Ted. C. J. Turlings
Institute of Zoology, University of Neuchatel, Rue Émile-Argand 11, CH-2007,
Neuchatel, Switzerland
Many chemical ecologist have given considerable attention to changes in plant chemistry after plants have been the subject of attack by herbivores or pathogens. It is generally assumed that at least part of the plants' reaction is a defense against the attacker(s) by producing toxic or repellent chemicals. Recent studies have also found that when arthropods feed on leaves, plants will start emitting relatively large amounts of specific volatiles. This phenomenon was demonstrated by entomologists that had first discovered that the natural enemies of the herbivorous arthropods use plant odors to locate their prey. A logical hypothesis that developed out of these studies was that the plants emit the volatiles as signals to attract the natural enemies of the herbivores. In this presentation, I will review some of the evidence that has been collected to support this hypothesis. I will also focus on some of the alternative explanations for the phenomenon of herbivore-induced emissions of volatiles in plants and try to convey that most likely the primary function of the plant responses is also a direct defense against pathogens and herbivores. The emission of chemicals may have been adapted over evolutionary time to enhance the additional benefits of attracting parasitoids and predators that can protect the plants against herbivores.
That the plant response is a general phenomenon and may serve several purposes, is reflected by the lack of specificity in the response. A wide variety of arthropods can induce a strong response, but mere mechanically damages trigger only weak responses. Elicitors that are, for instance, present in caterpillar regurgitate appear to be responsible for the induction. The reaction is very similar when different herbivorous species attack the same plant variety, but different plant species and varieties may emit a different blend of odors. The plants themselves, rather than the herbivores seem to contribute most to the variation in blends that are induced.
Overall all, the purpose of this presentation is to give a selective overview of what we know about herbivore-induced emissions of plant volatiles in the context of tritrophic interactions and to request and open mind as to the interpretation of what has been found so far. We are likely dealing with a phenomenon that serves several purposes. With the evidence that is available now it cannot yet be determined how it has evolved and whether the beneficial consequences of attracting natural enemies were selective forces that modified the plant responses.
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