Thiamethoxam Toxicity and Effects on Consumption Behavior in Orius insidiosus (Hemiptera: Anthocoridae) on Soybean

This study evaluated the effects of thiamethoxam in soybean plants and soybean aphids on the behavior of Orius insidiosus, a major predator of soybean aphids. Predatory insects are critical to the control of soybean aphids, and O. insidiosus can be an effective predator to suppress field populations of aphids. Experiments were designed to test toxicity of thiamethoxam to O. insidiosus through soybean leaves, soybean aphids that fed on treated leaves, and in the field. For soybean field experiments, plants were grown with uncoated seed, seed coated with fungicides (mefenoxam, fludioxinil, and sedaxane), seed coated with thiamethoxam, and seed coated with fungicides and thiamethoxam. The authors found significant increases in mortality as concentrations in soybean plant material increased in lab experiments, however these levels were determined to be unlikely to occur in the field or to overlap with O. insidiosus migration into fields. O. insidiosus could overlap with sublethal concentrations of thiamethoxam in soybean plants in fields. Results from O. insidiosus feeding on soybean aphids that consumed soybean leaves treated with thiamethoxam suggest that mortality could occur from predation on soybean aphids in treated fields. Mortality of O. insidiosus was variable in response to the concentrations that aphids were fed, which may reflect variability in the amount of insecticides consumed by and residing in the aphids. The number of aphids consumed by the predators declined at higher rates of thiamethoxam. In field experiments, the authors did not observe significant differences in the abundance of O. insidiosus amongst treatments but soybean aphids were also not observed in the fields so potential effects of consuming aphids could not be evaluated. The authors suggest that sublethal effects could still impact populations of predators especially with continuous exposure over space and time, although they will be difficult to evaluate in field studies. Residues in prey and vegetative tissue can have significant effects on O. insidiosus, but planting dates, arrival time, and the availability of other insect prey can all impact effects in the field. Further research to determine the concentration of neonicotinoids that early season prey accumulate would help determine risks to beneficial predatory species.