Modification of Nephila clavipes (Araneae Nephilidae) webs induced by the parasitoids Hymenoepimecis bicolor and H. robertsae (Hymenoptera Ichneumonidae)

Abstract

Some polysphinctine ichneumonid wasps induce alterations in the web construction behaviour of their host spiders, and then suspend their pupal cocoons from the resulting “cocoon webs”. Cocoon webs that have been described previously appear to be designed to increase the web's mechanical stability, and thus to protect the wasp's cocoon. This study describes the cocoon webs of Nephila clavipes that are induced by two wasp species, Hymenoepimecis bicolor and H. robertsae, and shows that the alterations induced by H. bicolor make the webs more resistant to destruction. The cocoon webs of both species include a hub-like platform from which the cocoon is suspended, and are usually protected by a nearby tangle of barrier lines of variable density. The web alterations induced by H. bicolor are apparently not a consequence of parasitised spiders being in a poorer nutritional state, because orbs spun by parasitised spiders preceding the final “cocoon webs” were not significantly smaller than those of unparasitised spiders. Cocoon webs themselves were all highly reduced, and some of those induced by H. bicolor resemble the “skeleton” webs that are occasionally made by unparasitised N. clavipes. We document a possible spider defence (molting) against polysphinctine parasitisation.