Host of the month - Birch

‘Host of the month’ is a series of Blogs and PDF’s that highlight a tree host and their associated priority pests and diseases that are best seen and recorded in that month. For March we’re looking at birch (Betula species) and bronze birch borer.

Birches are the predominant source of hardwood timber in Northern Europe and an important part of the Boreal ecosystem. Great Britain has three native species; silver birch (Betula pendula Roth.), downy birch (Betula pubescens Ehrh.) and dwarf birch (Betula nana L.), and several non-natives such as Himalayan birch (B. utilis var. jaquemontii) which are widely planted in parks and gardens. The shrub-like dwarf birch is restricted to the uplands of Scotland and northern England, but silver and downy birch are widespread and not always easily identified from each other. Reliable identification is sometimes made more difficult by the fact that individual trees do not always comply strictly with published species descriptions and they hybridise! In winter traits such as bark, tree form and twig characters are all helpful; in summer leaf margins, catkin scales and seed wings can also be used.

Priority pest – bronze birch borer

Bronze birch borer (BBB) is a non-native pest of birch trees which has not yet been found outside of its native range across the USA and Canada where it is considered the most serious pest of Birch species. Based on observations of European birch species planted in North America they have limited resistance and very high mortality. Combined with the wide distribution of birches and their ecological and commercial importance this high mortality BBB could have catastrophic effects if it reaches Great Britain.

Adult BBB are between 7 and 12mm long and a metallic copper-colour with the characteristic bullet-like shape of other Agrilus species. The female beetles lay their eggs in crevices in the bark of host trees and once hatched the larvae feed within the inner bark, effectively girdling the tree and causing dieback of foliage and branches above. Once mature the larvae pupate beneath the bark and emerge via characteristic D-shaped exit holes and feed on foliage of the host tree and other species.

Identification

Characteristic D-shaped exit holes may be the first external signs of attack but with increasing numbers of larvae leaf yellowing and subsequent branch dieback in the upper canopy soon follows. These may be accompanied by swellings and bumps as the tree responds to the cambium damage by tunnelling larvae, accompanied by rusty-coloured sap and staining on the outer bark.  In common with many pests and diseases that girdle stems the trees can also respond by producing abundant epicormic shoots.

Birch in the British Isles can be hosts for two other Agrilus species, the native A. viridis and the non-native A. olivicolor. Both leave similar D-shaped exit holes in the bark as they emerge but neither are fatal to the host tree.

For more information check the Observatree resource pages for bronze birch borer, or the Host of the Month for March. You can also test your knowledge with the Host of the month Quiz.

Reporting

March is an ideal time to seek out birch trees and see if you can identify any of the signs and symptoms of bronze birch borer. BBB is a priority pest so please report possible sightings via TreeAlert. Healthy tree data is equally important so please do report those too.