Does Pesticide-Induced Activity of Twospotted Spider Mite (Acari: Tetranychidae) Really Contribute to Population Increases in Orchards?

The effect of pesticides on the population density, dispersion, and activity of the twospotled spider mite, Tetranychus urticae (Koch) (Acari: Tetranychidae) was investigated under field and laboratory conditions on tart cherry, Prunus cerasus L. In the laboratory, the pesticides permethrin, phosmet, and phosalone significantly increased the time spider mites were active, whereas the pesticide azinphosmethyl did not significantly affect activity. In the field, by 12 d after pesticide application, mite populations on all pesticide-treated trees were significantly higher than those on the water-sprayed control trees. Analysis of these populations showed that no significant differences in dispersion occurred on any treatment between the evaluation 2 d before application and the evaluation 2 d after application. These facts, combined with the very low numbers of predators observed in this test, suggest that pest resurgence is probably not a result of increased mite activity, but is instead related to a pesticide’s ability to stimulate mite physiology, either directly or indirectly, through changes in host plant nutritional value.