Relevance of resource-indicating key volatiles and habitat odour for insect orientation

Olfactory orientation by insects may be guided by specific volatile blends released from sites where resources are present. Such blends need to be recognized against the odorous background of the habitat. The egg parasitoid Closterocerus ruforum is known to be attracted to plant volatiles induced by egg deposition of its herbivorous host, the pine sawfly Diprion pini. The parasitoid has to detect this volatile signal against a background of other pine volatiles (i.e. odour from pine without host eggs). Previous studies have shown that attractive resource-indicating odour (pine with host eggs) and nonattractive habitat odour (pine without eggs) differ only by enhanced quantities of (E)-β-farnesene in the resource-indicating odour. However, (E)-β-farnesene per se was not attractive. We studied the relevance of quantitative ratios of (E)-β-farnesene and other pine volatiles present in the habitat for parasitoid orientation by electrophysiological methods (EAG) and behavioural (olfactometer) assays. Seven of 12 terpenoid pine volatiles elicited an EAG response. In tests of the parasitoidʹs behavioural response to EAG-active volatiles, a blend of five components (including (E)-β-farnesene) was sufficient to attract the parasitoid but only when the ratio of (E)-β-farnesene and the other compounds matched that in odour from egg-laden pine. Since the quantities of volatiles from egg-laden and egg-free pine are not significantly different except for (E)-β-farnesene, our results show that the quantity of a resource-indicating key compound (here (E)-β-farnesene) is validated by its ratio to quantities of background volatiles emitted by both the resource and the habitat.