Acute Oak Decline (AOD) poses a threat to native oak trees in the United Kingdom. Three bacterial pathogens, Brenneria goodwinii, Rahnella victoriana and Gibbsiella quercinecans, are frequently isolated from necrotic stem lesions that characterise AOD, demonstrating necrogenesis to live bark in lesion formation. Larval galleries of the two-spotted oak buprestid, Agrilus biguttatus (Coleoptera: Buprestidae) commonly co-occur with stem lesions, which indicate the beetles could be involved in vectoring the AOD bacteria, by bringing the bacteria to the trees, and/or introducing the bacteria to live phloem through larval ingress and feeding. After emergence, beetles fly into the crown of trees, feed on leaves, and mate upon maturation. Gravid (mated) females then oviposit onto bark cracks on the outside of the stem, although the roles and compositions of semiochemicals as cues for locating suitable egg-laying sites is currently not known. The possible roles of the AOD bacteria on tree Volatile Organic Compound (VOC) emission, and bacterial VOCs on beetle behaviour, have also not yet been established. To discover the sequence of events in the epidemiology of AOD, we hypothesise that gravid females are more attracted to AOD bacterial VOCs in combination with bark VOCs, than to either separately. To test this, VOC collections from individual cultures of AOD bacteria, and a combination of all three bacterial species, were performed, and extracts analysed using gas chromatography/gas chromatography coupled mass spectrometry (GC/GC-MS). Interestingly, two abundantly produced VOCs specific to B. goodwinii elicited strong antennal responses by A. biguttatus in coupled GC-electroantennography (GC-EAG). Four-arm olfactometer tests using extracts from the bacterial headspace collections showed greater attraction of the beetles to a combination of synthetic bark blend + headspace extract from a mixed colony of B. goodwinii, G. quercinecans and R. victoriana compared to the bark blend alone, suggesting bacteria infected bark is more attractive to gravid females than non-infected bark. This work will reveal the role of VOCs in this host-pathogen-herbivore ecological system and may clarify the role A. biguttatus plays in bacterial transmission. Identified attractant VOCs can aid the development of semiochemicals-based lures to improve the monitoring and control of A. biguttatus, and the subsequent spread of AOD.

Investigating the role of acute oak decline (AOD) bacterial volatiles on the behaviour of the two-spotted oak buprestid Agrilus biguttatus (Coleoptera: Buprestidae)

ACS Fall 2021

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