Exploitation of herbivore.induced cotton volatiles by the parasitic wasp Bracon vulgaris reveals a dominant chemotactic effect of terpenoids

Plants emit a wide array of complex blends of volatile organic compounds that can be involved in plant communication with herbivores and their natural enemies. Bracon vulgaris Ashmead (Hymenoptera: Braconidae)is a gregarious larval ectoparasitoid that attacks the boll weevil larvae, Anthonomus grandis Boheman (Coleoptera: Curculionidae), an important pest in cotton plantations in Brazil. This parasitoid species has been studied as a potential biological control agent of A. grandis. However, little is known about B. vulgaris host foraging behaviour. We have previously demonstrated that female wasps respond to host-associated cues (boll weevil's aggregation pheromone)and host habitat odours, such as cotton volatiles induced by the presence of the boll weevil's pheromone. In the current study, we evaluated the electrophysiological and behavioural responses of B. vulgaris to constitutive and herbivore-induced plant volatiles (HIPVs)emitted by A. grandis-infested cotton plants at different phenological stages. The results demonstrated that B. vulgaris recognizes and responds to reproductive cotton HIPVs and that polar compounds might not be essential for its attraction. Electroantennogram (EAG)recordings and bioassays suggested that the compounds ?-myrcene, (E)-ocimene, (E)-4,8-dimethylnona-1,3,7-triene (DMNT), (E)-(1R,9S)-caryophyllene, and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT), as well as other minor components of cotton blend, can be used by B. vulgaris wasps in its host foraging behaviour. Our results show an important role of terpenoids in cotton indirect defence, which is discussed relative to the role of other minor plant volatiles.