The deadly decline of Britain's native oak trees may be driven by an unexpected accomplice: their own smell.
Scientists have discovered that trees affected by Acute Oak Decline (AOD) emit distinct odours that are highly attractive to the beetle Agrilus biguttatus, a key contributor to the decline. The findings suggest that beetles use chemical cues in these odours — emitted by infected leaves and the bacteria involved in AOD — to locate ideal sites to lay their eggs.
The study, published this week in Current Biology, sheds new light on the complex interplay between trees, microbes and insects in one of the UK's most concerning woodland health crises.
AOD is a fast-progressing condition affecting thousands of mature oaks across England and Wales. It is characterised by dark, weeping lesions on tree trunks and the presence of larval galleries from A. biguttatus, a jewel beetle whose larvae burrow through the tree's inner bark, disrupting nutrient flow.
Researchers found that female beetles were strongly attracted to airborne chemicals produced by infected oak leaves and by two bacterium species central to AOD: Brenneria goodwinii and Gibbsiella quercinecans. The effect was even more pronounced when the scent of the bacteria was combined with scents from oak bark.
Crucially, scientists identified three specific chemical compounds within the odour emitted by the bacteria, two of which are entirely new to science. These natural products appear to serve as olfactory cues, helping the beetles zero in on stressed and susceptible trees, although they do not directly trigger egg-laying behaviour.
Dr Jozsef Vuts, who led the study, said: "What we're seeing is a sophisticated chemical signalling system. The beetles are essentially using the scent of illness — from both the tree and the bacteria — as a beacon. It's a remarkable example of how insects can exploit microbial activity to guide their behaviour."
The team's findings point to a possible feedback loop in which the beetles are drawn to declining trees already infected by AOD bacteria, facilitating further damage as their larvae open new pathways for infection and decay.
Understanding this chemical dialogue could open the door to novel management strategies. By developing beetle traps based on synthetic versions of these odour compounds, or sensors that can detect the compounds, foresters may be able to monitor or even slow the spread of AOD in vulnerable woodlands.
More than 120,000 trees are estimated to be affected by AOD in the UK, many in ancient or ecologically important forests. As climate change places further stress on native species, the need for innovative approaches to forest health has become increasingly urgent.
"This research provides a new piece of the puzzle," Dr John Caulfield added. "If we can interfere with the beetle's ability to detect infected trees, we might be able to tip the balance back in favour of the oaks."
