Geographic and seasonal variability of resistance to insecticides in Cacopsylla pyri L-(Hom., Psyllidae)

Pear psylla (Cacopsylla pyri L.) resistance to insecticides was studied in adults using the topical application method. In the Avignon population, the winter forms tested (diapausing generation) showed a greater tolerance to insecticide than the summer forms. High resistance rates, ranging from 25- to 180-fold, were observed for three organophosphorous insecticides. Resist ance rates were seven limes lower for pyrethrinoids and amitraz and practic ally nil for carbamates. Inter-population resistance rates, for an identica l period, were different for the three organo-phosphorous insecticides and for deltamethrin in two localities. However, it was difficult to make compa risons in time due to seasonal variations. Rates of synergism by S,S,S-tributyl phosphorotrithioate (DEF) and piperony l butoxide (PBO) were low when applied 4 h before the insecticide (0.3- to 3.3-fold), except for deltamethrin with PBO (>60-fold). The presence of mix ed-function oxidases in winter forms is discussed with regard to bibliograp hical data concerning another psylla species from North America (Psylla pyr icola Foerster).