Bronze birch borer—Agrilus anxius
This flatheaded borer (family Buprestidae) infests only birch. It can cause dieback of limbs and death of entire trees.
Identification
Adult bronze birch borers are oblong, dark metallic, coppery beetles about 3/8 inch long. Their feeding causes inconspicuous chewing on leaf edges of alder, birch, and poplar. D-shaped, smooth-edged holes occur in bark where adults emerged after feeding and boring inside during their larval stage. Larvae occur hidden under bark. They are legless and pale with distinct segments and an enlarged head and grow up to 1/2 inch long.
The many similar Agrilus species include bronze poplar borer (A. liragus), which infests Populus species (aspen, cottonwood, and poplar). Emerald ash borer (A. planipennis), which occurs in the eastern and midwestern United States, has killed millions of ash trees. For information on other Agrilus species see Guide to Insect Borers in North American Broadleaf Trees and Shrubs from the USDA Forest Service. Birch can also be infested with other boring insects. For information on these see Pest Notes: Bark Beetles and Pest Notes: Clearwing Moths.
Life cycle
Flatheaded borers develop through four life stages: egg, larva, pupa, and adult. Adult females lay eggs in crevices and wounds of birch bark on sunny portions of limbs and trunks. The hatching larvae chew through bark and into phloem and xylem tissue, packing frass (excrement) in their tunnels as they feed and grow. Mature larvae tunnel in the outer sapwood, where they overwinter. In spring, larvae chew an oblong chamber beneath bark and pupate. Adults emerge in Northern California beginning in April, then mate and reproduce. There is one generation per year.
Damage
Foliage of borer-infested trees turns pale green, yellow, and then brown. Leaves drop prematurely and scattered limbs die. Swollen ridges develop on branches and the trunk where larvae tunnel beneath bark. Liquid oozes from bark, creating stained blotches. Severely infested trees die prematurely.
Solutions
To prevent infestations, provide trees with a good growing environment and proper cultural care. Birch require more water than most other trees, so good soil conditions, healthy roots, and appropriate irrigation are especially critical. Birch typically have an especially short life span in California in hot, dry interior areas; avoid planting birch where summers are dry and hot.
To minimize problems with bronze birch borer
- Do not plant birch where they will be exposed to heat stress, such as the south and west sides of buildings.
- Plant at the most suitable location, such as the east side of buildings where heat stress is typically less severe.
- Avoid planting white-barked birch species (e.g., Betula jacquemontii, B. pendula), which are especially susceptible to bronze birch borer.
- If planting birch, consider using Betula alleghaniensis, B. lenta, or B. nigra. Borers attack all birch, but these species appear to be less susceptible in California.
- Apply mulch to retain soil moisture and keep roots cooler.
- Provide sufficient irrigation, such as deep watering about every 1 to 2 weeks during prolonged dry weather.
- Do not fertilize birch unless nutrient deficiency has been confidently diagnosed as the cause of poor growth. Fertilization can increase birch’s demand for water, thereby increasing tree stress.
- Promptly remove dead and dying birch. Chip the wood or cut it into logs and solarize it under clear plastic to prevent adult borers from emerging and infesting nearby birch.
Regularly monitor valued birches from mid-April through summer, looking for bark swelling and stains. Prune off and dispose of any infested branches before mid-September. Except for removing borer-infested and hazardous limbs whenever they appear, prune birch during late fall through March when adult bronze birch borers are not flying.
If the critical cultural and environmental practices summarized above have been followed and bronze birch borer is a local problem, spraying birch thoroughly about twice (once during April and again about late May) with a pyrethroid insecticide (e.g., bifenthrin, permethrin) can kill adults and larvae hatching from eggs before larvae bore into wood. A professional applicator must be hired to apply an effective product using equipment that provides good spray coverage. To increase the likelihood of a satisfactory experience, see Pest Notes: Hiring a Pest Control Company.
Application of a systemic insecticide (e.g., dinotefuran) may provide some control if trees are relatively healthy and contain relatively few or no borers. Treatment can be warranted where nearby birch are infested and a source of borers. To minimize the potential adverse effect on pollinating insects, make any application of systemic insecticide soon after trees have completed flowering. See Homeowner Guide to Managing Bronze Birch Borer in the Upper Klamath Basin (PDF) for more information on this pest and how to protect pollinators when managing it with insecticides.
Severely infested trees will die regardless of treatment. Promptly remove birch that are highly infested, dying, or dead. Chip the wood or cut and solarize it under plastic to prevent borers from emerging to attack other birch. Do not rely on insecticide application alone or trees are likely to die from the cultural and environmental problems that caused them to become susceptible to attack by borers.
Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
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Adult bronze birch borer.
Emergence hole (right) of adult bronze birch borer.
Swollen wood reaction to tunneling by larvae.
Larva of flatheaded borer exposed under bark.
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