Bronze Birch Borer

The Bronze Birch Borer (Agrilus anxius) is a wood-boring beetle native to North America, known for its destructive infestations of birch trees. As the larvae feed beneath the bark, they can cause significant damage, often leading to the decline and eventual death of infested trees.

Host Plants

Bronze Birch Borers primarily infest birch trees (Betula spp.), including paper, gray, yellow, and European white birch. While they can attack healthy birch trees, they most commonly target stressed or weakened trees. Other species of trees can occasionally be infested, but this is less common.

Regions impacted

The Bronze Birch Borer is found throughout North America but is most common in the northern United States and southern Canada.

Description

Adult Bronze Birch Borers are slender, metallic beetles, typically measuring about 3/8 inch (1.25 cm) in length. They are dark, bronzy green, or black with a distinctive metallic sheen. They have elongated, slightly flattened bodies and the wing covers taper to a point at the end of the abdomen.

The larvae are legless, creamy white grubs with a distinctive, flat-headed shape and grow up to 1/2 inch long.

Life Cycle

• Egg: In late spring to early summer, female Bronze Birch Borers lay their eggs in cracks and crevices in the bark on the sunny side of the trunk of birch trees. The tiny, whitish eggs hatch in one to two weeks.
• Larvae: The larvae bore into the tree and begin to feed beneath the bark, creating meandering, serpentine galleries filled with frass (a mixture of sawdust and excrement). This feeding disrupts the flow of water and nutrients within the tree. The larvae overwinter in the tree and continue feeding in the spring. One or two years may be required to complete larval development.
• Pupa: In late spring to early summer, the mature larvae pupate within the tree. The pupal stage lasts about two weeks.
• Adult: Adults emerge from the tree in late spring to early summer, leaving behind distinctive D-shaped exit holes. They feed on birch leaves for a few days before mating and laying eggs, beginning the cycle anew.

There is typically one generation per year, although development can take two years in cooler climates.

Damage and Detection

Damage:

The Bronze Birch Borer can cause significant harm to birch trees, particularly those that are stressed or weakened. The primary damage is done by the larvae, which feed beneath the bark and disrupt the flow of water and nutrients within the tree.

The initial symptoms of an infestation can be subtle and may include sparse or yellowing foliage, particularly in the crown of the tree. Over time, as the larvae continue to feed, they create winding, serpentine galleries beneath the bark. This feeding activity can cause branches to wilt and die back, typically in the upper crown and progressing downward.

Eventually, a severe infestation can lead to the decline and death of the entire tree. This process can occur over several years, depending on the tree’s health and the infestation’s severity.

Larval galleries under peeled bark

D-shaped exit hole

Detection:

Detecting a Bronze Birch Borer infestation involves looking for both the damage caused by the borers and the presence of the borers themselves.

• D-Shaped Exit Holes: One of the most definitive signs of a Bronze Birch Borer infestation is the presence of D-shaped exit holes in the tree’s bark. These holes are created when the adult beetles emerge from the tree in late spring to early summer.
• Ridges and Trails: As the larvae feed beneath the bark, they create winding trails or galleries that can cause raised ridges on the bark. These ridges and trails are often visible on the trunk and branches of infested trees.
• Foliage Symptoms: Yellowing, sparse, or stunted foliage, particularly in the upper crown of the tree, can be an early sign of an infestation.
• Presence of Borers: The adult beetles, or the larvae may be found on or in the tree.

Early detection is crucial for effectively managing Bronze Birch Borer infestations and minimizing the damage they cause.

Prevention and Control

Managing Bronze Birch Borer effectively involves a combination of prevention strategies and, when necessary, control measures.

Prevention:

• Tree Health: The key to preventing Bronze Birch Borer infestations is to keep birch trees healthy and stress-free, as borers are particularly attracted to stressed and weakened trees. Ensure proper watering, especially during dry periods, and apply mulch to conserve soil moisture and moderate soil temperature.
• Planting Resistant Varieties: Some varieties of birch, such as river birch (Betula nigra) and Asian species, are more resistant to Bronze Birch Borer than the North American white-barked species. Choosing resistant varieties when planting can help prevent future infestations.
• Avoid Unnecessary Injury: Avoid unnecessary injuries to the tree, such as damage from mowers or trimmers, as wounds can provide an entry point for borers and other pests.

Control:

• Pruning: Infested branches or limbs should be pruned and properly disposed of during the fall or winter to eliminate overwintering larvae and pupae. Ensure to prune outside the infested area to prevent the spread of borers to other parts of the tree.
• Insecticides: Insecticides can be effective in controlling Bronze Birch Borers, particularly if applied when the adults are emerging and before they lay eggs. However, they should be used as a last resort due to their potential impacts on non-target organisms. Always follow label instructions when using insecticides.
• Tree Removal: In severe cases, it may be necessary to remove infested trees to prevent the spread of borers to nearby susceptible trees.

In conclusion, the best strategy against the Bronze Birch Borer is maintaining the overall health and vigor of the trees. Healthy trees can often resist and recover from borer infestations much more effectively than stressed or weakened trees.