Willow hybridization differentially affects preference and performance of herbivorous beetles

We examined the preferences and performances of five beetle species (four chrysomelids and one scarab) on two species of willows (Salix sericea and S. eriocephala) and their interspecific hybrids. Beetle species differed markedly in their responses. In preference assays, two chrysomelid beetle species (Calligrapha multipunctata bigsbyana and Plagiodera versicolora) preferred hybrids, two chrysomelids (Chrysomela scripta and Ch. Knabi) preferred hybrids and S. sericea, and the scarab beetle (Popillia japonica) preferred S. eriocephala. Experiments with purified salicortin indicated that salicortin concentration may contribute to these preferences. The relative performance (growth rate, pupal/adult weight and survivorship) of these beetles on the three willow taxa did not correspond with their feeding preferences. Three species exhibited intermediate performance on hybrid willows (the two Chrysomela spp. and P.japonica); the Chrysomela spp. performed best on S. sericea, while P.japonica performed best on S. eriocephala. One species performed equally well on all three taxa (C. multipunctata bigsbyana). The performance of PI. versicolora was not tested. Our results support the general pattern that willow taxa with phenolic glycosides are more acceptable to specialist willow herbivores while those taxa without phenolic glycosides are more acceptable to generalist herbivores. We also show that to predict the relative susceptibility of hybrid and parental plants to herbivores, consideration must be given to the inheritance of traits affecting both preference and performance.