The role of volatile cues in mediating tree host-bacteria-insect interactions in acute oak decline

Acute oak decline (AOD) is a decline syndrome affecting native oaks in the UK, characterized by bacterial stem lesions and larval galleries of the beetle Agrilus biguttatus. In similar tree host-bacteria-insect systems, volatile organic compound (VOC) semiochemicals-naturally occurring behavior- and development-modifying compounds-govern biotic interactions between insects and associated bacteria. However, the role of these VOCs in AOD has not yet been established. We demonstrate that leaves of declining oaks produce different VOCs compared with asymptomatic trees and that virgin female A. biguttatus prefer the odor of foliage from declining oaks compared with that from asymptomatic oaks. Moreover, in olfactometry tests, gravid females move toward VOCs emitted from pure cultures of AOD-associated bacteria (Brenneria goodwinii, Gibbsiella quercinecans, and Rahnella victoriana), and this preference is further enhanced by VOCs from oak bark. We identified three putative attractants from pure bacterial cultures and confirmed their presence in headspace extracts of necrotic bark lesions in situ. Two of these VOCs are newly discovered natural products characteristic of B. goodwinii, the bacterial species responsible for the majority of stem tissue necrosis in AOD. Moreover, bacterial VOCs did not induce A. biguttatus oviposition, suggesting their role in beetle behavior is only to provide olfactory cues for gravid females to locate suitable egg-laying sites on oak stems. We propose that adult A. biguttatus exploits systemically induced leaf and bacterial volatile cues to locate host trees for mating and egg-laying.