A2692 Birch disorder: Bronze birch borer R.C. WILLIAMSON and P.J. PELLITTERI Life cycle 3 Bronze birch borer (Agrilus beetles. They are ⁄8 inch long with anxius Gory) is a serious pest of birch Bronze birch borers belong to a blunt heads and pointed bodies. trees in the Midwest. It can severely group of beetles called the metallic Adult females live for 3 weeks and injure or kill most birch species; wood borers because of the adult deposit eggs under loose bark or in European white birch varieties are beetles’ iridescent color. They spend cracks or crevices on the trunk. particularly susceptible. the winter as larvae in small cells just Eggs hatch within 2 weeks, and Like most wood-boring beetles, beneath the bark. Full-grown larvae larvae bore into the tree and feed on the bronze birch borer is really a sec- are 1⁄2–1 inch long, white, and slender the inner and outer wood layers. ondary problem. It usually attacks with dark brown jaws. They have a Larvae usually reach maturity by late trees under stress or in a weakened slightly flattened, enlarged area fall. However, predicting wood condition because of drought, directly behind the head, and two borers’ exact stage of growth at any disease, nutrient deficiency, construc- brownish projections on the last given time of year is not possible; tion injury, or excessive exposure to segment of the body. factors like tree vigor and weather the sun. Planting birch in a poor loca- Larvae pupate in spring and conditions make it possible to find all tion is the most common reason for adults emerge during May, June, and larval stages during winter. problems with bronze birch borer in July. The adults chew their way Wisconsin. Vigorous healthy trees, through the bark, leaving character- planted in favorable surroundings istic 1⁄8-inch, D-shaped exit holes, are less attractive to the borer and which are useful in identifying birch more likely to survive an attack. borer problems. Adults are slender, Symptoms greenish- to bluish-black metallic The usual first sign of attack is a thinning or dieback of branches in the upper third of the tree. Leaves on infested branches may be unusually Adult bronze birch borer beetle. small or may show browning along the edges in May or June. Larvae cause the damage by feeding under the bark and girdling the branches. Girdling prevents movement of food and water to tissues above the attack site. Extensive feeding may kill the branch by July or August. Over time, the attacks progress downward to lower branches and the trunk. Typically, borers kill trees in 3–4 years. Trees planted in very poor sites or planted in borer-infested areas may die in 1 year. White birch severely weak- Typical D-shaped exit holes. ened by bronze birch borer infestation. BIRCH DISORDER: BRONZE BIRCH BORER Control Bronze birch borers are often an kill larvae already tunneling beneath Cultural ongoing problem. Borer damage the bark, but they prevent succeeding Among the white birches, native means environmental stresses or other generations from infesting the tree. stock appears less susceptible than factors have made trees susceptible to During this period, fertilize and water many introduced cultivars. But native attack in the past. Therefore, future trees as needed. trees often have less desirable attacks are possible. Imidacloprid (Merit) is a systemic growing characteristics. Betula platy- If more than a third of the tree has insecticide that can be applied as a phylla var. japonica ‘Whitespire’ is the died back, saving the tree will be diffi- soil injection or as a soil drench near most borer-resistant, white-barked cult. Commercial arborists have the base of the trunk. Treatment birch in Wisconsin. The Heritage river rescued some infested trees using a timing depends on the size of the tree. birch (Betula nigra ‘Heritage’) has a combination of tactics. Consultation For tree trunks larger than 4 inches in pale salmon-colored bark and has with an arborist may be helpful. diameter at chest height, treatments been relatively free from borer attacks. To prevent the breeding and must be made in the fall (late Selecting the proper site is very spread of adult bronze birch borers, September until early November). important. Avoid exposed terraces remove or destroy dead and dying Smaller trees can be treated in the and locations subject to severe soil trees. Use all birch firewood before spring. Treat only once per year. the next spring. Injecting chemicals such as Bidrin compaction or drought conditions. Chemical Planting a white birch by itself in a directly into trees is not recom- sunny, southerly exposed lawn will Lightly infested trees often return mended because the process creates surely lead to water or temperature to full health with a regular preven- new wounds and offers limited stress problems and ultimately to tive insecticide program. Two insecti- control (less than 50%). borer attack. Planting trees in semi- cides are available to homeowners Heavy infestations of other shaded, moist sites leads to fewer and professional applicators: perme- insects, particularly birch leafminer, problems. thrin and imidacloprid. reduce tree vigor and increase chances Try to keep trees healthy. Because Permethrin acts as a chemical of successful borer attack. Extension birches have a shallow root system, barrier that kills young larvae as they publication Birch Leafminer (A2117) water them during droughty periods try to tunnel into the bark. It must be explains how to manage this problem. or if they are growing on light soil. It in place before the adults lay eggs. References to products in this publication also helps to keep the soil around the Because eggs are laid throughout the are for your convenience and are not an birch cool by planting low-growing late spring and summer, spray three endorsement of one product over other similar products. You are responsible for bushes or shrubs. Fertilizer helps trees times—about May 20, June 15, and using chemicals according to the manu- withstand light infestations and fight July 15. Infested branches and trunk facturer’s current label directions. Follow off borer attacks. must be thoroughly sprayed. You may directions exactly to protect the environ- need a commercial arborist to help ment and people from chemical exposure. treat large trees. These sprays do not Copyright © 2003 University of Wisconsin System Board of Regents doing business as the division of Cooperative Extension of the University of Wisconsin-Extension. Send inquiries about copyright permission to: Manager, Cooperative Extension Publishing, 432 N. Lake St., Rm. 103, Madison, WI 53706. Authors: R.C. Williamson is assistant professor of entomology and P.J. Pellitteri is distinguished outreach specialist of ento- mology, College of Agricultural and Life Sciences, University of Wisconsin-Madison and University of Wisconsin-Extension, Cooperative Extension. Produced by Cooperative Extension Publications, University of Wisconsin-Extension. University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture and Wisconsin counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and provides equal opportunities in employment and programming. 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