The inheritance of deltamethrin resistance in Cydia pomonella (L.) has been investigated by crossing a resistant (Rv) and a susceptible (Sv) strain, derived from a population collected in south-eastern France in 1995. Deltamethrin resistance was suspected to be under the control of a kdr-type allele and an enhanced mixed-function oxidase (mfo). F(1) and F(2) progenies were therefore tested through dose-response and enzyme assays. Dose-response relationships indicated that resistance was inherited as an autosomal incompletely recessive (D=-0.199) character, involving at least two genes. Enzyme measures suggested the contribution of 1.2 genes to the expression of mfo, with incomplete dominance (D=0.460). Our results support the hypothesis of a polygenic response to deltamethrin selection in the Rv strain, including a major kdr-type allele with a minor effect of mfos. In the light of these findings, we consider the resistance in codling moth populations in south-eastern France as a product of an adaptive sequential selection process, occurring through the sequential addition of resistance genes.

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).

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Topical application bioassays, conducted on French populations of Cacopsylla pyri, showed resistance rates to deltamethrin ranging from 31‐fold at the adult stage to 135‐fold at the last larval stage. Synergism studies between deltamethrin and piperonyl butoxide (PBO), applied either topically or by tarsal contact with PBO, showed (i) that the rates of synergism were more than 10‐fold in all the populations and that they could increase to a thousand‐fold in some populations, (ii) that the synergistic effect was present to a similar extent at both the larval and adult stages, (iii) that there was no correlation between the synergistic effect and the resistance rates, and (iv) that no correlation was observed between the dates of collection and the rates of synergism. The results makes it impossible to determine whether resistance to deltamethrin is of genetic origin or results only from the induction of monooxygenases by allelochemical substances present in the food ingested by the species.

Laboratory selection with azinphosmethyl had little effect on the chemical resistance in Cacopsylla pyri strains, in comparison to the original wild populations. The resistance ratio relative to a susceptible strain varied from 10‐ to 40‐fold depending on the generation studied. Crosses between two resistant strains and the susceptible strain show resistance to be autosomally inherited and semi‐dominant in expression. Backcrosses between F1 and the susceptible strain were unable to distinguish unabigously between monogenic and polygenic inheritance. In the majority of experiments, however, the overall dose‐response relationship for backcross progeny was consistent with a single gene hypothesis. Additional bioassays showed azinphosmethyl‐resistant strain to cross‐resistant monocrotophos and phosmet, but not carbamates, pyrethroids, amitraz or the organophosphates chlorpyrifos and mevinphos.