Analysis: The ash dieback disease caused a dramatic decline of the common ash, but the spread of the emerald ash borer pest is now causing concern
In the last decade, Europe has witnessed the dramatic decline of one of its most beloved native trees, the common ash, due to ash dieback disease. Now, another pest, the emerald ash borer, currently in Ukraine and Russia, adds urgency to conservation efforts and requires vigilance across Europe.
Since first spotted in Poland in the 1990s, the invasive ash dieback fungus Hymenoscyphus fraxineus has spread throughout Europe mainly by wind-dispersed microscopic spores, putting the survival of common ash (Fraxinus excelsior) at risk. The presence of ash dieback was first noted in Ireland in 2012. Since then, researchers at Teagasc have been working on ash breeding and conservation in collaboration with national and European research organisations and industry.
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From RTÉ Radio 1's Morning Ireland, Joe Mag Raollaigh on how the ash dieback crisis has impacted Irish foresters
However, Eastern Europe is currently facing the spread of another major threat to ash: the emerald ash borer (Agrilus planipennis), a shiny green beetle that burrows under the bark of ash trees. The borer has become a major pest of American ash populations since it was first detected in the United States in 2002. The insect’s larvae feed on the inner bark, disrupting water and nutrient flow, eventually killing the tree.
This has led to mass ash mortality and is causing significant ecological, economic, and social impacts. In the United States, multiple native species of ash exist and show varying degrees of tolerance to the emerald ash borer and to the dieback fungus. In contrast, Europe has only one widespread native ash species, the common ash, which is vulnerable to both pests where they occur.
This makes Europe particularly susceptible to further threats affecting this unique tree. In Ireland, ash is well known as the preferred wood for manufacturing of hurleys for our traditional games of hurling and camogie. Moreover, ash trees are a significant component of European forests, supporting a wide range of wildlife and contributing to ecosystem services such as carbon sequestration and soil protection. The potential added threat from the emerald ash borer could further impair these important functions and destabilise our forests.
From Andrew The Arborist, a look at the emerald ash borer's life cycle, how they damage trees, how to detect signs of an infestation and treatments to use against them
Monitoring and biological control
Lessons from North America highlight the need for enhanced monitoring and swift response measures. The European and Mediterranean Plant Protection Organization (EPPO) network is actively monitoring the spread of the emerald ash borer in Eastern Europe. Recent results indicate that, so far, the borer remains confined to Russia and Ukraine – a hopeful sign, though not a guarantee. Authorities across Europe have already tightened regulations on the transportation of ash material to prevent accidental introductions of this pest.
Another crucial measure under evaluation is the use of biological control. This approach involves breeding and releasing parasitoid insects, tiny wasps that lay their eggs on the borer’s eggs or larvae, effectively using the host’s resources to develop their own offspring. The key challenge is ensuring these wasps effectively target the emerald ash borer without harming native beetle species. Researchers across Europe are exploring both parasitoids imported from Asia, which have already proven effective, and testing local species that may also provide protection against the borer.
Plants highly tolerant to disease
A further strategy to mitigate the threats posed by invasive pests is the development of highly tolerant plant varieties. In Ireland, Teagasc researchers have been working on the genetic improvement of ash since the early 2000s, initially focusing on growth and wood quality. With the arrival of ash dieback disease, research expanded to better understand how ash interacts with the dieback fungus in order to fight the disease more efficiently.
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From RTÉ Radio 1's Mooney Goes Wild, can we save the common ash tree?
Research work included the establishment and constant expansion of an ash collection, drawing samples from Ireland as well as from various European countries. This diverse collection is a key resource for tree breeding that targets tolerance to dieback as well as other traits of interest.
Crucially, researchers in Switzerland have foud evidence of cross-resistance between tolerance to ash dieback and the emerald ash borer. In other words, trees that are tolerant to one are more likely to be tolerant to the other, which is promising news for the future of ash breeding.
Diverse genetic ash collections
While most of Europe seems free from this borer for now, ongoing and concerted vigilance is required to protect us from its potential future range expansion. Research shows that the emerald ash borer can fly up to ten kilometres and may even hitch rides on vehicles, accelerating its spread. Early detection is a challenge, as current monitoring techniques might only catch the insects once their numbers are high in a location.
Collaborative measures, which include expanding diverse genetic ash collections, are crucial. These efforts will help mitigate the immediate threat of ash dieback while building a foundation of expertise and resources that will be crucial if the emerald ash borer eventually spreads towards Western Europe.
To secure our future forests, we must invest in better monitoring methods and in sustaining long-term tree breeding programmes. Currently, Teagasc is strongly focused on leveraging the genetic diversity of ash to establish a diverse, dieback-tolerant breeding population. If you see a healthy ash tree in your area, report its location and join the AshforFuture initiative – you can learn how to identify healthy ash and report it on the project webpage. Together, we can help safeguard and restore our natural heritage.
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Dr Ricardo Pimenta is a post-doctoral genomics researcher in Teagasc's Forestry Development Department.
The views expressed here are those of the author and do not represent or reflect the views of RTÉ
