Induction of plant synomones by oviposition of a phytophagous insect

Earlier investigations of host habitat location in the egg parasitoid Oomyz us gallerucae have shown that oviposition of the elm leaf beetle (Xanthogal eruca luteola) induces the held elm (Ulmus minor) to emit volatiles that at tract the egg parasitoid. In this study we investigated the mechanism of th is induction by testing the effects of differently treated elm leaves on O. gallerucae in a four-arm olfactometer. First we investigated which sequenc e of the herbivore oviposition behavior is necessary for the synomone induc tion. The following major sequences were observed: (1) Prior oviposition, t he gravid female gnawed shallow grooves into the leaf surface. (2) After gn awing upon the leaf surface, the female attached about 20-30 eggs with ovid uct secretion in the grooves. We experimentally mimicked the shallow groove s on the leaf surface by scratching the leaf surface with a scalpel (= scra tched leaves). Volatiles from such scratched leaves did not attract the egg parasitoid. However, as soon as eggs with oviduct secretion, or only ovidu ct secretion, was applied to these scratched leaves, they emitted attractiv e volatiles. Application of oviduct secretion and eggs on undamaged leaves did not elicit release of attractive synomones. Thus, an elicitor is locate d in the oviduct secretion, but becomes active only when the leaf surface i s damaged. Jasmonic acid is known as a mediator of plant responses induced by feeding of herbivorous arthropods, and we demonstrate that it mediates p roduction of elm synomones that attract O. gallerucae. The plant's reaction to oviposition was systemic, and leaves without eggs near leaves with eggs emitted attractants.