Abstract
Data on fenvalerate susceptibility, pyre throed use, and related information for 48 sites in British Columbia, Washington, Oregon, and California were analyzed to clarify the spatial scale and causes of fenvalerate resistance in pear psylla, Psylla Agricola Foerster. LC50’s for fenvalerate varied significantly among regions and subregions. The number of pyrethroid treatments per site explained a significant portion of the variation in fenvalerate LC50 over all sites, within regions, and within subregions. For a given number of pyrethroid treatments, the expected LC50 for fenvalerate varied significantly among regions and subregions. Within-season timing of pyrethroid treatments, continuity of pyrethroid use, intensity of pear production in the surrounding area, and number of pyrethroid treatments at neighboring sites were not significantly associated with LC50’s for fenvalerate. Prospects for managing resistance to fenvalerate are best in Oregon and California (where levels of resistance were generally lower) compared with Washington and British Columbia. Results showing that local variation in LC50 for fenvalerate was significantly associated with local variation in pyrethroid use suggest that growers can reduce local increases in resistance by limiting pyrethroid treatments.
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