Bronze Birch Borer
Birch is a popular landscape tree planted across the United States. The variations in bark texture and color, as well as foliage characteristics of the different species and cultivars, make them highly attractive landscape specimens. Unfortunately, birch are often stressed in the urban environment and become susceptible to a devastating insect, the bronze birch borer (Agrilus anxius). Fortunately, there are ways to reduce problems with this pest through proper tree and site selection, cultural practices and direct control of the bronze birch borer.
Adult borers are slender, metallic-coppery beetles about 3/8 inch (10 mm) long and are rarely seen. They are in the family Buprestidae and are often termed “metallic wood boring beetles." Larvae occur underneath the bark and are white, segmented, legless grubs with an enlarged area behind the head (giving them the name “flatheaded borers”). They are about 1/2 inch (10-15 mm) long when mature. All birch species can be attacked; however, some birch are more susceptible to bronze birch borer.
The larvae of the bronze birch borer feed in the phloem tissue [photosynthate (food) transporting tissue just beneath the bark] of the trunk and larger branches. This feeding girdles the phloem, reducing the amount of photosynthate (food) transported from the canopy to the roots. Eventually, this root starvation leads to decreased root growth and function. Unable to take up adequate amounts of water, the tree begins to die back, often starting in the upper crown.
- Foliage on some branches in the upper crown begins to yellow in midsummer, progressing to brown/dead leaves. This results in the death of smaller branches in the upper crown. Over time, large branches begin to die back and eventually the entire tree may die. However, brown leaves may not always be the result of bronze birch borer attacks. Another insect, the birch leafminer, also causes leaves to turn brown.
- •D”-shaped exit holes where adult borers have emerged from the tree begin to appear on the trunk and larger branches (see Figure 1).
- •Ridges begin to appear on the bark of the trunk and larger branches in response to larval feeding activity beneath the bark in the phloem tissue.
Management and Control
Reducing problems with bronze birch borer starts with understanding your birch tree’s requirements. The following steps will help promote healthy, vigorous birch trees that are less susceptible to borer attacks than unhealthy trees. Bronze birch borers are not capable of successfully attacking healthy trees.
1. Select an appropriate location, Species and followed up with good cultural practices, watering mulching, fertilizing, and pruning for your birch tree when planting.
Bronze birch borer overwinters in feeding galleries as a mature larva, pupating in early spring. Adult beetles begin to emerge from late May to early June in most northern climates. Emergence will typically last into July with peak emergence around mid-late June. Adult beetles feed on leaves and mate shortly after emergence. Female beetles then lay eggs in bark cracks and crevices, other protected locations, or in small niches they chew in the bark. Tiny larvae emerge in about a week and chew through the bark and enter the phloem tissue. Larvae make zigzag patterns in the phloem tissue as they feed through the growing season, permanently damaging the phloem tissue. Mature larvae then overwinter before emerging as adults the following spring.
If your tree becomes vulnerable to borers, chemical control may be helpful. Chemical applications should not be made on a preventive basis, but only if symptoms of borer activity appear. Insecticides can be applied to the bark and foliage of trees to prevent new attacks. Thorough coverage of the trunk and larger branches is required (this is often best done by a professional applicator). This treatment will not kill borers already present in the tree, but will prevent newly hatched larvae from entering the tree. Three applications should be applied at approximately two-week intervals (late May, mid-June, late June). Remember, good cultural practices should be tried first and are most important for reducing birch borer problems and the insecticide applications.
Existing borer infestations can be controlled by injecting an insecticide (Bidrin) into the tree. Bidrin is injected around mid-July after larvae have entered the tree. Bidrin is a restricted-use pesticide and can only be applied by a professional applicator. It is not recommended as a preventive measure because repeated injections will damage the tree’s vascular system. Also, this method of treatment should not be considered for trees that are heavily infested and near death.
Chemicals Commonly used to control Bronze Birch Borer: Dycarb, Turcam Dursban, Pageant, Bidrin, Astro