Effect of imidacloprid application timing on twospotted spider mite (Acari: Tetranychidae) on hop

Imidacloprid and other neonicotinoids have been associated with secondary pest outbreaks of twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae). Imidacloprid is commonly applied in hop for the suppression of various pest organisms. A 3-yr study was conducted in western Oregon to determine the impact of a single foliar application of imidacloprid on twospotted spider mites and their predators in hop when made in mid-May versus mid-June. Applications made in June generally resulted in greater densities of twospotted spider mites as compared to applications made in May or when plants were nontreated, although this effect was year-dependent. Predatory mites (Acari: Phytoseiidae) were present at low levels and seemingly were not impacted by imidacloprid application. Orius spp. (Hemiptera: Anthocoridae), Stethorus spp. (Coleoptera: Coccinellidae), and other macropredators were minimally reduced in one year on 2 dates. Given the small impact on the key predator groups, the mechanisms of imidacloprid-induced secondary outbreak of twospotted spider mite appears to involve factors other than direct toxicity to predators. Practically, avoiding the application of imidacloprid when twospotted spider mite densities are increasing (generally June) should minimize risk of secondary outbreaks of this organism.