Bronze Birch Borer

Problem type: Insect

Name of problem: Bronze Birch Borer

Plant name(s): Yellow, paper and grey birch; All birch species can be attacked.

Symptoms / Characteristics:
Small ridges and bumps on branches and trunk indicate larval feeding under the bark of the tree. Adults emerging from under the bark on the trunk and branches chew D-shaped emergence holes confirming the presence of the borers. Foliage in the crown of the tree will begin to yellow and wilt by mid-summer, then turning brown and dying, resulting in the death of some branches. Gradually larger branches will die back and eventually the whole tree may be killed.

Bronze birch borers normally require 2 years to complete their life cycle. Partially grown larvae spend the winter in a gallery just under the bark of the host tree and mature in the spring. Adults emerge around late June through August chewing characteristic D-shaped emergence holes in the trunk and branches. Adults are slender beetles measuring 6-11 mm long. The beetles are olive green to black with copper reflections. Female beetles are strong fliers and search for a suitable host upon emergence. Eggs are laid near the top of birch trees in bark cracks and crevices. Eggs hatch about 2 weeks later and the young larvae burrow directly into the bark and begin feeding in the cambium of the tree. Larvae are creamy white legless grubs measuring up to an inch long when mature and they remain under the bark until the following spring when they pupate and emerge as adults.

As larvae feed in the cambium layer, the food transporting tissue is damaged, reducing the transport of food from the canopy to the roots. As roots are affected, less water is absorbed and the canopy starts showing signs of wilting and die back.

Insects generally attack trees that are already weakened or stressed by other factors such as drought or other insect infestations. Healthy trees may be attacked but they usually have enough vigor to kill the larvae.

Control / Preventions:
As with most boring insects, the bronze birch borer is very difficult to control once an infestation is established. The best way to prevent attack is to plant birch trees in a suitable site and keep them healthy. Birches prefer partial shade and moist soil. Sufficient moisture is probably the most important factor in maintaining a healthy birch tree and is probably the one thing that is most often neglected.

The use of mulches will help conserve soil moisture as well as regulating soil temperatures. Fertilizers need only be used when nutrients are inadequate to keep trees healthy. Prune out and destroy all dead and dying branches but try to avoid pruning during adult emergence in July and August as female borers are attracted to fresh pruning wounds.

Chemical insecticides can provide some control, but complete eradication is not possible. Protectant surface sprays can be applied on a preventative basis to the trunk and branches of the tree. Thorough coverage of the bark is necessary to insure insecticide reaches under loose bark and bark crevices. Sprays should be applied before eggs are laid so the insecticide is present to kill the larvae hatching from eggs. Repeat applications at 2 week intervals is necessary until the end of August as adult may still be emerging and laying eggs.

Systemic insecticides can also be applied to control larvae already feeding in the tree. A professional applicator is required for most if not all pesticide applications.

Cultural controls are the most important factors in reducing bronze birch borer attacks, and are needed if chemicals are going to be used as chemicals will not be effective without them.