Two-lined chestnut borer

What is two-lined chestnut borer?

Two-lined chestnut borer is a native jewel beetle in the same genus (Agrilus) as emerald ash borer. NC State calls it a secondary pest that mainly attacks oaks already stressed by other problems, and ties it to oak decline. Its larvae tunnel and feed under the bark. UW-Madison lists drought, defoliation by caterpillars such as spongy moth, root diseases, construction or storm injury, girdling roots, soil compaction and road salt as stresses that set oaks up for attack.

Dieback usually starts at the tips of branches in the sunny upper crown and moves down. Leaves can wilt quickly, turn brown and stay attached. Decline often takes several years, and the symptoms can look like other problems, so a branch sample is needed to confirm the borer.

For contractors and homeowners planning work near big oaks, this pest is a reminder that root damage, soil compaction and grade or drainage changes can lead to tree loss years later. Protecting the root zone during construction is part of protecting the tree.

Signs and damage

• Dieback starting at branch tips in the sunny upper crown and working down
• Leaves that wilt suddenly in summer, turn brown and stay on the branch
• Sparse, small or off-color leaves
• D-shaped exit holes about 1/8 inch wide in the bark
• Winding tunnels packed with pale frass under peeled bark

Two-lined chestnut borer identification: what to look for

• Adult: Slender beetle about 0.2 to 0.5 inch long, metallic black with a bluish sheen and two pale golden stripes on the wing covers (often faint).
• Larva: Cream-colored, segmented, legless, up to about 1 inch long, with a large flattened head and a pair of spines at the tail.
• Damage: Tip-down crown dieback, sudden browning of leaves that stay attached, D-shaped exit holes about 1/8 inch, frass-packed tunnels under bark.
• Timing: Adults emerge in late May and are active until early July.

Life cycle

One generation a year. Larvae and pupae spend the winter inside infested trees. Adults emerge in late May and are active until early July. Eggs are laid in bark crevices and hatch in one to two weeks. The larvae feed under the bark, cutting off the flow of water and nutrients.

When to look: Adults fly late May to early July. Crown dieback and brown, hanging leaves are most visible in summer.

Two-lined chestnut borer look-alikes

A few things get mixed up with it:

• Emerald ash borer (Agrilus planipennis): A close relative that also leaves D-shaped exit holes, but NC State says it attacks only ash and white fringetree. Check the tree species first.
• Bronze birch borer (Agrilus anxius): Another Agrilus borer with D-shaped exit holes, but it attacks birch, not oak.

How to control two-lined chestnut borer

Cultural control

• Water oaks about one inch per week during dry spells and mulch properly.
• Protect the root zone: avoid soil compaction, root injury and grade or drainage changes near oaks during construction.
• Avoid wounding trunks and roots with landscaping equipment, and plant oak species suited to the site (NC State).
• Prune correctly and avoid bark wounds.
• Remove and dispose of infested wood before beetles emerge in mid-May.
• Cover oak firewood piles with plastic tarps from mid-May through mid-July.
• Manage defoliating caterpillars during outbreaks to reduce stress.

Chemical control

UW-Madison names systemic insecticides with the active ingredients dinotefuran, imidacloprid or emamectin benzoate, applied as soil drenches or injections, basal bark sprays or trunk injections in spring before adults are active. It says chemical control is practical only for a small number of high-value oaks in fairly good health, is not feasible for woodlots, and that trees with more than 40% canopy loss are hard to save. UW-Madison recommends a professional arborist for insecticide applications. Use only products labeled for this use; the label is the law. Use an ISA Certified Arborist. 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

• Protect the root zone of valued oaks during excavation, grading and heavy equipment work; avoid compaction and root injury.
• Avoid changing grade or drainage near oaks.
• Water oaks during dry spells and manage defoliating caterpillars during outbreaks.

When to call a pro

Call an ISA Certified Arborist if an oak shows top-down dieback or brown leaves hanging on in summer, especially after construction, drought or defoliation. An arborist can take a branch sample to confirm the borer and rule out diseases like oak wilt.

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

Questions about two-lined chestnut borer

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