Native to Asia, the emerald ash borer could wipe out all the ash trees in the UK and Europe, Mark Seddon reports.

Britain’s trees are facing multiple threats. as global trade has intensified, once-localised pests and diseases can move with alarming speed on the backs of roots, pots and wood products, putting a species of tree that has inhabited these shores for millennia at risk of being almost completely overwhelmed.

Forty years ago, a virulent strain of Dutch elm disease, imported on North American logs, virtually wiped out the English elm. More recently, there has been the disastrous effect of bleeding canker on our beloved horse chestnuts and there’s been oak blight and the march of the processionary oak moth. Three or four years ago, we learnt that imported ash saplings were likely to be responsible for fungus-induced ash dieback, Hymenoscyphus fraxineus, becoming established and that huge numbers of these numerous and totemic trees were under threat.

However, little attention has focused on what is now probably the most significant threat to the native ash. It comes in the shape of a small, brightly coloured native of East Asia—the emerald ash borer (Agrilus planipennis). It’s currently wreaking havoc in the forests of North America, threatening to destroy up to 99% of the eight billion ash trees in the USA alone.

Unfortunately, in North America, the pest, which is believed to have arrived in Michigan several years ago in wooden crates from China, has no natural enemies. The grubs, which emerge from eggs laid in bark in spring, soon girdle the sap wood, choking the trees to death by depriving them of water. It spreads at up to 12½ miles a year and is often aided by the transportation of infested timber— mature trees are usually dead within two to four years.

In Europe, the emerald ash borer is reportedly moving quickly west of Moscow, where no controls have been put in place. The European and Mediterranean Plant Protection Organisation’s pest risk assessment indicates a strong likelihood of it ‘becoming established in the European Union and causing significant damage’. The EU has placed import restrictions on ash wood from regions where it’s present, including Russia, China, Japan, Mongolia and North and South Korea, but not for any movement within the EU.

In January, the Forestry Commission produced a contingency plan, which can be found online (www.forestry.gov.uk). It makes for sober reading, recommending that, if trees become infected, a 20km (121⁄2 mile) cordon sanitaire should be established, plus a complete ban on the movement of ash timber. Although the commission believes that eradication might be possible if there is a highly localised outbreak, worryingly, its plan seems largely to rest on slowing the spread of the beetle in the event of it becoming established in mature trees.

The commission dampens hope that experimental work in America on parasitic wasps, which normally feed on the grubs of the emerald ash borer in their native habitat, might be extended here. Other than annual injections of a powerful insecticide for particularly cherished ash trees, there is no alternative such as planting strains of the tree that are resistant to the pest’s devastating antics—and it is heading our way. Vigilance will be key.

Telltale signs

Rapid die-back of trees; vertical splits develop in the bark in which the winding S-shaped galleries of emerald ash borer grubs can often be seen. Older trees may initially sprout new shoots below infestations. When mature, the bright, metallic-green insects emerge from small D-shaped holes in the bark. Bark becomes lighter in colour and lifts from the tree as it dies.