Do parasitoids use herbivore-induced plant-chemical defenses to locate hosts ?

Herbivore-induced plant chemical responses are usually considered direct defenses against the herbivorous attackers. Recent findings show that a third tropic level can be involved as well. Herbivore-injured plants release relatively large amounts of volatiles that are attractive to natural enemies of the herbivores. The host-searching behavior of the generalist parasitoid Cotesia marginiventris is mediated by such plant produced volatiles. Since plant and entomophage both profit from this interaction it has been suggested that the plant responses serve to recruit the entomophages. Here we argue that induced production of plant chemicals evolved first as a direct defense against herbivores, and that the attractive function likely evolved secondarily. Results of a plant palatability experiment are given to provide preliminary support for a defensive function. Plants that were induced to release volatiles that are known to be attractive to the parasitoid became less palatable to the herbivore beet armyworm.