Electrophoretic analyses of 12 loci on nine populations from Morocco, northern Spain, and southern France have revealed heterogeneity among localities (Fst = 0.064, Nm = 3.65) and a highly significant G test for polymorphic loci. The nine populations clustered into two groups on the UPGMA phenogram of genetic distances. Ecophysiological characters were examined (photoperiodic thresholds for diapause induction, development time, and number of larval instars at 25°C, under various photoperiods) for four of these populations. According to these ecophysiological characters, the populations could be divided into two groups congruent with those obtained with the genetic analysis. Although the species is known as being sedentary, we can put forward the hypothesis that insects could fly, not across the Pyrenees, but via the Atlantic or Mediterranean coast, thus creating a genetic flow between populations. Moreover, despite a larval diapause, the high mortality rate observed within certain subpopulations might contribute to genetic differentiation from residual populations surviving hard winters.