The bronze birch borer (Agrilus anxius) is an insect that poses a significant threat to birch trees across prairie provinces. This wood-boring beetle is notorious for infesting and killing various species of birch, particularly those already weakened by environmental stress. If left unchecked, the birch borer can lead to the widespread decline of birch populations in both urban and natural landscapes. The beetle prefers to attack stressed or weakened birch trees, as healthy birch trees have strong natural defenses that make it difficult for the larvae to establish. Trees under drought stress, suffering from root damage, or aging are especially vulnerable to infestation. The most commonly affected species include paper birch (Betula papyrifera), European white birch (Betula pendula), and river birch (Betula nigra).
Pest ID
The bronze birch borer is named for its distinctive, iridescent bronze-colored body. Adult beetles are approximately 7-15 mm in length with a narrow, elongated shape. The larvae, which cause the actual damage, are creamy-white, legless grubs with a flattened head and an overall size that can reach up to 30 mm.
The insect typically has a one-to-two-year life cycle, depending on environmental conditions. Female beetles lay eggs in the cracks and crevices of the birch bark during late June and early August. Each female can lay up to 100 eggs. After hatching, the larvae burrow beneath the bark and feed on the cambium layer of the tree. Their feeding disrupts the tree’s ability to transport nutrients and water, eventually leading to dieback and death. The larvae overwinter in their galleries and continue feeding the following spring. Mature larvae pupate within the tree, transitioning into adult beetles in late spring. The new generation of adult beetles emerges from D-shaped exit holes in the bark, typically between May and July, and begins the cycle again.
Signs and Symptoms
Early detection of birch borer infestation is critical in preventing tree mortality. Some of the most common signs include:
- Presence of D-shaped exit holes on trunk and branches are one of the most distinctive signs of birch borer activity
- Upper canopy dieback often starts in the upper branches, leading to progressive dieback as larvae continue to damage vascular tissues.
- S-shaped serpentine galleries under bark of an infested tree created by feeding larvae.
- Affected branches exhibit yellowing and sparse foliage due to restricted nutrient flow.
- Splitting or loose bark is a result of the internal damage caused by the larvae.
Management and Control
Birch borers are highly destructive and can kill a tree within a few years of infestation. The tree’s vascular system is compromised as the larvae feed, leading to symptoms of drought stress and nutrient deficiency. Young trees or those in poor health succumb faster than well-maintained, mature trees. If left untreated, birch borers can contribute to widespread tree loss, impacting both ecosystems and urban landscapes where birch trees are commonly used for ornamental purposes. Managing birch borer infestations requires a combination of preventive and remedial strategies.
- Tree selection – Choose resistant species like river birch (Betula nigra), which shows greater resistance to bronze birch borers than other birch species.
- Proper watering – Maintain consistent soil moisture to prevent drought stress, which weakens trees and makes them more susceptible to infestation.
- Fertilizing – only when is required
- Mulching – Apply a layer of organic mulch around the base of the tree to help retain moisture and regulate soil temperature.
- Pruning and sanitation – Remove and destroy infested branches to limit the spread of the beetle population. Prune during the dormant season to avoid attracting more beetles.
- Use of preventive insecticides containing imidacloprid, dinotefuran, or permethrin can be applied in early spring before adult beetles emerge.
- Encourage natural predators such as woodpeckers, parasitic wasps, and certain predatory beetles feed on birch borer larvae, helping to keep populations in check.
