Bronze birch borer (Agrilus anxius Gory) larvae are flattened, creamy-white, legless grubs with light brown heads. Adults are slender, olive-green to black beetles with metallic bronze reflections.
This native beetle attacks all white-barked birch species, including native paper birch and European cutleaf weeping birch. While its typical host tree would be mature birch trees 40 years or older, it will attack even young birch trees when population outbreaks occur.
Such an outbreak was triggered in Grande Prairie in the early 2000s by the onset of a drought cycle. The outbreak has not yet ended.
European cutleaf weeping birch have no resistance to this pest and are not recommended for planting in Grande Prairie. Paper birch, which evolved with the borer, have the ability to increase their sap pressure to push the beetle larvae out, so long as they remain well-watered. Unfortunately, due to the drought cycles that Grande Prairie experiences on a regular basis, the trees lose their ability to fend off the beetle and eventually die from its attacks as well.
- Dead limbs at the top of the tree.
- D-shaped bore holes are visible below dead limbs.
- Successive years of attack progress down the tree until it dies.
- Ensure your birch trees are well watered during dry periods.
- Leave low branches in place to keep the tree's shallow roots cool.
- Don’t dig in the root zone.
- Do not store or transport infested birch wood.
- As soon as damage is observed in the top limbs of the tree, prune out all deadwood at least 60 cm below the lowest bore holes to ensure all larvae are removed. Burn or chip the wood immediately.
- Do not plant any white-barked birch species, as all are susceptible to this insect. Dark-barked birches (river birch (Betula nigra) and water birch (Betula occidentalis) are considered resistant to bronze birch borer and recommended as substitutes.
- We recommend planting Hardy weeping willows such as 'Lace' weeping willow (Salix babylonica) instead of birch trees. They will give the beautiful weeping form desired by those who love cutleaf weeping birch trees and are not affected by the bronze birch borer.