Bronze birch borer

What is bronze birch borer?

Bronze birch borer is a native wood-boring beetle related to emerald ash borer. The adults are small and dark with a bronze shine. The larvae do the damage: they tunnel in winding, frass-packed galleries just under the bark, which girdle branches and eventually kill trees.

Damage usually starts at the top. The upper branches thin, the leaves turn yellowish and sparse, and the bark over the tunnels gets lumpy. Over time the whole tree can die. Purdue notes birch is a fairly short-lived tree and that lawn conditions, unlike its natural habitat, often weaken it.

European white and Himalayan birch are the borer's favorites. Darker-barked birches such as river birch, including the lighter-barked 'Heritage', resist it. UMass and Purdue both stress keeping birches vigorous and choosing resistant species.

Signs and damage

• Dying upper branches and thinning at the top of the crown (Purdue describes symptoms on white, paper and yellow birch)
• Yellowish, sparse leaves that look like drought stress
• Ridges and bumps on the bark of branches and trunk over the tunnels
• D-shaped exit holes, mostly on upper branches and the trunk
• Bleeding spots on the trunk and sprouts growing from the trunk

Bronze birch borer identification: what to look for

• Adult: Black beetle about 1/2 inch long with a bronze shimmer on the back. Likes sun and is often seen on the sunny side of trunks in late May and early June.
• Larva: Cream-colored, flat, slender, with a head slightly wider than the body and a pair of small brown pincers at the tail.
• Damage: Winding (S-shaped) tunnels packed with sawdust-like frass under the bark, bark ridges and bumps, D-shaped exit holes, top-down dieback, trunk sprouts.
• Timing: UMass says adults emerge in about early to mid-June; Purdue says they fly when black locust blooms, and the University of Kentucky reports emergence starting in mid-May in Kentucky.

Life cycle

Larvae spend the winter in the wood, mostly partly grown. Purdue says larvae pupate in the wood in late April or early May. Adults come out around the time black locust blooms (Purdue), about early to mid-June (UMass), leaving D-shaped holes. Females lay eggs in bark cracks; eggs hatch in two weeks or less, and larvae bore in to feed between the wood and bark. The cycle takes one to two years.

When to look: Adults are out from late May through June (Purdue sees them on sunny trunks in late May and early June). Crown thinning and top dieback show in summer.

Bronze birch borer look-alikes

A few things get mixed up with it:

• Emerald ash borer (Agrilus planipennis): A close relative with the same D-shaped exit holes, but it attacks ash, not birch. Penn State lists bronze birch borer as an EAB look-alike.
• Two-lined chestnut borer (Agrilus bilineatus): Another Agrilus borer with D-shaped holes, but it attacks oaks.

How to control bronze birch borer

Cultural control

• Plant resistant birches such as river birch (including 'Heritage'), monarch birch (UMass) or Japanese birch (Purdue) instead of European white or Himalayan birch.
• Water birches in lawns every 7 to 10 days when rainfall is under about an inch to an inch and a half a week, letting water soak the root zone (Purdue).
• Reduce other stresses; control aphids, scales and leafminers that weaken the tree.
• Avoid pruning birch until late summer, around mid-September (UMass).
• Remove dying trees and branches before adults emerge in early May.

Biological control

• Woodpeckers and parasitic wasps are important natural enemies in forests but usually don't give enough control on stressed landscape trees (UMass).

Chemical control

Purdue lists imidacloprid (alone or with clothianidin) as a homeowner soil drench from April 1 to May 15, dinotefuran as a professional trunk or soil treatment from May 1 to June 15, and permethrin sprays in mid-May to mid-June aimed at emerging adults, with bifenthrin listed among commercial sprays. UMass also lists emamectin benzoate as a trunk injection. The University of Kentucky says injection may be the only practical option for tall trees. Apply after bloom and when pollinators are least active. Use only products labeled for this use; the label is the law. Use an ISA Certified Arborist or licensed applicator for trees. Muthler Landscaping does not treat trees or apply pesticides; see our pesticide applicator license guide.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Choose river birch or another resistant species for new plantings.
• Keep birches watered in dry spells.
• Avoid wounding birch bark with mowers or trimmers; the University of Kentucky says fresh trunk wounds attract egg-laying borers.

When to call a pro

Call an ISA Certified Arborist if a birch has top dieback, bumpy bark or D-shaped holes, or if dying branches need to come out before adults emerge in May.

Related guides: Tree database: mature size and planting distance · Pennsylvania tree identification · Firewood rules by state · All pests and diseases

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