REGIONAL RESISTANCE TO INSECTICIDES IN PSYLLA PYRICOLA FROM PEAR ORCHARDS IN OREGON

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Abstract
Using a slide-dip bioassay, Psylla pyricola Forester from intensively sprayed, minimally sprayed (IPM), recently abandoned, and long-abandoned pear orchards in the Hood River, Rogue River, and Willamette valleys of Oregon were surveyed for resistances to azinphosmethyl, endosulfan, Perthane®, and fenvalerate. Lethal-concentration (LC50) values showed resistance levels from 12- to 41-fold to azinphosmethyl and 5- to 12-fold for endosulfan in selected populations. Resistance patterns did not correlate with orchard-treatment histories in the Hood River and Rogue River valleys, where even long-abandoned orchards harbored resistance psylla. There was better association between treatment histories and resistance levels in populations from the more widely scattered orchards in the Willamette Valley. A regional analysis of mean LC50 values indicated that psylla resistance was significantly higher to azinphosmethyl in the Rogue River Valley and to endosulfan in the Hood River Valley. Regional resistance levels for these compounds reflect differences in intensity of past use. A regional hypothesis is proposed to explain the patterns of resistance developed in the Hood River and Rogue River Valleys, where pear production is intensive as opposed to the more localized patterns of resistance in the Willamette Valley. Applications of results to the management of resistance are discussed.