Will Europe lose its ash trees?

The emerald ash borer has been detected in Hungary – the first confirmed record in the EU. How significant is this?

The fact that EAB has been detected in both Hungary and Slovakia is unfortunate and significant. Based on experiences from North America, we know that this is an extremely damaging forest pest – maybe the worst that has ever invaded Europe. I anticipate that it will relatively quickly kill most ash (Fraxinus) in Europe. The species has been spreading west across Russia and has been present in Ukraine for 5 years or more so we anticipated that it would spread into the European Union, but now it is here and we will need to confront the reality of its devastating impact.

EAB will have devastating impacts in natural forests since ash is a common component of European forests. However, the greatest impacts will be in urban areas – ash comprises a large fraction of urban tree plantings across much of Europe. The rapid dieback and death of ash in urban areas is a major challenge for city managers because so many dead trees will represent a significant public safety hazard and removal of large numbers of trees will be costly.

What can Europe learn from the invasions in North America or other countries?

Eradicating new invasions of damaging non-native insects can be difficult but if successful, it is a valuable approach for preventing their impacts. There are many instances in which non-native insect species have been successfully eradicated. One important thing that we have learned from North America is that it is unfortunately not possible to eradicate the emerald ash borer. The problem is that there are no tools for detecting it at low densities – by the time its presence is discovered, it has already been there for many years and is too widespread to eradicate.

Where might comparisons between North America and Europe be misleading? Could differences in climate, ash species, forest structure, natural enemies or management lead to different outcomes across Europe?

I expect the biology and impacts of EAB in Europe will be very similar to what has been observed in North America. European host ash species are highly susceptible and the climate is generally suitable for this insect. Probably the only hope for the impacts of EAB being less than in North America is the fact that there is a native parasitic wasp species, Spathius polonicus, that is native to Europe and capable of using EAB as a host. Based on experiences from Russia, we expect this parasitoid to attack EAB and it may provide some level of control, but that may only be partial control and may not happen for several years.

How might ash dieback and the emerald ash borer affect European ash together?

Ash dieback is already negatively affecting ash in Europe. I anticipate that EAB will be more “aggressive” and cause much more rapid tree death across large areas. Widespread infection of ash may contribute to more rapid death in host ashes and this may intensify the EAB epidemic. The presence of ash dieback also unfortunately obscures our ability to detect areas with newly established EAB infestations because the initial symptoms (crown dieback) are similar.

What will most influence its spread in Europe: climate, ash distribution, natural dispersal or wood transport?

Ash is present throughout central Europe so I anticipate EAB will spread quickly. Long-distance spread is facilitated by people accidentally moving life stages. This happens frequently through the movement of infested wood (e.g. fuelwood) or nursery stock.

What are the key scientific uncertainties?

Even from North America, we do not know the long-term future for ash in natural forests. In both North America and Europe, ash produces large numbers of seedlings so we anticipate that there will be a new generation of ash growing following the initial EAB epidemic. It remains to be seen how well they will survive once they become large enough for EAB to attack them.

Does this early detection offer a chance to study an invasion from the outset? What data and samples should scientists collect now?

Unfortunately, it is impossible to detect this insect at low densities. Traps exist but they are extremely weak at attracting beetles so by the time EAB is found in traps, populations have already been present and widely spread. Larval populations can be directly sampled though it is a laborious process requiring peeling of bark. I think that studies of larval parasitism will be very important since, as I mentioned before, parasitoids offer our best hope for minimizing this problem.

Which signs should trigger expert inspection or official reporting?

Any detection of live EAB (either in a trap or in a tree) triggers a response. Currently, the European Commission requires that national plant protection organizations conduct surveillance and any detection of EAB must be reported.

What should plant health authorities prioritise in the coming weeks and months?

The most important and effective practice is to limit the spread of EAB by preventing domestic movement with cut wood or nursery stock. By prohibiting the movement of such material, it is possible to slow the spread of EAB, though it is not possible to stop its spread.

What should forest managers, municipalities, nurseries and arborists do now – and what should they avoid?

These and other community members should avoid moving dead or living ash wood or cuttings because this facilitates EAB’s long-distance movement. Urban tree managers should immediately stop planting European and North American ash species, they should identify high-value urban ash that can be protected using insecticidal injections and prioritize immediate removal of ash that are already in poor health.

Can the public do anything to help minimise the spread of EAB?

Even though it may be difficult for the general public to discover new populations of the emerald ash borer, there are important things they can do to help minimize the problem. Specifically, they can help slow the spread of the pest by not moving logs or firewood, or garden waste that might include ash. Often, people use dead or dying trees from their gardens for fuel and sometimes it is moved quite some distance – this is how EAB gets moved around and the public can help by refraining from this practice.

Is urgent European cooperation needed, and if so, in which areas?

There is already good cooperation within Europe. The European Commission, the European Plant Protection Organization (EPPO) and the European Food Safety Authority (EFSA) are already acting to coordinate the management of this pest’s invasion.

What is realistic now: eradication, containment, slowing the spread or protecting selected trees and ash genetic resources? What determines the best strategy?

Out of these choices, the best strategy is to slow EAB’s spread by prohibiting movement of infested wood and nursery stock. The other approach that has value is to limit the species’ impacts on urban trees via insecticidal injections. These injections have very little non-target environmental effects and a single treatment is effective at protecting a tree from EAB attack for 2-3 years. Though they are not practical for use in natural forests, they are a great option in urban areas where the high value of trees can justify their cost.