Do generalist predators prevent spider mite (Tetranychus urticae) outbreaks in potato fields?

Late-season spider mite (Tetranychus urticae) outbreaks are common in Washington State potato crops, requiring expensive pesticide applications. Progress in controlling these pests has been limited by our uncertainty about what causes mite densities to increase. It is likely that a leading cause of spider mite outbreaks is the removal of predators following early-season insecticide applications that target other pests. To test this hypothesis we intensively surveyed mite and predator populations on treated and untreated potato fields, searching for spider mite DNA within the stomachs of their predators to determine which predator species are most important in spider mite suppression. Predator abundance and diversity was greatly reduced in fields treated with broad-spectrum insecticides. Results suggest that predators in the genera Geocoris and Nabis were eating spider mites at low pest densities and when other potential prey species also occurred. These relatively consistent, focused attacks on spider mites by the predatory bugs may be important in diffusing spider mite outbreaks before the pests reach damaging levels. Reducing the frequency of broad-spectrum insecticide applications, and/or adopting the use of selective insecticides that harm spider mites but not predators, could allow growers to capture these benefits of natural pest control.