Aphid-deprivation from Brassica plants results in increased isothiocyanate release and parasitoid attraction

In response to insect herbivory, plants release volatile blends that differ quantitatively, and in many cases also qualitatively, from those of undamaged plants. These altered blends can attract the herbivore's natural antagonists, and such herbivore-induced volatile blends have often been interpreted as co-evolved plant-insect antagonist signals. When comparing volatile profiles of infested Brassica oleracea var. gemmifera L. plants (Brassicales: Brassicaceae), on which Brevicoryne brassicae L. (Hemiptera: Aphididae) aphids are feeding, with previously infested plants (i.e., deprived of aphids four hours prior to volatile collection), we found that the emission of a particular volatile compound, the glucosinolate breakdown product allyl isothiocyanate, was significantly higher in the latter. We used dual choice olfactometry to evaluate attractiveness of plants and aphids to Diaeretiella rapae (M'Intosh 1855) (Hymenoptera: Braconidae), a parasitoid of B. brassicae. Previously infested plants deprived of the herbivore attracted significantly more parasitoids than infested plants, although aphid odours per se proved to be attractive. Mechanical damage approximating aphid stylet insertion remained without effect on emission of allyl isothiocyanate and on parasitoid response. The unexpected higher parasitoid attraction to previously infested than infested plants is discussed here from the perspective of the plant and the parasitoid, as well as from the herbivore. Results suggest that while the host-finding mechanism of D. rapae has not evolved to allow this parasitoid a discrimination between herbivore-infested and previously infested plants, the herbivore seems able to suppress emission of a key volatile, thereby reducing its own olfactory detectability to its specialised natural antagonist