Bronze Birch Borer
Agrilus anxius
A slender bronze beetle whose larvae tunnel beneath the bark of birch, girdling branches and killing trees from the top down. Stressed and sun-exposed birches are the most vulnerable, and white-barked species suffer worst.
🔎 How to spot it
Adults are slender, dark, iridescent greenish-bronze beetles about a quarter to half an inch long. The flattened white larvae are slightly over half an inch long with an enlarged area behind the head. The tell-tale signs are D-shaped exit holes in the bark and raised ridges or bumps on the trunk and limbs where larvae have tunneled.
🥀 Damage it causes
Larvae feed in winding galleries in the phloem just under the bark, girdling branches and cutting off the flow of nutrients and water. Foliage in the upper crown yellows in midsummer, branches die back, and over time the whole tree can be killed. Raised bark ridges and rusty staining mark the larval tunnels.
🛡️ Prevent it
Keep birches healthy and cool, since larvae are reported not to survive in vigorous trees: plant in cool, moist, shaded sites and avoid hot, open, south or west exposures. Water during drought and mulch the root zone. Choose resistant species such as river birch where pressure is high, and prune out dead or damaged branches by early May.
🧯 If it is already here
Existing infestations are hard to cure once larvae are in the wood. Thoroughly spray the trunk and branches with a registered insecticide during the first and third weeks of June to kill newly hatched larvae before they bore in, or use a professionally applied soil or trunk injection. Tree vigor remains the foundation of control.
💡 Good to know
The life cycle takes one to two years, most often two, and the borer overwinters as larvae under the bark. White, paper, and gray birches are the most susceptible, while river birch and other brown-barked species are more resistant. Early attacks begin on thinner upper branches.
For educational and informational purposes only. Pest control advice is general guidance drawn from university cooperative extension sources; always identify a pest positively and read and follow the label on any product before use, especially around food crops, children, and pets.