Bronze birch borer
Click for larger image Adult bronze birch borer (Coleoptera) and D-shaped emergence hole on paper birch (Betula spp.). S. Katovich, USFS, Bugwood.org

The bronze birch borer, Agrilus anxius, is a beetle that is native to North America. It can be a serious pest of forest and shade trees, particularly several species of birch. The varieties most commonly attacked are the paper birch Betula papyrifera, the European white birch B. pendula, and the gray birch B. populifolia. Injury is due to larval feeding tunnels under the bark which girdle the trunk or branch of the tree. Generally, the most susceptible trees are those weakened by disease, age, defoliation, adverse weather conditions, or previous borer infestations. Healthy, young trees are rarely attacked.

Symptoms and Diagnosis

Early symptoms of borers include chlorotic ("yellowing") leaves, sparse foliage, and browning tips of the upper branches. Infestations and dieback usually start at the top of the tree and work downward. Infested branches often are somewhat swollen and brown, with ridges appearing around the smaller ones. Branches die because of the girdling effect of the larval feeding tunnels. These winding, irregular tunnels ("galleries") under the bark interrupt the flow of water and nutrients through the damaged area. They are also packed with sawdust left over from feeding. Slender white grub-like larvae about 3/4 inch long may be found in the tunnels. External signs of these hidden feeding tunnels are conspicuous scarred areas or zigzag ridges caused by the healing process in infested trees. A definite sign of adult borers is the distinctive D-shaped escape holes (1/8 inch diameter) that they chew through the bark as they emerge in late spring.

Life Cycle

In early spring, when tree sap begins to flow, overwintering larvae begin to feed and migrate close to the surface of the bark. There they pupate, mature into adults, and emerge through the bark (making the distinctive D-shaped holes). Adult emergence in Missouri is typically in late spring and lasts for six weeks. Adults are a metallic green-bronze color, about 1/2 inch long, and have a blunt head. After emergence they feed on leaves (with no significant damage), mate, and lay eggs under bark flaps or in cracks in the trunk or branches. Newly hatched larvae then bore into the bark and begin feeding.

The light colored larvae are long and flattened. They appear to have a wide head but really have an enlarged segment just behind a tiny head. Two dark spines on the very end of the last body segment are another distinguishing feature. Larvae feed and make galleries under the bark until winter and generally require two years to complete a life cycle.

Integrated Pest Management Strategies

1. Maintain plant vigor. Proper care and maintenance of birch trees decreases the likelihood of infestation. Sickly or weakened trees are more likely to be attacked by borers so keep trees growing vigorously with adequate water and fertilizer. In healthy trees the sap flow can act as a defense and drown borers as the sap fills holes bored by the larvae.

2. Prune infested branches. Prune branches known to be infested with larvae in early spring (best if done before April 1). Prunings should be properly disposed of well away from the infested tree.

3. Apply insecticides. Pesticides registered for use include: acephate (Orthene), azadirachtin (Bio Neem, Margosan-O), bendiocarb (Turcam, Closure), chlorphyrifos, imidacloprid (Merit, Marathon), and permethrin. Apply to the shoots and bark to kill adults and newly hatched larvae before they bore into the bark. Adults do not feed significantly on the leaves so it is not necessary to treat the foliage. Timing of insecticide applications is important because once the larvae bore into the bark they are out of reach of insecticides. First insecticide application should be when adults appear (look for the D-shaped holes) in early to mid-May. Frequently it is time to apply insecticides when bridal wreath (Spiraea x vanhouttei) finishes blooming. Some insecticides will need repeat applications so follow the label recommendations.

4. Plant resistant varieties. Replace borer-susceptible birches with river birch, B. nigra, the most borerresistant birch. 'Heritage' is a light bark cultivar that will do well in hot climates.

Organic Strategies

Strategies 1, 2, and 4 are strictly organic approaches.

More images:

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Adult bronze birch borer (Coleoptera) and D-shaped emergence holes on birch (Betula spp.). S. Katovich, USFS, Bugwood.org
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Bronze birch borer adult found on birch (Betula spp.). PA DCNR-Forestry Archive, Bugwood.org
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Infestations of bronze birch borers (Coleoptera) cause dieback, usually starting at the top of the tree and work downward; the European white birch (Betula pendula) is a favorite host.
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Dieback, as shown on this European white birch (Betula pendula) is a common symptom of damage from the bronze birch borer (Coleoptera)