Twospotted spider mites, Tetranychus urticae Koch, on new strawberry plantings near Watsonville and Salinas, Calif., were surveyed to detect the influence of immigrants from nursery plants or surrounding vegetation on resistance development to cyhexatin and formetanate. Susceptible mites (cyhexatin LC50 = 0.0039% AI; formetanate LC50 = 0.0018% AI) from nurseries initially colonized new strawberry fields, after they had survived cold storage in diapause (mean = 0.068 mites per plant) and on plants thereafter transplanted (mean = 0.0051 mites per plant) in fields.Often, these mites were subsequently outnumbered by resistant mites (mean cyhexatin LC50 = 0.064% AI; mean formetanate LC50 = 0.086% AI) that immigrated from surrounding vegetation. Occasionally, susceptible mites predominated until acaricides were applied. At three field sites, resistance to cyhexatin and formetanate resulted from immigration of resistant mites from surrounding vegetation. Such immigration resulted in moderate resistance levels to cyhexatin and formetanate by mid-March. At one isolated site without significant immigration of resistant mites, LC50’s of only 0.0035 and 0.0074% AI occurred for formetanate and cyhexatin, respectively, during the same time period. Alternative crop-production practices to improve management of immigrant T. urticae and subsequent acaricide resistance in this species are discussed.
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Miller et al. (1985) studied this question.
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