THE FOOD PLANT PREFERENCES OF PHRATORA-VITELLINAE (COLEOPTERA, CHRYSOMELIDAE) .A. FIELD OBSERVATIONS

In the Upper Rhine (Petite Camargue, Saint Louis, France) study site, P. vitellinae are found mainly on Salix nigricans on which they are bivoltine. Some beetles are occasionally observed on S. purpurea, especially in spring, but never on S. alba, S. caprea and S. cinerea. In other European localities, S. nigricans or S. hegetschweileri at higher elevations are frequently recorded as host-plants. Some populations are also found on S. purpurea, Populus tremula and P. trichocarpa .times. deltoides, usually when neither S. nigricans nor S. hegetschweileri are present. All the Salicaceae on which P. vitellinae was observed in the field have leaves rich in phenylglycosides and with glabrous lower surfaces; at least in spring, P. vitellinae was never recorded on those Salix spp. with leaf undersurfaces densely covered with trichomes. At the time of emergence of the beetles from hibernation, S. nigricans leaves are more developed than those of S. purpurea and S. cinerea. Those S. nigricans trees with the longer shoots in spring support the largest number of beetles at their emergence. The preference of P. vitellinae and S. nigricans might be reinforced by these phenological differences. In the Petite Camargue, the same S. nigricans trees support the largest populations of beetles from one generation to the next and from year to year. This suggests that S. nigricans does not respond to defoliation with induced resistance to the beetles. All shoots on the branch seem to be equally suitable and attractive for oviposition. The older leaves at the base of the shoot and the younger ones at the top are less favored by ovipositing females. Survival of the egg batches and the groups of freshly emerged larvae is lower in the distal shoots of the branch and on the younger leaves. This might be due to a higher degree of exposure to predators.