Bronze birch borer (BBB, Agrilus anxius) is a specialist wood borer of birch trees (Betula species). Its adult form is a beetle, and it is native to North America.
Adult beetle, approximately 7-12mm long. Source: Whitney Cranshaw, Colorado State University, Bugwood.org.
Bronze birch borer larvae, or grubs (the immature life stage), affect host birch plants by boring into and feeding on the inner bark and cambium (the first layer of tissue under the bark). This action restricts or blocks the movement of water and nutrients through the plant, with serious consequences, including death, for some host plants.
Although BBB is not known to be present in the UK, there is a risk of its being accidentally introduced, and a Pest Risk Analysis was produced in 2011 by the European & Mediterranean Plant Protection Organization (EPPO).
The UK Plant Health Risk Register gives details of the pest’s risk rating and other information.
All species of birch are susceptible to the pest, including Great Britain’s native silver and downy birch (Betula pendula and B. pubescens).
In nature, the pest is spread locally by adult beetles moving from tree to tree, although no clear data on the extent of adult spread is available.
Long-distance spread can occur by the movement of infected plants for planting and wood of birch species, with or without bark.
BBB is currently found only in North America, where it is widely distributed, but more common in warmer, southern regions.
Initial symptoms of an infestation appear in the upper crown of the tree, with leaf yellowing and branch dieback. Other evidence of infestation is the presence of 3-5mm-wide, ‘D’-shaped exit holes formed by the emerging adult beetles. Sinuous larval galleries are found under the bark.
D-shaped exit holes of A. anxius. Source: Whitney Cranshaw, Colorado State University, Bugwood.org.
Oozing, rust-coloured sap, and staining, can also be observed on the outer bark, along with swellings and bumps where the tree has healed inside.
BBB is subject to European Union importation controls on birch material coming in from North America. The control strategy primarily aims to keep the pest out of the UK if possible. Pending landings of birch material from North America must meet UK landing requirements, and must be pre-notified to the UK plant health authorities to enable inspection.
However, if it did become established in the UK, to minimise spread control would focus on the targeted removal of the most seriously affected trees in the first instance, particularly those suffering from extensive canopy thinning and dieback.
Our contingency plan sets out the steps that will be taken in the event of an outbreak of bronze birch borer being discovered in forests in Great Britain.
Although BBB is not known to be present in the UK, there is a risk of its being accidentally introduced. We therefore urge the public, and especially tree and plant professionals, to remain vigilant for signs of it, and to report suspicious symptoms to us.
Bronze birch borer is a quarantine organism, so there is an obligation to report any trees suspected of being infected by it. Please report suspected cases to us with our Tree Alert on-line pest and disease reporting form.