Advance of the emerald ash borer beetle in Europe

Life cycle of the insect

Female emerald ash borer beetles, Agrilus planipennis, lay 40-90 eggs between June and October on the bark and in cracks in the bark of ash trees. They mainly do this in the upper parts of the crown on branches around 5 to 10 cm in diameter. The eggs hatch within 10 days and the small larvae eats their way through the bark to the nutritious part of the tree, the cambium. In this area the larvae wind their way through the cambium. As the insect grows to a maximum of 36 mm the diameter of the feeding tunnel also increases.

Diameter galleries increase with growth of larvae

The tunnel winds horizontally and slopes vertically downwards over about 20 cm. Under warm conditions, as in much of the United States, the insect has a complete cycle in one year. Under colder conditions, such as Canada and Russia, a complete cycle can take two years. The larvae winter in trunk of the tree and pupate in April/May. From June to October the beetles eat their way through the bark and leave a diagnostic D-shaped exit hole behind. The D-shaped exit hole is actually a feature of all boring beetles, Agrilus. The beetle lives roughly 22 days, eats leaves in the upper crown and usually flies a maximum of 800 metres and lays its eggs on another ash, thus completing the cycle.

Species sensitivity in ash

Emerald ash borer beetles are monophagous, and as would be expected predominantly specialise in ash trees. Like a juicy steak to some people, green ash Fraxinus pennsylvanica is to the emerald ash borer beetles. In reality American- or white ash Fraxinus americana, is also very sensitive. From both species just a few survive, possibly due to a different constitution or better resistance after infestation. Within diverse hectares of ash forest or plantations it’s only a few trees which survive. The phenomenon that a few trees survive disease has already been recorded in the past, by Andrea Harper from the University of York in relation with ash dieback disease.

Fraxinus quadrangulata is quite tolerant and native in Kentucky

The blue ash Fraxinus quadrangulata, in comparison fares more positively with a higher tolerance. The experience in America shows that this species is far less sensitive than green ash and American ash. Around half of the blue ash population survives. It is assumed here that the stronger blue colouring in the cambium some kind of influence has on the attractiveness to the ash borer beetles.

In the Netherlands we mostly have the common ash Fraxinus excelsior, often also in various cultivated forms. In common ash it is not yet fully known how sensitive they are to ash borer beetles. It is expected that they will be sensitive especially in combination with reduced resistance due to ash dieback disease. In addition to ash there are also cases known in America with white fringetree, Chionanthus virginicus. This species does not appear to be high on the menu but can be affected. This specie is very restricted in the Netherlands.

Consequences

After the larvae have attacked the upper parts of the crown and found their way through the cambium, thin canopy cover and dieback can be visible. An alert observer will usually only see signs after more than one season after the initial infestation. It is known that an obvious sighting is usually only seen 3-5 years after the first infestation where die back occurs in the crown starting from the top. In this period diverse life cycles have been completed and eggs have been laid on multiple occasions. The tree reacts with epicormic shoots on the trunk under the broken cambial zone. Eventually the ash dies, this whole process can take 5 years but there are also known cases where the whole plantation has died within 2 years of first. It is likely that if an ash shows signs of being infected that it will eventually die.

Dying ash in Kentucky

Distribution

The original distribution was in Northeast China, North and South Korea and in the far east of Russia. There the beetles also attack ash trees including Fraxinus mandshurica and Fraxinus chinensis, but the consequences are more limited. The ecology of the whole environment in particular with predators maintains a balance with the beetles. Especially ichneumon wasps, both as larval and egg parasites are the predominant predators. How the beetles reached Detroit (United States) in 2002 or Moscow (Russia) in 2007 is not known. That could have been through infected vegetation or untreated wood. In these cases as well there were several years between the moment of infection and first visible signs were noticed. By this stage multiple generations have already had chance to spread and infect other trees. The natural distribution is usually relatively restricted to a maximum of 800 metres. It is a different story in the United States where new cases are nearly always next to busy transport routes/motorways. This means that beetles are travelling in or on transport.

Humans are an important source within the succes of the spread of emarald ash borer

It could also occur by means of being carried on felled trees. In America this has created an increase in distribution of 20 km per year. But there were also enormous jumps of 1200 km away from the known infected areas. In Russia a spread of 4km per year is recognized, although logs for fires is not recognised as part of the distribution. In reality the outbreak in Saint Petersburg is 470 km from the known area in Moscow. The infection Saint Petersburg means the beetles are only 140 km from the EU border in Estonia. It would seem to be only a matter of time before they reach the EU, if they aren’t already here.

Experience Russia and United States

In 2017 I went to Russia to learn more about the distribution and strategy to deal with the problem. I visited the outbreak areas and the University of Moscow. The university is actively involved in monitoring the distribution and studying of management measures. Moscow is seen as the central outbreak zone of 2007. Just walking through Moscow there were signs of peak mortality in many places, but there were also seemingly healthy ash trees also still standing.

A plantation of green ash, Fraxinus pennsylvanica next to a railway line had died back to just 5 metres above ground level where epicormic regrowth was visible. It was obvious that these trees were not going to survive. The eating tunnels were also visible in the lower parts of the trunk. I thought it was unusual that the majority of ash trees in Russia were American ash species. After my visit to Moscow I visited Voronezh, a place roughly 650 km south of Moscow near the border with Ukraine. To my surprise I barely saw any living ash trees here.

Massive dieback in Voronezh, above September 2013 (source Google Streetview) below September 2017 during my visit. Green leafs of epicormic growth and other tree species.

The trees I did see were in their last stages of life, with several metres of epicormic shoots on the trunk. Quite a contrast with Moscow! I immediately doubted if Moscow was the centre of the outbreak zone. In 2017 the EPPO already confirmed the presence of the beetle in Ukraine. This route is also towards the EU, so it can be expected.

Late 2022 I visited a tree care company in Kentucky (United States). The consequences to the ash trees here are great. Many ash trees stand alongside the motorways in rows of trees and forests. The majority of the ash trees are dead or dying. The majority of dead ash trees are Fraxinus pennsylvanica and Fraxinus americana.

Steminjection with emmamectin

An effective method has been developed to fight the beetles. This involves a preventative trunk injection where the chemical ‘Emmamectin’ with the active ingredient ‘emmamectine bentozoa’ is stem injected in the spring. This product works systematically, is ingested during digestion and both the larvae and the beetle die. The manufacturer says that the product is effective for 3 years. In reality applicators want to shorten this period to 2 years. The effects on non-target species are negligible according to the applicators. Considering the broad range of the product this is certainly something I am concerned about. Serious effects on non-targeted organisms can definitely be expected. It needs to be taken into account that any tree not treated is likely to die, so any organisms dependent on a living ash tree will also suffer. There are also programmes where ichneumon wasps from China are reared in America. Up until now it’s too early, and experimental, to say if this will have an effect on the emerald ash borer population.

Quarantine policy

Experience in the United States shows that ash free zones, varying from 800 metres around each outbreak to around 2400 metres in Maryland, haven’t been able to halt the beetles. In Canada even more drastic measures have been taken with a 10km ash free zone between 2 lakes where 85,000 ash trees have been preventatively felled. That didn’t work: about a year later the beetles were found behind this zone. Canada reacted by felling another 50,000 ash trees. This didn’t make a difference and eventually it was decided not to carry out anymore felling. The beetle was already 100km further within 3 years.

As we speak the EU is developing quarantine policy. Experience shows that quarantine zones can’t stop the beetles. Early detection seems to be the most effective method. This can be done by stripping the bark of ash trees to make them attractive to beetles and by installing traps, at least 1 per hectare. Both methods are very labour intensive. Installing and analysing involves a lot of man hours.

It appears to be impossible to stop the beetles reaching Europe. Their arrival from the east seems inevitable. Identification and immediate removal at source would slow down the advance, but not prevent it. And yet I still think it’s important to train people who work every day with trees. Scientists also deserve to play a role but they just don’t spend enough time in the field and are usually more project based. In addition planners should be aware that ash will probably disappear as a species from our landscapes in the future. Also here species variety is key. Let’s make sure we don’t just plant elms to solve the problem. We should have learnt from the past with elm disease when we mainly replaced the dead trees with ash trees!