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Pear psylla, Cacopsylla pyri (L.), adults collected from commercial pear orchards around Toulouse (France) were selected in the laboratory for resistance to monocrotophos. Resistance was obtained only when the insecticide selection pressure was continuous, but when some generations were not selected, resistance did not develop or even reverted. Resistance appeared to be stable after 30 generations of selection and stabilized at a level of 140-fold. The monocrotophos-selected strain was highly cross-resistant to the organophosphate phosmet (1 55-fold) and cross-resistant to the formamidine amitraz (1 4-fold).

This study determined toxicities of three insecticides/acaricides representing three classes (formamidines, avermectins, and norpyrethrates) to a prey‐predator complex consisting of anthocorid bug (Anthocoris nemoralis F.) and its phytophagous prey, pear psylla (Cacopsylla pyri L.). The anthocorid bug LD50s were generally lower than those of the pear psylla indicating greater susceptibility in the predator than the prey. Determination of residual activities of the test insecticides indicates different selectivities to the beneficial insect. From this study, abamectin alone may be sprayed to control the pear psylla when the anthocorid bug populations are active. Reasons for the susceptibility variations to insecticides in phytophagous‐entomophagous systems are suggested and discussed.

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