August 29, 2003
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Common Name(s): Bronze Birch Borer Scientific Name: Agrilus anxius Gory, Coleoptera: Family Buprestidae. Identifying Characteristics for Damaging Stage(s): The damaging stage is the larva, a slender, flattened, ½ inch long, grublike stage with a slight swelling behind the head and two short spines at the tip of the abdomen. Adults are beetles, each about 3/8 inch long, slender, and metallic bronze with a greenish tinge. Damage/Nature and Symptoms: Infested birch trees exhibit rough, swollen areas in the bark of branches and trunks, as well as scattered, small half moon shaped holes in the bark. Once the bark is peeled back, several very long and rather extensive, wandering galleries can be observed in the outer cambium layer and sapwood. Galleries, packed tightly with sawdust, are made by tunneling larvae and extend up, down and around the trunk, resulting in a girdled tree. Early signs of damage include wilting in the top of the tree, with eventual death and drying of infested branches.
Distribution and Life Cycle Distribution: Widespread Summary of Life Cycle: The bronze birch borer passes the winter as a mature larva in a cell made beneath the outer bark. In the spring these worms enter the pupa stage, emerging as adult beetles through D-shaped emergence holes in bark, beginning in late May, peaking in mid-June and extending through July. Each beetle lives about three weeks. Once mated, females lay eggs in bark crevices or niches made by chewing into the bark surface. Newly emerged larvae invade the bark and bore beneath, extending individual galleries by as much as 3 feet during their development. The galleries wander around, crisscrossing the cambium layer, effectively girdling the tree. By September and October, larvae are mature. There is usually a single generation each year, but the cycle further north may take two years to complete. Management Methods: Inspection/Survey Methods: Periodically examine birch trees for evidence of wilting, damage to bark and D-shaped emergence holes. Non-Chemical Management Strategies: Since native stands of birch are extremely rare in Nebraska, a prairie state, we might conclude that conditions here are marginal for establishment. Therefore, if birch trees are to survive, they must be carefully managed. A first recommendation for homeowners is to plant other, more adapted tree species that are less prone to insect attack, such as oaks, lindens, maples, crab apples, and conifers. If birches are to be planted, however, several cultural practices can reduce the chance of borer infestation. Healthy trees that are maintained in vigorous growing condition are not as attractive to borers. Trees must be properly watered, fertilized, and protected from pests, particularly during the first two or three years after planting and during drought periods that cause extreme stress. Birches should be planted where they are shaded in the afternoon, avoiding southern and western exposures. These trees are best suited for partially shaded, protected, and more moist situations. They must be watered regularly. Wrapping of young tree trunks will also help to prevent borer infestation. Planting a ground cover under the canopy or mulching should help keep roots cool and moist. The river birch appears to be better adapted to Nebraska home landscapes. Trimming damaged branches and eliminating weak trees is important. Infested limbs, branches and trunks should be trimmed in the fall and burned, chipped and composted to reduce emerging borer populations the following spring. Chemical Management Strategies: Borer damage must be prevented because once they gain access to cambium, sapwood, and heartwood, little can be done to control them. Chemical treatments should be applied when Spirea (Van Houttei) blossoms turn brown and again in about three weeks. Chemical barrier treatments are effective only if applied as residual sprays prior to egg-laying. If residual treatments are in place, young larvae are killed while attempting to invade the wood. Proper timing is essential. The initial application of a residual barrier treatment for bronze birch borer must be in place on the trunk and branches of birches no later than mid-June in Nebraska. Treatment should be repeated in 12 - 14 days. For the latest information on available chemical controls, consult the Pesticide Selection Guide. References: Baker, W. L., 1972. Eastern Forest Insects. USDA Forest Service Misc. Pub. No. 1175, US Forest Service, 642 pp. Metcalf, C. L., W. P. Flint and R. L. Metcalf, 1962. Destructive and Useful Insects. 4th Ed.: McGraw-Hill Book Co., New York NY. 1087 p. White, Stephan C. and Glenn A. Salsbury, 2000. Insects in Kansas. Kansas Department of Agriculture, Topeka KS. 521 pp.
