Effects of neonicotinoid seed treatments and drought stress on abundance and fecundity of spider mites on soybean plants

Neonicotinoid insecticides are often used as a seed treatment for row crops such as soybean, and their use in soybean production has recently increased. Neonicotinoids are incorporated into the plant and expressed systemically, allowing for targeted effects on herbivores. Their impact on non-target arthropods, however, remains the focus of much research. Spider mites (Acari: Tetranychidae), which are not susceptible to neonicotinoids, are of particular interest because they tend to increase in abundance after neonicotinoid use. The goal of this research was to document the propensity for spider mite outbreaks on soybean plants after treatment with a neonicotinoid insecticide thiamethoxam. Moreover, because spider mites tend to be more abundant under drought conditions we investigated the impact of neonicotinoid use and drought stress alone, as well as when combined to determine whether there was an additive effect of the insecticide and drought stress on spider mite abundance. In greenhouse experiments, we examined how two levels of thiamethoxam seed treatment (treated and untreated) and two levels of drought stress (present and absent) affected spider mite abundance and fecundity. We found that spider mites were significantly more abundant on plants exposed to the neonicotinoid insecticides but were not consistently affected by watering regime. This research provides insights into the relationship between neonicotinoid insecticides, spider mites, and plant physiology and has important implications for sustainable management of spider mites in soybean fields.