Abstract
Resistance to the pyrethroid, fenvalerate, was studied in pear psylla, Psylla pyricola Foerster, from several pear-production areas of the northwestern USA. Bioassays showed that a psylla population from Wenatchee, Washington, was highly resistant (242-fold) compared with a susceptible strain. Populations from Yakima, Washington, and Hood River and Medford, Oregon, were moderately resistant (26-fold), and a population from Corvallis, Oregon, showed only low levels of resistance (tenfold). Fenvalerate resistance appears to be polygenic and semi-recessive or intermediate in nature. Low levels of resistance are maintained in F 1 and backcross progency. Piperonyl butoxide synergized fenvalerate in all resistant strains, but caused its greatest effect on highly resistant psylla. Use of piperonyl butoxide in overcoming fenvalerate resistance in the field is discussed. Because pear psylla can rapidly develop pyrethroid resistance, careful monitoring is recommended.
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