consumption of soybean aphid by both predators was not affected at evaluated concentrations of thiamethoxam in the insect prey. However, laboratory studies on toxicity of thiamethoxam on Orius insidiosus suggest potential toxic effects of this neonicotinoid based on the time of arrival of the predator to the field and the type of exposure to neonicotinoids. Toxicity studies of mixtures of different classes of pesticides used in seed treatments are rarely available. To our knowledge this is the first study that evaluates the interaction of mefenoxam on acute toxicity of thiamethoxam. Studies of mixture toxicity of seed mixtures are imperative to minimize the risk of pesticides to beneficial insects by a careful selection of products with lower toxicity.

Subject Area


Recommended Citation

Camargo Gil, Carolina, "Ecological risks of the conventional insecticide/fungicide seed treatment mixture of thiamethoxam and mefenoxam in soybean on beneficial insects" (2016). ETD collection for University of Nebraska-Lincoln. AAI10102754.