Several species of Birch trees in the Pacific Northwest host a destructive new pest, the Bronze Birch Borer.
Confirmed in 2003, Borer populations are on the increase in Portland and SW Washington areas. The good news is that if caught in time, we may be able to treat your trees to suppress infestations.
What is a Bronze Birch Borer?
A wood boring beetle, the adult is a copper or bronze-colored slender beetle (hence its name). Adult beetles may feed on leaves but it’s the larvae that are responsible for damage. The life cycle takes 1 to 2 years to complete and starts by the insect overwintering as larva in the tree. When the weather warms in spring, they resume feeding.
Bronze Birch Borer life cycle
They pupate in the tree and in late spring, the adult insect chews a D-shaped hole in the bark to emerge. They lay their eggs in bark cracks or under bark flaps. After hatching, the larvae bore into the branches and/or trunk. They are unseen as they feed on the nutrient and water-conducting vascular tissue under the bark.
As the larvae feast, they make wandering trails through the cambial layer (between the wood and the bark). This essentially girdles the tree and the canopy suffers die back as the infestation advances.
The tree tries to compensate for the die back by growing new (epicormic) branches below the damaged tissues. The bark will split over dead vascular tissues and (if not treated), the tree may die within only two years of the onset of the symptoms.
Signs of bronze birch borer infestation
- Die back of branches, most commonly near the crown, which increases in severity as infestation worsens.
- In later stages, D-shaped exit holes (1/8 “ in size) left by the borer often having rust-colored stains.
- Bark may appear lumpy where the tree tried to heal or grow over larval galleries.
- Leaves distal to the girdling may become yellow (chorotic) and wilt, which usually begins on affected limbs in early spring.
What trees are affected?
Bronze Birch Borers attack all birch species though some are more prone to attacks than others. Borers are unable to survive in healthy trees and therefore, usually attack trees that are already stressed or in decline, such as those weakened by age or have had previous insect attacks.
You can decrease the risk of infestation by planting more resistant varieties, such as heritage birch, heritage river birch, river birch and red birch. Moderately susceptible species include gray birch, whitespire birch, yellow birch, sweet birch, black birch, cherry birch, paper birch, white birch, and canoe birch.
Species that are more susceptible include whitebarked Himalayan birch, European white birch, silver birch, European white weeping birch, Young’s weeping birch and ornamental cutleaf varieties.
Treatment for Bronze Birch Borers
If you suspect any of your trees have been infected, it’s crucial to contact General Tree right away. We’ll have one of our tree experts do an inspection as affected trees may be treated though outcome depends on the severity of the infestation.