Combined effects of drought stress and neonicotinoid seed treatment on Banks grass mite (Oligonychus pratensis) in corn

Spider mites, a cosmopolitan pest, can proliferate with elevated temperatures and prolonged dry conditions. More recently, neonicotinoid insecticides have been implicated in spider mite outbreaks in the Intermountain West. Drought conditions are prevalent and predicted to persist, while neonicotinoids are one of the most widely used seed treatments in corn. Drought-stressed plants can have increased plant nutrient availability, while neonicotinoids appear to modify plant physiological pathways leading to increased mite abundance. What it is not clear is how drought-stress and neonicotinoids interact and what the outcomes are for spider mite outbreaks. We conducted two field studies to examine the effect of drought stress (+,-) and neonicotinoid treated corn seed (+,-) on Banks grass mites. In a cage experiment, corn treated with clothianidin was irrigated at 100% or 50% of the total water lost by evapotranspiration (ET). 200 mites were added to 20 six-week old plants. In a parallel non-cage study, we evaluated the effect of an additional neonicotinoid, thiametoxam, and plants irrigated at 100% ET and 25% ET on resident mite populations. For both studies mites were monitored and recorded weekly throughout the season. We found that the abundance of spider mites increased when corn was drought stressed (P=0.016). Moreover, the effect of drought on corn was exacerbated by neonicotinoids especially when plants were heavily water stressed (25% ET) (P<0.01). Growers may face more frequent mite outbreaks given the predictions for drought episodes and current neonicotinoid use, and will need to adjust their management of mites.