Invasive insect
Emerald ash borer
Agrilus planipennis
Emerald ash borer signs: metallic green beetles, D-shaped exit holes, S-shaped galleries and woodpecker blonding on ash trees.
Published Last reviewed
An ash tree with a thinning top and pale patches where woodpeckers have stripped the bark may have emerald ash borer. The insect is a small metallic green beetle. Its larvae feed just under the bark, in the living layers that keep the tree alive. The adult is not the easiest thing to find. The exit hole and the gallery behind the bark are better evidence.
Emerald ash borer identification
The adult is metallic green with a coppery-red abdomen. APHIS describes it as about 1/2 inch long; Penn State Extension gives 1/3 inch. That is a real disagreement between the sources, so do not rely on size alone. The beetle is slender and bullet-shaped, and the color is the stronger clue.
The larva is the stage that kills the tree. It is white to cream, 1 to 1 1/4 inches long, with 10 abdominal segments and a pair of brown, pincer-like appendages at the rear. It feeds in the phloem and cambium, the layers just under the bark.
Emerald ash borer uses green, white and black ash. Penn State Extension describes a one-year life cycle. Adults emerge in late May and early June, peak in mid-June to early July, and leave D-shaped holes. Females lay 60 to 90 eggs, which hatch in 7 to 10 days. Larvae feed for several weeks, overwinter in the bark and pupate in late April or early May.
Signs of emerald ash borer
Start with the tree, not the bug. A quick check:
- Look at the top of the canopy. Dead branches near the top and a thinning crown are early symptoms.
- Look for new shoots on the trunk. Epicormic sprouts are a sign of stress.
- Look for vertical cracks in the bark.
- Look for pale, jagged patches where woodpeckers have peeled the bark.
- Look for D-shaped holes on the trunk and branches.
- If a branch is already dead, peel back a little bark and look for S-shaped galleries packed with sawdust-like frass.
| What you find | What it points to |
|---|---|
| D-shaped exit hole on an ash | Emerald ash borer is the main suspect |
| S-shaped gallery with sawdust-like frass under ash bark | Emerald ash borer |
| Round, 1/4-inch exit hole with sawdust-like frass | A clearwing borer or a longhorned beetle |
| Many tiny round holes about 1/16 inch across | Eastern ash bark beetle |
The exit hole is the fastest separator, but it is not perfect. Bronze birch borer and two-lined chestnut borer are close relatives that also make D-shaped holes. The tree species matters. If the holes are D-shaped and the tree is an ash, treat emerald ash borer as the main suspect. If the holes are round, look at the size and the frass before you assume EAB.
Emerald ash borer look-alikes
Several native insects attack ash or look like EAB. Clearwing moths and longhorned beetles leave round holes, not D-shaped ones. The eastern ash bark beetle leaves holes so small that they look like a pepper shaker was used on the bark.
The two Agrilus relatives are the harder call. Bronze birch borer and two-lined chestnut borer are metallic wood borers in the same genus, and they also leave D-shaped holes. They are look-alikes, not ash borers. That is why the first question is not “What made this hole?” but “What tree am I standing under?”
What homeowners can do
1. Assess the tree before you spend money. A healthy ash with a full canopy and a few exit holes may be a candidate for treatment. A tree that is already dead, dropping large limbs or leaning toward a target is a removal decision. Ask a local extension agent or an arborist.
2. Treat early if the tree is worth saving. Penn State Extension says imidacloprid can be effective as a soil drench, soil injection or stem injection in May or early June. The treatment has to start before the tree is heavily infested, and some products last more than one year. Follow the product label; the label is the law.
3. Remove trees that are dead or hazardous. Tree removal is the main option for trees that are too damaged to save. Use a professional for anything near a house, road or power line.
4. Do not move ash firewood. Buy firewood where you burn it, or use certified heat-treated wood. EAB travels in infested firewood, logs and nursery stock.
5. Report a suspected find. APHIS asks residents to use its online report form or call 1-866-322-4512.
6. Check the rules before you move wood. The federal domestic quarantine was removed in 2020 and the final rule took effect on January 14, 2021. State rules can still apply.
For more on invasive insects, see the insects hub and the spotted lanternfly page. The Japanese beetle page covers a different insect that feeds on garden plants. The prevention guide covers keeping new pests out, and the plants hub lists the trees and plants they affect. Photo credits and licenses sit on the image credits page.
Identification
Look for the combination of characters below. A single feature is rarely enough, especially when a native look-alike is present.
- Adult beetle
- About 1/3 to 1/2 inch long, metallic green, with a coppery-red abdomen. APHIS gives about 1/2 inch and Penn State Extension gives 1/3 inch, so use the color and shape rather than the ruler.
- Larvae
- White to cream, 1 to 1 1/4 inches long, with 10 abdominal segments. The last segment has a pair of brown, pincer-like appendages.
- D-shaped exit holes
- Adults chew out through the bark and leave holes shaped like a capital D. Look for the straight side and the curved side on the trunk or branches.
- S-shaped galleries
- Under the bark, larvae feed in winding, S-shaped tunnels packed with sawdust-like frass. You usually need to peel or cut away bark to see them.
- Canopy symptoms
- Dead branches near the top of the tree, a thinning canopy, vertical cracks in the bark and new shoots sprouting from the trunk.
- Woodpecker blonding
- Woodpeckers peel off the outer bark and punch through the rest to eat larvae. The pale, jagged patches they leave are often one of the first visible signs.
Look-alikes
-
Bronze birch borer
Agrilus anxius
Also a metallic wood-boring beetle with D-shaped exit holes. It is listed as an EAB look-alike, not an ash borer, so the host tree is the first clue.
-
Two-lined chestnut borer
Agrilus bilineatus
A close relative in the same genus that also makes D-shaped exit holes. The exit hole alone will not separate it from emerald ash borer.
-
Banded ash clearwing and ash/lilac borer
Podosesia aureocincta and Podosesia syringae
Native clearwing moths that attack ash. Their larvae push sawdust-like frass from the trunk, and adults leave round, 1/4-inch exit holes rather than D-shaped ones.
-
Redheaded ash borer and banded ash borer
Neoclytus acuminatus and Neoclytus caprea
Longhorned beetles that also attack ash. They leave round, 1/4-inch exit holes.
-
Eastern ash bark beetle
Hylesinus aculeatus
A tiny bark beetle that peppers the bark with round exit holes about 1/16 inch across.
Range and status
Native range: Eastern Russia, northern China, Japan and Korea
Regulatory status by state
No state regulatory classification has been verified for this species yet. Check the current noxious-weed list for your state before making a regulatory decision.
Why it matters
Emerald ash borer larvae feed in the phloem and cambium, the living layers just under the bark. Infested trees can lose most of their canopy within two years and die within three to four years. APHIS says the beetle has killed tens of millions of ash trees in North America and has been detected in 38 states and the District of Columbia.
Management
Check the tree before you treat
Look for D-shaped holes, S-shaped galleries, canopy dieback and woodpecker damage. A tree that still has a healthy canopy may be worth treating. A tree that is already dead or badly broken may be a removal decision. Ask a local extension agent or an arborist to assess the tree before you spend money.
Treat early if the tree is worth saving
Timing: May or early June.
Penn State Extension says imidacloprid can be effective as a soil drench, soil injection or stem injection in May or early June. Start before the tree is infested. Some treatments last more than one year. Follow the product label; the label is the law.
Remove trees that are dead or hazardous
Tree removal is the main option for trees that are already dead or too damaged to save, according to Penn State Extension. Use a professional for a tree near a house, road or power line.
Do not move ash firewood
Buy local firewood or certified heat-treated firewood. People move emerald ash borer long distances in infested firewood, logs and nursery stock.
Report a suspected find
APHIS asks residents to use its online report form or call 1-866-322-4512.
Check the rules before you move wood
APHIS removed its domestic emerald ash borer quarantine in 2020, and the final rule took effect on January 14, 2021. State rules can still apply, so check with your state agriculture department before you move ash wood or nursery stock.
Sources
Facts on this page were checked against the sources below. Each link shows the date the source was accessed.
- Emerald Ash Borer — USDA Animal and Plant Health Inspection Service Accessed November 3, 2026
- Emerald Ash Borer — National Invasive Species Information Center Accessed November 3, 2026
- Emerald Ash Borer — Penn State Extension Accessed November 3, 2026
- Native Woodborers and Emerald Ash Borer Lookalikes — Oklahoma State University Extension Accessed November 3, 2026