Rank, N. E., A. Köpf, R. Julkunen-Tiitto & J. Tahvanainen. (Ecology, in press). Host preference and larval performance of the salicylate-using leaf beetle Phratora vitellinae
Larvae of Phratora vitellinae (Coleoptera: Chrysomelidae) convert salicyl glucosides from the host plant into a larval defensive secretion with salicylaldehyde. This secretion repels generalist predators. Willows vary greatly in the concentrations of salicyl glucosides in their leaves. One may predict that P. vitellinae prefers and survives better on plants that contain more salicyl glucosides. We determined the amount of larval secretion, host preference, larval growth, and larval survival of P. vitellinae on Salix myrsinifolia, S. pentandra and S. phylicifolia. We also measured feeding rates of three natural predators on P. vitellinae larvae feeding on different hosts. Salix pentandra and S. myrsinifolia contained substantial amounts of salicyl glucosides, but S. phylicifolia contained very little of them. Phratora vitellinae larvae produced more secretion on S. pentandra than on S. myrsinifolia. They produced little secretion on S. phylicifolia. Adult beetles preferred S. myrsinifolia over S. pentandra and S. pentandra over S. phylicifolia. Larvae grew most rapidly on S. myrsinifolia and S. pentandra. Their growth was slowest on S. phylicifolia. The larval survival was similar on S. myrsinifolia and S. phylicifolia, but it was significantly lower on S. pentandra. The natural predators fed equally well on P. vitellinae feeding on S. myrsinifolia and S. phylicifolia. Thus, the host preference of P. vitellinae did not correspond to larval survival on these hosts, but rather to larval growth. Larval survival of P. vitellinae was not related to the amount of defensive secretion. Natural predators were not repelled by the host-derived defensive secretion. We discuss the implications of these findings for the evolution of host plant use in this herbivore.
Key words: Phratora vitellinae; Salix; three-trophic level interactions; larval defensive secretion; host suitability; larval performance
Key phrases: host plant preference vs. larval performance; host plant chemistry vs. larval survival; natural enemies and host use
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