P. H. Westigard

Seasonal susceptibility to insecticides in insecticide-resistant pear psylla, Psylla pyricola (Homoptera: Psyllidae)

Seasonal changes in susceptibility of adult winter and summer forms of insecticide-resistant pear psylla, Psylla pyricola Foerster, to several insecticides were evaluated using slide-dip and topical application bioassays. Insecticides tested were azinphos-methyl, endosulfan, fenvalerate, methiocarb, Perthane, amitraz and avermectin. Throughout the period of testing, LD50 values generally were lowest for avermectin followed by fenvalerate < methiocarb < endosulfan < azinphos-methyl < Perthane. The topical application method showed that winter forms of pear psylla were more tolerant to azinphos-methyl, methiocarb, endosulfan, Perthane and fenvalerate than summer forms. Susceptibility to most insecticides increased for both forms as the season progressed. The slide-dip test showed similar trends to the topical application method for azinphos-methyl, methiocarb and endosulfan. However, slide-dip tests for formulated fenvalerate showed lower susceptibility in summer forms than in winter forms. This was consistent with field control results obtained with this compound. With both bioassays, seasonal tests with amitraz showed little effect on adults. Applications of results to control and resistance management of pear psylla are discussed.