Interactions between neonicotinoids and water stress lead to Banks grass mite outbreaks in corn

Drought conditions are prevalent in the Intermountain West and are predicted to persist, while neonicotinoids are widely used insecticides as seed treatment in corn. Spider mites, a cosmopolitan pest, are thought to proliferate with elevated temperature and prolonged dry conditions and with applications of neonicotinoids. Drought is thought to increase plant nutrient availability and neonicotinoids appear to modify plant physiological pathways, both leading to an increase in spider mites. Most of these studies have focused on two-spotted spider mites and on young corn plants in controlled environments. What it is not clear is how drought and neonicotinoids interact and what the outcomes are for Banks grass mites and corn throughout the growing season. We conducted a manipulative field study in two seasons using corn plants grown in 2×2×2 m cages either with or without neonicotinoid seed treatments (control, clothianidin, and thiamethoxam). Drought stress (+,-) was applied to cages by replacing either 100% (optimal irrigation) or 50% (drought stressed) of the total water lost by evapotranspiration. Banks grass mites were added to plants and recorded weekly throughout the season. A parallel greenhouse study was also conducted. We found that the combination of drought stress and clothianidin-treated seed increased mite density during the season until plants bloomed (P=0.03 F=2.74), compared to non-treated seed and clothianidin-treated seed with optimal irrigation. Interestingly, mite density increased with thiamethoxam-treated seed regardless of irrigation treatment. The combined effect of drought and neonicotinoid treatments can elevate mite density, but the responses vary depending on the neonicotinoid active ingredient.