Berrada_1994_Pestic.Biochem.Physiol_48_41

A resistant strain (TL) of pear psylla, Cacopsylla pyri (L.), was obtained by laboratory selection with monocrotophos. In comparison with a sensitive strain (LS), resistance mechanisms in TL were investigated on the basis of both the effects of two synergists (piperonyl butoxide and S,S,S-tributyl phosphorotrithioate) and the in vitro analysis of the activity of four enzymes known to be often involved in organophosphate resistance (acetylcholinesterase, mixed-function oxidases, esterases, and glutathione S-transferases). The presence of a modified acetylcholinesterase, less sensitive to the inhibition by the insecticide (70-fold) combined with a slight increase in the oxidative metabolism (5-fold) was found to be responsible for the high resistance level developed in TL (150-fold).