Systemically induced plant volatiles emitted at the time of "danger"

Feeding by Pieris brassicae caterpillars on the lower leaves of Brussels sp routs (Brassica oleracea var. gemmifera) plants triggers the release of vol atiles from upper leaves. The volatiles are attractive for a natural antago nist of the herbivore, the parasitoid Cotesia glomerata. Parasitoids are at tracted only if additional damage is inflicted on the systemically induced upper leaves and only after at least three days of herbivore feeding on the lower leaves. Upon termination of caterpillar feeding, the systemic signal is emitted for a maximum of one more day. Systemic induction did not occur at low levels of herbivore infestation. Systemically induced leaves emitte d green leaf volatiles, cyclic monoterpenoids, and sesquiterpenes. GC-MS pr ofiles of systemically induced and herbivore-infested leaves did not differ for most compounds, although herbivore infested plants did emit higher amo unts of green leaf volatiles. Emission of systemically induced volatiles in Brussels sprouts might function as an induced defense that is activated on ly when needed, i.e., at the time of caterpillar attack. This way, plants m ay adopt a flexible management of inducible defensive resources to minimize costs of defense and to maximize fitness in response to unpredictable herb ivore attack.