Host specificity and host recognition in a chemically-defended herbivore, the tenthredinid sawfly Rhadinoceraea nodicornis

The sawfly Rhadinoceraea nodicornis Konow (Hymenoptera: Tenthredinidae) is a member of a closely related group of species, the tribe Phymatocerini, which feed on the Liliales and Ranunculales. It is known to sequester steroid alkaloids from its host plants, species in the genus Veratrum (Liliales: Melanthiaceae), and to use them as a defence against predators. There are known chemical relationships between the hosts of R. nodicornis and hosts of related sawfly species. We tested whether the R. nodicornis larvae would accept hosts of closely- and more distantly-related sawflies, but found that they accepted only plant species in the genus Veratrum. This specificity was apparently innate, as it was independent of early larval experience. A feeding bioassay showed that the steroid alkaloids from Veratrum nigrum were phagostimulatory for R. nodicornis larvae, suggesting that they may be involved in host recognition. We discuss the possibility that the evolution of recognition of specific compounds may represent the mechanism of host radiation within the Phymatocerini.