The involvement of plant volatile chemicals in the hostseeking behavior of parasitoids has an ecological and an evolutionary aspect. By facilitating the parasitization of herbivores feeding on a plant these synomones aid both the plant and the parasitoid. As such, plant and parasitoid would be expected to coevolve, resulting in some finely developed systems of signal and response. Examples of tritrophic interactions in which such coevolution may have occurred include situations in which a parasitoid is attracted to volatile chemicals produced by a plant only when that plant has been damaged by herbivores. This makes the signal more meaningful to the parasitoid than a signal produced by all plants at all times. The plant also presumably benefits by not wasting resources to produce a signal that is not needed. However, another possible explanation is that substances produced as a result of injury are part of an induced resistance response of the plant to herbivory; thus the primary purpose of the material would be to decrease foliage palatability or otherwise directly harm the herbivore. A parasitoid might evolve to use these materials, but still have little or no impact on the evolution of the plant responses. Indeed, most work on coevolution in insects and plants has emphasized the plant—herbivore interactions, yet there is little solid information about tritrophic interactions. However, coevolution between parasitoid and plants is still a theoretical possibility, and researchers are beginning to study this interaction.
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