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 organo-phosphorous insecticides. Resistance rates were seven times lower for pyrethrinoids and amitraz and practically nil for carbamates. Inter-population resistance rates, for an identical period, were different for the three organo-phosphorous insecticides and for deltamethrin in two localities. However, it was difficult to make comparisons in time due to seasonal variations.
Rates of synergism by S,S,S-tributyl phosphorotrithioate (DEF) and piperonyl 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 mixed-function oxidases in winter forms is discussed with regard to bibliographical data concerning another psylla species from North America ( Psylla pyricola Foerster).