Host location by Gelis festinans , an eggsac parasitoid of the linyphiid spider Erigone atra

Interactions were investigated between the spider Erigone atra (Blackwall) (Araneae, Linyphiidae) and the insect parasitoid Gelis festinans (Fabricius) (Hymenoptera, Ichneumonidae), both found in an agricultural wheat field surrounded by grass edges. The searching behaviour of G. festinans was studied in laboratory experiments. Odours from detached wheat and grass leaves were preferred by male and female parasitoids when tested against clean air in Y‐tube olfactometer experiments. Female parasitoids, collected from the field, and their lab‐reared female offspring, preferred wheat odours when given the choice between wheat vs. grass odours. Male parasitoids showed no significant preference for wheat or grass leaves. Both field‐captured and lab‐reared female parasitoids responded to silk from the spider E. atra with an increased searching behaviour, while showing no response to silk from the spiders Lepthyphantes tenuis (Blackwall), Bathyphantes gracilis (Blackwall), Oedothorax retusus (Westring) and O. fuscus (Blackwall) (Araneae, Linyphiidae). Female parasitoids spent significantly less time on surfaces that were pre‐searched by themselves or conspecific females than on unsearched areas, even in the presence of E. atra webbing. Parasitized eggsacs were distinguished from unparasitized ones; superparasitization was not observed.