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 characterising AOD, showing necrogenesis to live bark in lesion formation. Larval galleries of the two-spotted oak buprestid, Agrilus biguttatus (Coleoptera: Buprestidae) commonly co-occur with these lesions, indicating beetles may be involved in vectoring AOD bacteria by bringing them to trees, and/or introducing bacteria to live phloem through larval ingress/feeding. Post-emergence, beetles fly to the tree crown, feed on leaves, and mate upon maturation. Gravid (mated) females then oviposit on bark cracks on the outside of the stem. Semiochemical cues from healthy trees show a role for beetles in host location, although changes to the composition of cues following AOD infection are unknown. Our hypotheses are therefore as follows: 1) AOD infection causes changes to Volatile Organic Compound (VOC) production from tree foliage, which attract beetles more so than healthy tree VOCs, and 2) AOD bacteria produce VOCs which attract gravid female beetles to bark lesions, as sites for oviposition. To test hypothesis 1, VOC collections from healthy vs. AOD infected trees were performed and analysed using gas chromatography (GC)/GC-coupled mass spectrometry (GC-MS), showing unique VOC production in infected vs. healthy trees. The role of these VOCs on beetle behaviour, using four arm olfactometer tests, will be discussed. To test hypothesis 2, VOC collections from AOD bacteria were performed and analysed by GC-MS. AOD bacteria produce unique VOC profiles, including two B. goodwinii specific VOCs which elicit strong antennal responses by A. biguttatus in coupled GC-electroantennography (GC-EAG). Four-arm olfactometry using extracts from the bacterial headspace collections showed greater attraction of gravid female beetles to a combination of a synthetic bark blend + headspace extract from mixed colonies of AOD bacteria, compared to the bark blend alone, suggesting bacteria infected bark is more attractive than non-infected bark. This work will reveal the role of VOCs in the host-pathogen-herbivore ecological system and may clarify the role of A. biguttatus in bacterial transmission. Identified attractant VOCs can aid the development of semiochemicals-based lures to improve the detection and control of A. biguttatus, and the spread of AOD.

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

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