HOST LOCATION IN OOMYZUS-GALLERUCAE (HYMENOPTERA, EULOPHIDAE), AN EGGPARASITOID OF THE ELM LEAF BEETLE XANTHOGALERUCA-LUTEOLA (COLEOPTERA,CHRYSOMELIDAE)

Eggs of the elm leaf beetle Xanthogaleruca luteola are often heavily a ttacked by the chalcidoid wasp Oomyzus gallerucae. We studied the chem ical signals mediating interactions between the egg parasitoid, its ho st, and the plant Ulmus campestris. Olfactometer bioassays with O. gal lerucae showed that volatiles of the host-plant complex attract the pa rasitoid. In order to determine the source of attractive volatiles wit hin this host-plant-complex, we tested separately the effect of odours of eggs, gravid elm leaf beetle females, faeces of the beetles and el m twigs (with undamaged leaves and leaves damaged either mechanically or by feeding of the beetles). Odours of faeces of the elm leaf beetle were attractive, whereas neither volatiles from eggs nor from gravid females acted as attractants. Volatiles from undamaged or damaged plan ts did not elicit a positive reaction in O. gallerucae, whereas volati les from feeding-damaged plants onto which host eggs had been deposite d were attractive. This latter result suggests that it is not feeding but deposition of host eggs onto elm leaves that induces the productio n of plant volatiles attractive to the egg parasitoid. Investigations of the search patterns of O. gallerucae within the habitat by laborato ry bioassays revealed that the egg parasitoid encounters host eggs by chance. Contact kairomones from faeces were demonstrated to be importa nt in microhabitat acceptance, while contact kairomones isolated from the host eggs are relevant for host recognition.