Do generalist predators prevent spider mite outbreaks?

Late-season spider mite (Tetranychus urticae) outbreaks are common in Pacific Northwest potato crops, requiring expensive pesticide applications. Progress in controlling these pests has been limited by our uncertainty about what causes mite densities to increase. One hypothesized reason is that early-season spray for aphids and other pests devastate natural enemy communities, freeing mites from biological control. We investigated this possibility using molecular gut content analysis, tracking predation of spider mites throughout the season in production potato fields located throughout central Washington. We found that the predatory bugs Nabis alternatus and Geocoris bullatus were the most consistent spider mite predators in non-outbreak fields, where no spider mite feeding damage was apparent. Bembidion ground beetles and Orius bugs were less-common predators under low-mite-density conditions. From previous work we know that Nabis and Geocoris densities often are reduced 90% or more in potato fields treated with broad spectrum insecticides. This observation, combined with the observed importance of these two predator taxa for spider mite suppression reported here, provides circumstantial evidence in support of a weakening of spider mite biological control in heavily-sprayed potato crops. Ongoing work will examine the contributions of other possible contributors to mite problems in potatoes, including (1) the common use of neonicotinoid insecticides, known to “flare” spider mites on other crops, and (2) immigration of spider mites into potatoes from crops that senesce earlier in the season.