Rank, N.E. and J.T. Smiley. 1994. Host-plant effects on Parasyrphus melanderi Curran (Diptera: Syrphidae) feeding on a willow leaf beetle Chrysomela aeneicollis Schaeffer (Coleoptera: Chrysomelidae). Ecological Entomology 19:31-38.

1. Generalist predators are repelled by chrysomelid (Chrysomela spp., Phratora vitellinae L.) larval defensive secretions that are obtained from salicin in their host plants. But little is known about the effect of these secretions on specialist predators.

2. In this study, we describe the feeding behaviour of a fly, Parasyrphus melanderi Curran (Diptera: Syrphidae), which feeds on Chrysomela aeneicollis Schaeffer (Coleoptera: Chrysomelidae). Parasyrphus melanderi lays its eggs on C. aeneicollis egg clutches, and its larvae consume C. aeneicollis eggs and larvae.

3. Chrysomela aeneicollis hatching rates were significantly lower (20%) on clutches with fly eggs than on clutches without them (40%). Half of the clutches with one fly egg had survival rates below 5%, and when two fly eggs were present (4 clutches), the entire clutch was consumed.

4. In nature, P. melanderi eggs were three times more abundant on a salicylate-rich willow species S. orestera Schneider, than on the medium-salicylate S. geyeriana Anderss. (1.8 versus 0.6 eggs per clutch). On 18% of the S. orestera clones, all the beetle clutches contained fly eggs. In laboratory-choice tests, P. melanderi larvae fed equally rapidly on C. aeneicollis larvae that were chemically defended (feeding on S. orestera) as on larvae that produced no secretion (feeding on the salicylate-poor S. lutea Nutt.). This predator does not appear to be deterred by C. aeneicollis's defensive secretion. We discuss the implications of specialist predators on determining host suitability to herbivorous insects.

Key words: Parasyrphus melanderi, Syrphidae, predation, three-trophic level interactions, Chrysomela aeneicollis, Salix spp.

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