Tree borer damage: ID by gallery pattern

Wood-boring insects -- beetles and moths whose larvae feed in the cambium and wood of trees -- are responsible for some of the most serious tree losses in North American landscapes. Their damage is often hidden under bark until it is severe, and by the time crown symptoms are visible, the.

—- title: "Tree borer damage: ID by gallery pattern" slug: how-to-identify-borers-in-trees hub: problems category: "Identification guide" description: "Identify tree-boring insects by their exit hole shape, gallery pattern under bark, and which tree species they attack. Covers emerald ash borer, bronze birch borer, flatheaded apple tree borer, and more." date: 2026-06-10 updated: 2026-06-10 author: "Thomas A." reading_time: 8 —-

Wood-boring insects — beetles and moths whose larvae feed in the cambium and wood of trees — are responsible for some of the most serious tree losses in North American landscapes. Their damage is often hidden under bark until it is severe, and by the time crown symptoms are visible, the infestation may be too advanced to manage. Knowing what exit holes look like, what gallery patterns indicate, and which borer species attack which tree hosts narrows the ID quickly.

Exit hole shapes

Exit holes are left by emerging adults. Their shape is determined by the cross-sectional shape of the emerging beetle or moth.

D-shaped exit holes (Buprestidae, flatheaded borers): The flat-headed woodborers (Buprestidae) are oval in cross-section, and their exit holes are D-shaped — flat on one side. Per Penn State Extension, this D-shaped hole shape is the most reliable marker for Buprestid beetles. Emerald ash borer (EAB) exit holes are D-shaped, 1/8 inch wide.

Round exit holes (Cerambycidae, roundheaded borers; Scolytinae, bark beetles): Most Cerambycid (longhorned beetle) larvae are cylindrical and leave round exit holes. Bark beetle galleries also produce round entry and exit holes but much smaller (1–3 mm). Per Penn State Extension, round holes of 0.25–0.5 inch diameter suggest a Cerambycid; smaller round holes suggest bark beetles.

Oval exit holes (Cossidae, carpenterworms; Lepidoptera): Carpenterworm moths and related species leave oval, slightly irregular exit holes.

Gallery patterns under bark

Per Penn State Extension, peel back loose or damaged bark to examine the galleries (tunnels carved by the larva through the cambium and inner bark):

S-shaped or winding galleries (EAB, other buprestids): Emerald ash borer larvae create winding, S-shaped galleries that zigzag through the cambium and phloem. Per Penn State Extension, the galleries are packed with frass (sawdust mixed with excrement) and are 4–10 inches long.

Straight, parallel galleries radiating from central hub (bark beetles): Bark beetle galleries have a distinctive pattern: a central mating chamber (hub) with egg galleries radiating outward, then small, short larval galleries perpendicular to the egg gallery. Per Penn State Extension, this "hieroglyphic" pattern is unique to bark beetles and visible when bark is peeled from a declining conifer.

Wide, open galleries (Cerambycids): Roundheaded borer larvae make wide, irregular tunnels, often packed with coarse frass. Per NC State Extension, Cerambycid damage often extends deeper into the wood than buprestid damage.

Species by host tree

Emerald ash borer (Agrilus planipennis)

Host: All native Fraxinus (ash) species. No other tree genus.

Signs: D-shaped exit holes, 1/8 inch; S-shaped galleries in cambium; crown dieback from top down; epicormic sprouting from trunk base; woodpecker blonding. Per Penn State Extension, D-shaped exit holes on an ash tree are EAB until proven otherwise.

Bronze birch borer (Agrilus anxius)

Host: Paper birch (Betula papyrifera), European white birch (B. pendula); moderate impact on other birches.

Signs: D-shaped exit holes, slightly larger than EAB (3–4 mm); winding galleries in cambium similar to EAB but on birch; crown dieback starting at top; irregular bark ridges from gallery pressure beneath. Per Penn State Extension, the ridging of bark over galleries is a useful early sign in birch.

Flatheaded apple tree borer (Agrilus mali in older literature; Chrysobothris femorata more broadly)

Host: Apple, cherry, and many other hardwoods — particularly stressed trees.

Signs: D-shaped exit holes; winding galleries near the soil line and lower trunk. Per Penn State Extension, flatheaded apple tree borer (Chrysobothris femorata) attacks a wide range of hardwoods and is most damaging on trees that are stressed by drought, transplanting, or sunscald.

Dogwood borer (Synanthedon scitula)

Host: Flowering dogwood (Cornus florida), occasionally other Cornus.

Signs: Round, 3–5 mm exit holes; larvae feed in the root crown and lower trunk cambium; rough, crumbly, dark frass mixed with sawdust around the trunk base. Per Penn State Extension, the dogwood borer is a clearwing moth (Sesiidae), not a beetle — the adult mimics a yellowjacket wasp. It most commonly enters through mower wounds and lawn equipment damage at the base of the trunk.

Pine bark beetles (Ips spp., Dendroctonus spp.)

Host: Pines (species-specific by beetle species); stressed trees most vulnerable.

Signs: Very small (1–3 mm) round entry and exit holes in bark; "pitch tubes" (white or amber blobs of resin) where the beetle bored through the outer bark (the tree's defense mechanism); characteristic "gallery pattern" under bark with central hub and radiating egg galleries. Per Penn State Extension, pitch tubes are the tree's response to invasion — trees with adequate vigor can pitch out invading beetles; severely stressed trees cannot.

Blue stain fungi: Bark beetles introduce blue-stain fungi that colonize the sapwood. Per Penn State Extension, the blue-gray staining in the sapwood beneath infested bark is visible when bark is peeled and confirms bark beetle activity.

Exit hole and gallery comparison table

Management principles

Per Penn State Extension:

• Early detection: The gallery and exit hole signs appear before serious crown symptoms. Periodically inspect high-value trees by peeling loose bark at suspicious spots.
• Preventive insecticide injections: For EAB and bronze birch borer on high-value trees, preventive systemic insecticide injections (emamectin benzoate, propiconazole) before or early in infestation are effective. Treatment after more than 50% canopy dieback is generally futile.
• Reduce stress: Most borers preferentially attack stressed trees. Proper watering, mulching to the drip line, and avoiding soil compaction reduce vulnerability.
• Wound avoidance: Many borers enter through mechanical wounds from lawn equipment. Avoid mowing or trimming near tree bases.

Recommended gear: Best dogwood cultivars (Cornus) — our buyer's guide covering picks for every budget, ranked by Extension publication consensus and personal use.

Frequently asked questions

I see sawdust at the base of my oak tree. Is this a borer? Coarse sawdust at the trunk base is often caused by a Cerambycid borer. Per Penn State Extension, examine the trunk for round exit holes (0.25–0.5 inch) and probe the frass pile for larval galleries. Oak borer species include the red oak borer (Enaphalodes rufulus) and the two-lined chestnut borer (Agrilus bilineatus) — the latter, a Buprestid, leaves D-shaped holes and is most common on stressed oaks. Sawdust at the base could also be from carpenter bees or carpenter ants in dead wood — those are not in the cambium.

Can I save a tree with more than half the canopy dead? Per Penn State Extension, a tree with more than 50% crown dieback from a systemic borer infestation (EAB, bronze birch borer) generally cannot be saved with insecticide treatment. The cambium damage is too extensive. Removal is the appropriate action for structural safety and to prevent the pest from spreading.

What are the first symptoms of EAB before crown dieback begins? Per Penn State Extension, the earliest signs are: (1) woodpecker blonding (bark removal by woodpeckers foraging for larvae) on the upper trunk — visible as lighter-colored strips; (2) D-shaped exit holes in branches as first-generation adults emerge; (3) epicormic sprouts on the lower trunk. These can precede visible crown dieback by 1–2 years, allowing time for effective treatment.

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Sources:

1. Penn State Extension — Borers
2. Penn State Extension — Emerald ash borer
3. Penn State Extension — Bronze birch borer
4. Penn State Extension — Dogwood borer
5. Penn State Extension — Bark beetles
6. NC State Extension — Tree borers