Ecotypic differentiation is well described in the mosquito Culex pipiens, separating populations breeding in subterranean and human‐made sites (hypogeous habitat) from those in open‐air sites (epigeous habitat). The pattern of population differentiation observed at the Aat‐1 locus has been suspected to be associated with such ecotypic differentiation via habitat‐dependent selection, but this supposition is still the subject of debate in the literature. We analysed differentiation patterns for Aat‐1 and another four loci among populations from both habitat types in the French Alps. We showed that the Aat‐1A allele is favoured within hypogeous habitats but selected against within epigeous habitats. Comparisons of our results with other data reported in the literature indicate that the Aat‐1A allele is generally evolving under habitat‐differential selection, but that the precise balance of migration and selection that determines equilibrium allele frequencies varies greatly across Europe. The nature of this habitat‐dependent selection, and its resulting (geographically varying) equilibrium point, are discussed in relation to the biology of this mosquito species.

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In the mosquito Culex pipiens L., the two associated esterases A2 and B2 are responsible for resistance to organophosphorus (OP) insecticides in many countries of Africa, Asia and North America. We report here their presence and geographic spread in French populations based on the analysis of 168 samples collected from 1984 to 1990. First detected in 1986 in one sample from southern France, these esterases were progressively found in new geographic locations, so that in 1990 their distribution covered at least four contiguous French regions. RFLP analysis of the amplified B2 gene indicates that A2‐B2 arrived in France most likely through migration. This genetic invasion is discussed in the light of the recent occurrence of A2‐B2 in the mosquito genome, and of the consequences of this new resistance factor in natural populations already possessing other insecticide resistance genes.

Insensitive acetylcholinesterase (AceR) and five over-produced esterases (A1, A2 and B2, and A4 and B4) involved in detoxification are responsible for resistance to organophosphorous insecticides (OPs) in Culex pipiens L. from the Rhône-Alpes region, where C. pipiens control is mainly accomplished with the OPs temephos and chlorpyrifos using 0.15 mg/liter doses. The strong linkage disequilibria observed between esterases A1 and Est-20(0.64), esterases A4 and B4, and esterases A2 and B2 indicate that these genes were introduced in the Rhône-Alpes region. AceR and esterase A1, which appeared in the south of France 3 yr before the start of mosquito control in Rhône-Alpes, had the highest frequencies. All resistant genotypes were shown to be killed by 0.15 mg/liter temephos in natural breeding sites, but not by 0.15 mg/liter chlorpyrifos. These results are discussed in relation with mosquito control strategies.