Bronze Birch Borer Diagnostic Guide
Native to North America, damage caused by the Bronze Birch Borer (BBB), Agrilus anxius, dates back to the late 1800’s and continues to present day. This pest is known to attack all native and non-native species of birch, although susceptibility varies. The BBB is opportunistic and thrives in birch trees that are weakened or stressed by agents such as drought, soil compaction, root injury, other insects, and old age. Birch trees tend to prefer cool and moist growing sites, which are not typically found in most residential and urban areas. With frequent stress caused by dehydration, high temperatures, and compacted soils, landscape birch trees are commonly attacked by this pest.
Trees at Risk
All species of birch may be attacked, but some species are more susceptible than others. Among the most susceptible are whitebarked Himalayan birch, European white birch, European white weeping birch, yellow birch, sweet birch, paper birch, whitespire birch and grey birch.
Signs of Damage
- Small branches in the upper 1/3 of the crown will be thin and begin to die back.
- Leaves on infested branches will be stunted, yellow, or show marginal chlorosis in May or June.
- Infested birch trees typically die in 3-4 years.
- D-shaped exit holes.
- Ridges may appear on the bark of the trunk and larger branches in response to larvae feeding beneath the bark in the phloem tissue.
- Zigzag galleries are often formed.
- “Coffee”-like stains on trunk and limbs.
- Adult borers are slender, about 3/8 inch (10 mm) long, metallic-coppery colored beetles (but are rarely seen).
- Larvae occur underneath the bark and are white, segmented, legless grubs with an enlarged area behind the head and are 1/2 inch long.
- Larvae feed laterally around the branch on phloem tissue, etching the xylem.
The best protection from the bronze birch borer (BBB) is prevention. Because this pest attacks weakened and stressed trees, care and attention should be given to maintaining tree health by watering during drought, mulching, and minimizing damage to the root zone that could cause root injury and stress. For new plantings, consider other species that are more stress tolerant.
As a rule of thumb, if more than a third of the tree is infested the chances of saving it are significantly reduced. Infested branches and deadwood should be removed, so give attention to what the tree will look like before deciding on subsequent treatments.
High value trees at risk for infestation by the BBB may be treated preventatively with Xytect™ insecticide. A soil drench in the fall or early spring will provide season-long protection. Xytect™ will also provide protection from the birch leaf miner, which causes additional stress by feeding in leaves. Trees infested with BBB may be similarly treated, although trees with serious decline are unlikely to be saved.
Emamectin benzoate† can also be used at a rate of 10ml/DBH” during the growing season. This treatment is good for 2 years.
†Tree injection treatments recommended if soil applications are not feasible.
- Preventative treatments are most effective.
- If the top third of the tree shows complete dieback, saving the tree is difficult.
- For infested trees, soil applications of Xytect™ will be effective against the next generation. An improvement in the tree typically occurs the 2nd full season following treatment as the tree repairs previous damage and prevents future damage.
A Diagnostic Guide is designed to help you identify a pest issue and management solutions. Always refer to product label for all rates and approved uses. Some images courtesy forestryimages.org. Use of the images does not imply endorsement of treatments by forestryimages.org