Host plant preference based on salicylate chemistry in a willow leaf beetle (Chrysomela aeneicollis)

Chrysomela aeneicollis (Coleoptera: Chrysomelidae) uses salicin from its host plant (Salix spp.) to produce a defensive secretion, salicylaldehyde. Because it requires salicin for this secretion, I predicted that C. aeneicollis should be attracted to willows which possess salicin and other salicylates. To test this prediction, I determined the host-plant preferences of C. aeneicollis among four potential hosts which occur in the Sierra Nevada range of eastern California. These species have very different salicylate chemistries but do not differ in nutritional quality for C. aeneicollis. In oviposition-preference tests, gravid females showed no preference between a salicylate-poor species, S. lutea, and a salicylate-rich species, S. orestera. However in feeding-choice tests, both larvae and adults preferred S. orestera over S. lutea. This preference was not affected by the species on which the larvae were reared. In other feeding tests, adults preferred S. orestera over two medium-salicylate species, S. boothi and S. geyeriana, regardless of which host species they had been feeding on in nature. In a final feeding test, adults were stimulated to feed by salicin itself. In nature, the relative abundances of C. aeneicollis adults and egg clutches among these species correspond to the adult feeding preference in the laboratory. Additionally, multiple regression analyses showed that adult abundance was not related to among-clone differences in leaf toughness or nutritional quality, but rather to salicin content and plant size. Thus for C. aeneicollis, both laboratory and field results demonstrate a preference for salicylate-rich willows which is partly responsible for the increased level of attack on them.