Laboratoire de Biologie Animale et Cellulaire, Université Libre de Bruxelles, 1050-Bruxelles, Belgique.
Chrysomela lapponica is a Palearctic leaf beetle displaying a disjunct boreo-subalpine distribution. At the present time, six populations have been identified with certainty throughout Europe. Both Salicaceae and Betulaceae are reported as natural food plants for Chrysomela lapponica. In some populations larvae and adults feed exclusively in nature on Betulaceae and in others, only on Salicaceae. In the laboratory, development is impossible or limited on plants of the other family than that of the natural food-plant. Feeding habits influence larval defense. Specimens feeding on Salix secrete mainly salicylaldehyde derived from host-plant phenolglucosides, whereas those feeding on Betula secrete isobutyrates and 2-methylbutyrates of mixed origin. The acids are neosynthesized by the insects from amino acids and the alcohols are derived from the food plant. Such striking differences suggest that the taxon C. lapponica comprises distinct populations in the process of speciation, and possibly sibling species. In order to understand this process of sepciation, we have realized a quantitative analysis of larval defensive secretions of five populations fed in the laboratory on two differents plants (S. fragilis containing phenolglucosides and S. caprea containing only traces of them). Hybrides between some populations were analysed in the same way. Genetic divergence between populations was evaluated by isozyme electophoresis.