The food plant preferences of Phratora vitellinae (Coleoptera: Chrysomelinae) : B. A laboratory comparison of geographically isolated populations and experiments on conditioning.

The specialisation of the beetles from the Petite Camargue on Salix nigricans was confirmed by laboratory food-plant trials. When S. nigricans is not present in the choice offered, other Salix species are accepted and the following ranking, in decreasing order of preference, can be established: S. nigricans, S. purpurea, Populus nigra, P. tremula, S. alba, s. caprea and S. cinerea. S. nigricans leaves are the richest in salicin of the 4 most acceptable plant species. The 3 least acceptable species have the undersurface of their leaves covered with trichomes.The food-plant preferences of 2 populations from central Europe and 2 from Belgium differ both in the field and in the laboratory. The level of dietary specialisation also differs from population to population. The populations originating from localities (Oignie in Belgium and the Petite Camargue in central Europe) where their favorite food-plants in the field and in the laboratory (respectively P. tremula and S. nigricans) are abundant, show clearer preferences than the populations from localities (Grammont in Belgium and Herrliberg in central Europe) where these plants are scarcer or even absent, due to human influence.The Salicaceae tested, as seen by the more specialised beetles of Oignie and the Petite Camargue, include both a strongly preferred plant (P. tremula or S. nigricans) and two or three strongly disliked species (S. alba, S. caprea and S. cinerea). for these beetles, laboratory preferences and field observations of host plant species are in agreement. It is interesting that S. nigricans, although not present in Belgium, is the second most preferred food of the Belgian beetles in laboratory trials. Similarly, P. tremula, the preferred food of the Belgian beetles, is well accepted by the central European beetles.For the less specialised beetles of Grammont and Herrliberg, the same Salicaceae include strongly disliked speicies but no single strongly preferred species. In the Herrliberg population, with rather poorly defined preferences, preference for the favorite species of the more specific population from the same geographical area (Petite Camargue) can readily be produced in the laboratory by conditioning.