Effect of pesticides on twospotted spider mite (Acari: Tetranychidae) populations on red raspberries in western Washington

Abstract Twospotted spider mite, Tetranychus urticae (Koch), numbers were monitored in 18 red raspberry fields for 3 years. The growers' pesticide records were examined at the end of each year. Fields having more than 25 mites per leaflet received 2.25–3.55 times as many insecticidal sprays as did those with mite populations of less than 25 mites per leaflet. Only 20% of the fields receiving 0–1 sprays attained more than 25 mites per leaflet whereas 91% of the fields that received more than two sprays had that number of mites. There was evidence that pyrethroid insecticides caused sharp increases in mite numbers. Numbers of fungicide applications also tended to correlated with mite numbers but this was probably because growers who used insecticides liberally usually also used fungicides liberally.