I am going to start this article with a preamble. I am not an entomologist so what I have written will certainly contain some inaccuracies. What I am interested in doing, besides talking about the emerald ash borer’s direct damage, is explaining all the effects that the death of millions of ash trees has had in terms of changing the urban landscape, microclimates, the rise in energy consumption, plant perception in certain areas and especially, the incidence of certain cardiovascular and respiratory tract diseases.
Even if the pest is currently confined to the United States and Canada, its effects are not dissimilar from other pandemics in the past (chestnut blight, Dutch elm disease, etc.). Moreover, unfortunately, we cannot rule out its arrival in Europe.
The Agrilus planipennis is an exotic insect, native to eastern Asia and was accidentally introduced into North America around 1990, most likely in wooden packaging (pallets or crates). From a systematic point of view, it is a member of the Buprestidae family of beetles, which has caused and is causing serious damage to American ash trees. However, European ash trees are also susceptible (Fraxinus excelsior and F. angustifolia). In China, it attacks F. chinensis while F. mandshurica appears more resistant probably thanks to the presence in the phloem of numerous phenolic compounds, including hydroxycoumarin and other compounds that might be mechanisms of resistance to the parasite. In research carried out about 10 years ago, they turned out to be present only in the mandshurica species and not in the others that were tested and that are much more susceptible.
As mentioned, the Agrilus is native to China, Japan, Taiwan, Korea, Mongolia, and the Russian Far East. Moreover, since its accidental introduction, it has now invaded a large part of North America. In Asia, it is moving westward spreading at the speed of over 40 km per year, and has reached the Moscow region.
An emerald ash borer infestation is usually difficult to detect until symptoms have become severe. The trees show a general yellowing and smaller foliage, canopy dieback, typically from the top downward. Younger trees may be killed within a year, while larger trees can survive for four years before dying. The presence of epicormic shoots, small longitudinal cracks in the bark, or woodpecker holes may indicate the presence of the beetle.
The symptoms of an Agrilus infestation are similar to those caused by a variety of root rot and the collar that can cause delays in budding, microphylls, and a general wasting leading to death. The ash trees in Europe have also been attacked by a fungus (Chalara fraxinea) which has similar symptoms of decay. Harmless in its country of origin, this fungus has shown a significant impact in terms of its rapid spread and aggressiveness on the host, hence its inclusion on the European and Mediterranean Plant Protection and Organization (EPPO) alert list. Trees with the above symptoms should be immediately reported to the relevant authority.
After the death of millions of elms occurred in the 1960s and 1970s, ash trees were planted extensively in American cities. Furthermore, they are also economically important for commercial timber production. Since the infected plants were first detected, it is estimated that the parasite has already directly killed at least 10 million plants (however, there is even talk of 25 million) and it threatens to kill the majority of the 8.7 billion ash trees present in North America, with devastating environmental and economic costs, both direct (non-timber production, removal and destruction of plant needs), and indirect for lost ecosystem services resulting from the death of plants.
The parasite has not been detected in Europe but monitoring, especially in the UK, is continuous. In Italy its presence has been repeatedly reported, but in reality it is always a case of being mistaken for the cantharides (Lytta vesicatoria), known for its production of cantharidin, an aphrodisiac, anti-inflammatory, as well as a poison. Similar in appearance to the Agriles it also lives by feeding on the leaves of ash trees – in this case European trees (Fraxinus excelsior and Fraxinus angustifolia but not Fraxinus ornus), elders, maples, poplars, and other trees (including those being cultivated). Sometimes it is noted that this insect may also feed on olive leaves (Olea europaea). There is harm especially on the young plants on which it irreparably damages the sprouts, hampering the plant’s development. However, the damage it causes is limited and easily controllable.
The loss of such a large number of plants, as we said, not only has immediate effects, but also long term ones. These effects so far have been underestimated in calculating the impact of this pest, but from the standpoint of human health and the related economic impact, they could be ruinous.
For example, it has been calculated that if just Wisconsin were to lose its approximately 5 million ash trees in urban areas, the cost of removing and replanting of trees would be around $3 billion and about $270 million dollars in lost ecosystem services in regard to pollution reduction, higher heating and especially cooling costs and, above all the management of excessive rainfall and the subsequent runoff.
A simulation carried out in the twenty-five US states where Agrilus has been found estimates the loss of more than 10 million trees and a cost of around $11 billion for treatment, removal and replanting.
The damage caused by the emerald ash borer is not linked just to felling trees, transporting them to special dumps and replanting, but also to health problems that may worsen because of tree loss. Recent research has highlighted effects on health and wellbeing that further increase the problem. In fact, there has been increased mortality connected to cardiovascular and lower respiratory system diseases in the areas infested by this insect. The magnitude of this problem will increase as the infection has advanced and has appeared more striking in areas inhabited by people with an above average household income. In the 15 US states where research was carried out, the damage caused by the beetle have been linked to 6113 deaths caused by diseases of the lower respiratory tract and 15,080 deaths as a consequence of cardiovascular problems.
For these reasons, the emerald ash borer clearly represents a real threat not only with regard to the landscape of a continent and because of the direct and indirect costs, but also a public health problem at a level that could become global. Managing this pest therefore requires maximum cooperation between the countries involved and those that soon may be. Epidemics, even in the plant world, are like a fire: a small fire can be controlled, but when it blazes up, it is almost impossible to extinguish, as the numbers go up exponentially.