Latin Name - Agrilus anxius

Family - Buprestidae

Order - Coleoptera

Bronze birch borer is a serious pest of birches in the prairie provinces.

Life cycle

Bronze birch borer is a member of the Buprestidae family, which are distinguished by a “bullet” shape and a flat head, which leaves characteristic D-shaped exit holes. Though the beetles are native to Canada, their natural range overlaps with areas with a natural birch presence. Adult beetles lay eggs in clusters on bark crevices which hatch in roughly two weeks. New larvae move into the inner bark and feed in criss-crossing galleries, interrupting the vascular function of the wood. Later in the season, the larvae bore into the sapwood to overwinter.

Larvae spend time in the stems and branches and go through several larval stages. Once the larvae mature, they pupate in the bark before chewing through the wood and exiting the tree as adults, leaving behind a D-shaped exit hole. In warmer climates, the life cycle can take one year, but in colder climates it make take up to two.

Main Hosts

Bronze birch borer may attack any species of birch, but the susceptibility varies by species and cultivar:

High Risk:

• European White Birch; Silver Birch (Betula pendula)

• Jacquemonti Birch (Betula jacquemontii)

Moderate Risk:

• Paper Birch (Betula papyrifera)

• Yellow Birch (Betula alleghaniesis)

• Sweet Birch (Betula lenta)

• Gray Birch (Betula populifolia)

• Japanese White Birch (Betula platyphylla)

Low Risk:

• River Birch (Betula nigra)

The most commonly attacked birch species in Lethbridge is the European white or silver birch, but paper birch often show symptoms as well. River birch are less frequently planted in Lethbridge, but tend to show much greater resistance.

Signs and Symptoms

The presence of bronze birch borer can be confirmed by looking for:

1. Flagging in the branch tips

2. Thinning crown

3. Branch dieback

4. Galleries in the inner bark

5. Swelling on the wood

6. D-shaped exit holes

7. Increased presence of woodpeckers/woodpecker feeding

8. Visible sap flow (in the absence of pruning/mechanical damage)

Unfortunately, most of the untreated mature birches in the City of Lethbridge may be presumed to have borer, or at least be at high risk.

Stages of infection may be classified into one of five stages of infestation. Stage one consists of a healthy, uninfected tree. Stages two through three show mild to moderate flagging in the branch tips and suggest a mild infestation. Trees in these stages are candidates for the application of a systemic insecticide and pruning.

Stages four and five are characterized by severe crown loss and borer presence in the trunk and large branches. These trees are beyond recovery and should be removed.

Mitigation and Treatment

Bronze birch borer prefers to attack isolated trees, rather than trees grown in groups. As many birches are planted as solitary lawn trees, these trees are attractive to the beetles. Further, bronze birch borer typically kills trees that are already in a weakened state. Because birch have shallow roots that are sensitive to heat and drought, birches planted in Southern Alberta are often already stressed. Mortality is generally caused by a combination of several factors, so taking steps to plant birches in sheltered locations with enough water can help mitigate borer damage. Mulch should be placed around the drip-zone of the tree, but not piled against the trunk.

Pruning of infested branches can prolong the life of mildly infested trees, but is not a long-term solution. Pruning should be avoided from May to June as the beetles are active during this time and are attracted to the scent of freshly wounded trees. Further, no more than 25% of live growth should be removed to limit the stress experienced by the trees.

Alternatively, trunk injections of systemic pesticides are available. Systemic trunk injections should be used while the tree is still healthy as the vascular system must still be functional in order to move the insecticide throughout the tree. There are several manufacturers of pesticide, but they may only be administered by a certified pesticide applicator. Tips Trees does not perform any pesticide application but is happy to connect you with reputable providers.