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Adaptive changes in populations encountering a new environment are often constrained by deleterious pleiotropic interactions with ancestral physiological functions. Evolutionary responses of populations can thus be limited by natural selection under fluctuating environmental conditions, if the adaptive mutations are associated with pleiotropic fitness costs. In this context, we have followed the evolution of the frequencies of insecticide-resistant mutants of Cydia pomonella when reintroduced into an untreated environment. The novel set of selective forces after removal of insecticide pressure led to the decline of the frequencies of resistant phenotypes over time, suggesting that the insecticide-adapted genetic variants were selected against the absence of insecticide (with a selective coefficient estimated at 0.11). The selective coefficients were also estimated for both the major cytochrome P450-dependent monooxygenase (MFO) and the minor glutathione S-transferase (GST) systems (0.17 and negligible, respectively), which have been previously shown to be involved in resistance. The involvement of metabolic systems acting both through xenobiotic detoxification and biosynthetic pathways of endogenous compounds may be central to explaining the deleterious physiological consequences resulting from pleiotropy of adaptive changes. The estimation of the magnitude of the fitness cost associated with insecticide resistance in C. pomonella suggests that resistance management strategies exclusively based on insecticide alternations would be unlikely to delay such a selection process.

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.