Beetle threat to UK ash trees hit by ash dieback disease 'may not be as grave as first feared'
PUBLISHED: 00:01 14 November 2019
David Cappaert, Bugwood.org
European ash trees may be fighting back more effectively against a damaging beetle than expected, a new study suggests.
When the threat of fungal disease Ash dieback emerged in Europe and the UK in the last decade, the outlook appeared gloomy for Ash trees, compounded by the arrival a beetle which has not yet in the UK but is damaging trees in eastern Europe.
But the latest scientific research reveals that European ash has "moderately good" resistance to the Emerald Ash Borer (EAB) - a beetle which has severely affected ash species in the USA and some parts of Russia.
Tests on a selection of ash species show that European ash - while not immune to initial attack by the EAB - has the resources to restrict the beetle's development.
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The study - which involved scientists including from the John Innes Centre in Norwich - found the frequency with which the larvae developed to later stages in European ash was much lower than in the highly-susceptible black ash. In fact, European ash had similar resistance to that of Manchurian ash, which co-exists with the beetle in East Asia.
Previously, researchers were concerned that if EAB arrived in Britain, any native European ash trees that survived ash dieback might be finished off by the beetle.
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But Professor James Brown of the John Innes Centre, one of the study authors, said the picture was looking more positive.
"In the long term we predict that ash in the UK will gradually evolve greater resistance to ash dieback as a result of natural selection. We expect that healthy trees may suffer some harm from emerald ash borer but not be severely damaged," he said.
However, weaker trees hit badly by ash dieback might be finished off by the beetle. It is estimated around 5% of ash in Great Britain has good resistance to ash dieback fungus (Hymenoscyphus fraxineus).
Researchers found no evidence for genetic variation in responses to EAB in European ash saplings - which means breeding for EAB resistance may not be an option. They conclude that efforts to save ash trees should focus on excluding the beetle.
"If the beetle were to arrive in UK it would encounter an ash population weakened by exposure to ash dieback," said Professor Brown. "The combined effect may prove highly destructive initially to woodland and urban plantings."
Experiments were carried out in controlled conditions at The Ohio State University, USA, with the work led by David Showalter and Pierluigi Bonello. The collaboration also included NIAB EMR in Kent and The Royal Botanic Gardens at Kew.
The Department for Environment, Food and Rural Affairs' (DEFRA) chief plant health officer Nicola Spence said they were setting up an early warning system to ensure a swift response if the pest is found in the UK. "I welcome this latest research from the John Innes Centre, which is good news for the future of our native ash trees and our work to protect those trees showing tolerance to ash dieback," she said.