Bronze Birch Borer
Insect Profile and Management

Two European white birch trees in Reno suffering from dieback. ©Aspect Tree Care

Insect Profile

What is the Bronze Birch Borer?

The Bronze Birch Borer is an insect pest of birch trees, and sometimes attacks other tree species. It feeds on the inner bark and sapwood, and can cause crown dieback and tree death. [1] It’s important to keep susceptible trees as healthy as possible, and in many cases, it’s also necessary to use pesticides.

What types of trees are susceptible?

In Northern Nevada, the European white birch is most affected. [2] Two of the common cultivars of this species are the cutleaf weeping birch and Young’s weeping birch. Paper birch and gray birch are also highly susceptible to the borer, but they aren’t commonly found. [1]

It may also attack other trees like aspen, willow, poplar, and cottonwood. [2] River birch, especially the Dura heat river birch cultivar, is resistant to the borer. [1] The borer prefers stressed and weakened trees. [1] Birches growing in grassy lawns can be severely stressed by the competition for water and shallow watering. [2]

Consequences and Severity

Bronze Birch Borer can cause major dieback, increase the likelihood of branch failure, and cause tree death. Trees that do survive can experience enough dieback to lose their aesthetic appeal, and provide much less shade.

Insect life cycle

The life cycle of Bronze Birch Borer is about 1-2 years in Northern Nevada. [2] After overwintering as a larva (not feeding, not causing damage), it resumes feeding in early spring with warmer temps. In May or June, it pupates and emerges as a mature adult. [2] Adults feed, causing little damage, and lay eggs on the tree from May through August, with a peak usually in July.[1] Newly hatched larvae begin feeding starting in May, causing substantial damage, and continue throughout summer and fall, before overwintering in colder temps.[1]

Diagnostic Clues

Dying branches and sparse or yellowing foliage in the upper tree crown are the first noticeable symptoms. [2] Once the borer pupates from larva into adulthood, it creates a distinctive D-shaped exit hole and leaves the tree. [1] Holes are very small, about one-fifth of an inch in diameter. [3] This is a sign that confirms the presence of the borer. Trees may also ooze rust-colored sap which stains the bark, and show swelling and bumps. [3] You may also notice wilting and broken branches.

Management

Susceptible trees must be managed with cultural controls to reduce tree stress so they can protect themselves from an attack. If there are limitations to providing the necessary care like watering, mulching, soil and root management, chemical control may be needed. Chemical control alone may not be effective. There are several different types of pesticides, formulations, and methods of application that we recommend and provide, based on the situation, time of year, etc.

Cultural and Mechanical Control

Birches do well in moist, cool, shady sites.[3] They should only be planted in sites that are protected from sun and wind, and with good soil drainage.

The most important treatment is deep watering and mulching.[3] Watering in the winter, although somewhat difficult, is very important. [3] Within the drip line of trees, soil needs to be kept moist to a depth of 18 inches.[3] Mulching is also very important. It keeps soil cool and moist. It reduces competition with grassy lawns for water and nutrients. Mulched areas around trees provide a buffer from lawn maintenance equipment, protecting trees from stress-causing damage.

Soil should be assessed to check for problems related to tree stress. Soil compaction, poor structure, low soil organic matter, inadequate drainage, etc are common soil problems in Northern Nevada that contribute to tree stress. Birches have a low tolerance for poorly draining soils.[4]

Root collar excavation and other root management work may be needed to improve tree health.

Biological Control

Woodpeckers are natural predators of the borer, and may play a small role in controlling populations if there is suitable habitat nearby.

Chemical Control

Systemic insecticides

There are several different pesticides that can be used to control Bronze Birch Borer. They can be applied by soil drench, trunk injection, or bark banding. In order to protect pollinators from harm, only a professional arborist licensed in pest control should provide treatments. Aspect Tree Care is also a Nevada Licensed Pest Control Business. We may recommend applying one of the following:

TREE-äge by trunk injection, as it will kill both the larvae and adult beetles that cause damage, and provides two years of control. [5]

IMA-jet by trunk injection, as it will kill the larvae that cause the most damage, and provides one year of control. [5]

Azasol is an OMRI Listed® organic botanical pesticide that can also be used to eliminate the larvae inside the tree. [5]

A systemic treatement applied by basal bark spray or soil drench.

It is recommended that treatments be applied before the vascular is badly damaged. Arborjet recommends treatment when your tree still appears healthy, with less than 40% dieback.[5]

Treatment may be applied during the growing season, May through September.[5] Treatment in spring will prevent adult beetles from feeding and laying eggs, and kill larvae feeding under bark.[5] Treatment in summer will also work if done earlier in the day [5]. Trunk injection in the fall can protect the tree for the following season.[5]

Closeup of a trunk injection. In this photo, we're injecting a pine tree to control bark beetles, but the same treatment is used to control Bronze Birch Borer in birch trees.