Eggs of the elm leaf beetle Xanthogaleruca (Pyrrhalta) luteola (Coleoptera, Chrysomelidae) experience heavy parasitization by the egg parasitoid Oomyzus gallerucae (Hymenoptera, Eulophidae) in the field. We investigated the tritrophic interactions between the elm leaf beetle, its host plant, the field elm (Ulmus minor = U. campestris = U. procera), and its egg parasitoid. We found that oviposition of X. luteola induces the elm leaves to release volatiles that attract the egg parasitoid (induced symonones). Studies on the mechanism of this synomone induction revealed that neither intact elm leaves nor leaves damaged by feeding beetles released attractive volatiles. But oviduct secretion of X. luteola which glues the eggs onto the leaves was proved to elicit the emission of the attractive synomones. The eggs are always glued onto a small epidermal wound, which is inflicted to the lower leaf surface by the female prior to oviposition. The oviduct secretion only functions as synomone elicitor when applied onto such a wound. Scratching a leaf by a scalpel to mimic the wound and application of oviduct secretion results into the release of synomones. Our studies on the specificity of the synomone induction show that the attractiveness of induced volatiles was specific both for the Ulmus species and the herbivore species depositing eggs. Further steps in the egg parasitoid's host location process are mediated by kairomones from host faeces and egg masses. These kairomones were also shown to be host specific, since O. gallerucae clearly discriminates between host and non-host (e.g. Galerucella lineola) cues during host finding and host recognition. The tritrophic system studied here is characterized by oligophagous and monophagous relationships on the second and the third trophic level. These intrinsic characteristics of the tritrophic system might have been a prerequisite for the development of such selective responses of a parasitoid towards specific infochemicals and of the specific indirect defense reaction of the plant to oviposition of the chrysomelid host.

Index terms: elm leaf beetle, Oomyzus gallerucae, oviposition behavior, synomones

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