- The threat
- Susceptible trees
- The science
- Reporting suspected cases
Emerald ash borer (EAB, Agrilus planipennis) is an exotic beetle pest. It is a member of the beetle family Buprestidae, and causes significant damage to ash trees (Fraxinus species).
A native of eastern Asia, it was accidentally introduced into North America, probably in the 1990s, and most likely in imported wooden packing material. Ash trees have been widely planted in urban situations there, and are economically important as a commercial timber crop. The death of many ash trees, within two to three years of first showing signs of ill health, is causing considerable concern.
There have been no discoveries of the pest in the UK, but we remain vigilant and ask you to report any suspected sightings.
EAB is native to China, Japan, Taiwan, Korea, Mongolia and the Russian Far East. In July 2002 it was identified as the cause of the decline and mortality of ash trees which had been causing concern for a number of years in Detroit. Shortly after this it was also found in Windsor, Ontario, Canada. The pest is now established across many thousands of square miles of the USA and Canada. It has also spread westwards from Asia, and has reached the Moscow region of Russia. It is has been spreading west and south from Moscow at a rate of up to 41 kilometres (25 miles) a year.
Ash is an important broadleaf tree in the UK, the second most commonly planted genus, and makes up nearly 15% of all broad-leaved woodlands. Its wood is strong but flexible, with many uses including the manufacture of ladders, flooring, handles, sports goods and furniture. Although there is no evidence to date that EAB is present in the UK, the increase in global movement of imported wood, wood packaging and dunnage poses a risk of its accidental introduction.
The UK's ash trees are already under threat from Chalara ash dieback, which is caused by the fungus Hymenoscyphus fraxineus (formerly known as Chalara fraxinea), first identified in the UK in 2012.
Our contingency plan outlines the steps which would be taken if an outbreak of emerald ash borer were discovered in the UK.
Emerald ash borer is included on the UK Plant Health Risk Register.
EAB infestation is usually difficult to detect until the symptoms become severe. Trees show a general yellowing and thinning of foliage, dying branches and crown dieback, typically from the top down. Small trees can be killed in one year, but larger trees can take up to four years to die. Sprouting epicormic shoots, small longitudinal splits in the bark, or woodpecker activity can indicate beetle presence. Characteristic serpentine insect galleries can be exposed when pieces of bark fall from damaged trees which have been infested for 1-2 years.
Symptoms of EAB infestation are similar to those caused by a variety of root and butt rots which can cause late flushing, thinning foliage and decline leading to eventual death. Ash can also suffer from a condition called dieback (Chalara ash dieback is only one form of dieback of ash), involving the death of scattered twigs, branches or limbs, especially in eastern, drier regions. Although not fully understood, this might be partially caused by root disturbance. Trees with symptoms like these which also show any signs of infestation by emerald ash borer should be reported immediately.
Look out for:
- initial thinning or yellowing of the foliage
(general, or limited to certain branches);
- bark fissures, 5-10cms in length, caused by the
growth of callus tissue produced by the tree in
response to larval feeding;
- woodpecker activity. Woodpeckers strip away
small patches of bark so that they can
extract the borers;
- larval galleries. Typical galleries meander, bend
sharply, and are packed with frass; and
- D-shaped holes in the bark, about 3mm in diameter, produced by emerging adults.
EAB has been found in several North American, Asian and European species of ash, including the European species widely found in the UK, Common ash (Fraxinus excelsior) and Narrow-leaved ash (F. angustifolia). In China, the beetle colonises Manchurian ash (F. mandshurica) and Chinese ash (F. chinensis).
It does not attack Mountain ash, also known as Rowan (Sorbus aucuparia).
In China the beetle develops through its full life cycle in one year over most of its range, and this appears to also be the case in North America. In colder, northern areas of China, the cycle can take two years. Adults appear from mid-May to late July, and feed on ash foliage. They are slender, elongate beetles 7.5 - 13.5mm long, and are metallic emerald green. Adult females live for about 22 days, and males slightly less. The females each produce 60-90 eggs. These are laid singly or in small clusters into bark crevices, and hatch in 7-10 days.
The larvae (grubs) burrow through the bark after hatching from the eggs, and begin feeding in the living water- and nutrient-conducting tissues of the tree. They produce sinuous tunnels as they feed through four larval stages (called instars), and reach a final size of 26-32mm. Feeding tunnels can be 20-30cm long. As the larvae increase in size, the galleries enlarge and fill with brown frass (excretions), and they can eventually girdle and kill branches and entire trees. Larvae feed aggressively until temperatures fall in the autumn. After that they spend the winter in the inner bark in thick-barked trees, or in the outer wood of trees with thinner bark, before emerging as adult beetles between May and July.
Scientists from our Forest Research agency have visited Russia to study the pest there, and have co-written a scientific paper (external site) on the distribution, impact and rate of spread of emerald ash borer in the Moscow region.
If you think you have spotted signs of emerald ash borer please tell us using our Tree Alert form