Summary. It is well known that feeding by Pieris brassicae caterpillars on cabbage leaves triggers the release of volatiles that attract natural antagonists such as the parasitoid Cotesia glomerata. The temporal dynamics in the emissions of parasitoid attracting volatiles has never been elucidated in this system. In a time course experiment, caterpillar infested leaves attracted the parasitoid within one hour after infestation. At such an early stage of infestation, as much as fifty percent of the parasitoids flew towards the infested plant in a wind tunnel bioassay, while only five percent flew towards the non-infested control plant. Three hours after infestation and later, the response to the volatiles from the infested plant reached its maximum and then continued at a constantly high level for the remaining 14 hours of the experiment. Chemical analyses of volatiles collected from infested leaves at short time intervals during the first 24 hours identified a total of ten compounds, comprising green leaf volatiles, terpenoids, and a nitrile. Significant increase of emission within the first 5 hours following initial herbivory was detected for (Z)-3-hexen-1-ol, (Z)-3-hexen-1-yl acetate, cineole and benzylcyanide. Subsequently, a coupled bioassay-chemical analysis procedure was developed allowing for testing and analyzing the same sample for future identification of the bioactive compounds. This was achieved by using stir bar sorptive extraction for the analysis of solvent extracts of caterpillar-damaged leaves.
Feeding by Pieris brassicae caterpillars on the lower leaves of Brussels sprouts (Brassica oleracea var. gemmifera) plants triggers the release of volatiles from upper leaves. The volatiles are attractive for a natural antagonist of the herbivore, the parasitoid Cotesia glomerata. Parasitoids are attracted 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 emitted green leaf volatiles, cyclic monoterpenoids, and sesquiterpenes. GC-MS profiles of systemically induced and herbivore-infested leaves did not differ for most compounds, although herbivore infested plants did emit higher amounts of green leaf volatiles. Emission of systemically induced volatiles in Brussels sprouts might function as an induced defense that is activated only when needed, i.e., at the time of caterpillar attack. This way, plants may adopt a flexible management of inducible defensive resources to minimize costs of defense and to maximize fitness in response to unpredictable herbivore attack.
The final steps of a parasitoid host selection process, host handling and oviposition, might be affected by the habitat cues to which parasitoids are exposed, and not only by the host itself. The habitat-related factors promoting parasitoid host-handling and reproductive success were investigated in a laboratory colony of Hyssopus pallidus, a larval parasitoid of the codling moth, Cydia pomonella, an important pest of apple. Parasitoid host handling was addressed in computer-monitored behavioural bioassays during 8 h. Naive females showed more intensive host handling behaviour (frequent host examination) when offered host larvae in combination with apple fruits or in combination with an artificial diet devoid of fruit material than when offered host larvae alone. The exposure of parasitoids to fresh apple during host handling resulted in an enhanced behavioural response equivalent to that one obtained by giving an oviposition experience prior to the bioassay. The progeny produced by parasitoids exposed to plant cues for 8 h was almost double that of parasitoids exposed to artificial diet or no cues. Parasitoids exposed to no cues produced the same amount of progeny than parasitoids exposed to apple cues only with an increased time of exposure (32 h). The data demonstrate that the odour emitted by the host-food plant represent not only a habitat location signal, but triggers and enhances parasitoid host handling behaviour and reproductive success.
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