The hot, dry growing season has led to infestations of two-spotted spider mites in many areas of Minnesota. Over the past two weeks, increasing numbers of fields with economic infestations have been observed. Most, but not all, have been in areas with obvious drought stress. Soybeans appear to have the heavier infestations although spider mites also be found in corn and other crops.
Complicating these developments are reports of inconsistent performance of chlorpyrifos (e.g., Lorsban), presumably due to populations of resistant mites, which were first documented in Minnesota in 2012. Since then, chlorpyrifos applications to soybean and other crops have continued selection for resistant populations.
Two-spotted spider mite populations are typically kept in check by predators, fungal disease, and adequate moisture. Warm, dry weather and moisture stress of the crop favor mite reproduction. When conditions are favorable for the mites, their rapid life cycle and high reproductive rates can quickly create large populations and damage crops. Other factors may accentuate mite populations, including some insecticides and fungicides. Once spider mite infestations are established and building, a single rainfall event or even short-term change in weather is not likely to stop an infestation.