Corn seedlings release large amounts of terpenoid volatiles after they have been fed upon by caterpillars. Artificially damaged seedlings do not release these volatiles in significant amounts unless oral secretions from the caterpillars are applied to the damaged sites. Undamaged leaves, whether or not they are treated with oral secretions, do not release detectable amounts of the terpenoids. Females of the parasitic wasp Cotesia marginiventris (Cresson) learn to take advantage of those plant-produced volatiles to locate hosts when exposed to these volatiles in association with hosts or host by-products. The terpenoids may be produced in defense against herbivores but may also serve a secondary function in attracting the natural enemies of these herbivores.
M O
OST STUDIES ON THE SIGNIFI- cance of herbivore-induced pro- duction of secondary metabolites in plants focus on the direct ecological inter- actions between plants and the herbivores that feed on them (1-3). Only a few investigators (4-6) have suggested active interac- tions between herbivore-damaged plants and the third trophic level of insect parasitoids and predators. There are many examples of these insects being attracted to plant odors (7), but only recently have studies indicated an active involvement of plants (5,6). Dicke and co-workers presented the first convincing evidence for an active release of volatiles by herbivore-infested plants that attract natural enemies of the herbivorous attackers (6). As yet, no herbivore-specific factor that induces characteristic changes in plants, used by foraging entomophagous insects, has been pinpointed.
It is common that parasitic wasps learn to respond to specific odors that are associated with their hosts (8). The often observed flexibility in these responses has been attributed to the variability in space and time of reliable cues that may best guide the wasps to available hosts (9). Their ability to learn should allow parasitoids to distinguish among odors of plants with different types