Emerald ash borer

What is emerald ash borer?

Emerald ash borer is a wood-boring beetle native to Asia. It was first found in the US in southeastern Michigan in 2002 and reached western Pennsylvania in 2007. The damage is done by the larvae. They feed in the layer just under the bark that carries water and nutrients through the tree.

The tunnels cut off the tree's supply lines. Dieback usually starts at the top. One-third to one-half of the branches can die in a single year, and most of the crown is often dead within two years of the first symptoms. Heavy infestations can kill a tree in as little as three years.

Penn State says removal is the main option for most infested ash, because control choices are limited. Healthy, high-value ash can be protected with insecticides, but treatment works best when it starts before the tree is infested and must continue for as long as you want to keep the tree.

Signs and damage

• Thinning canopy and dead branches starting at the top of the tree
• Woodpecker holes where birds dig for larvae
• D-shaped exit holes about 1/8 inch across on trunk and branches
• Vertical splits in the bark over S-shaped tunnels packed with sawdust-like frass
• Clusters of new shoots (sprouts) growing from the trunk

Emerald ash borer identification: what to look for

• Adult: Bright metallic green, slender, bullet-shaped, about 1/3 to 1/2 inch long. APHIS notes a coppery-red abdomen under the wings.
• Larva: White to cream, up to about 1 to 1 1/4 inches long, with 10 abdominal segments and a pair of small brown pincer-like parts at the tail end.
• Damage: S-shaped (winding) tunnels packed with frass under the bark, D-shaped exit holes, bark splits, woodpecker holes, top-down dieback and trunk sprouts.
• Timing: In PA adults come out late May to early June and are most active mid-June to early July.

Life cycle

One generation a year in Pennsylvania. Adults chew out through D-shaped holes from late May to early June, with activity peaking mid-June to early July. Each female lives about three weeks and lays 60 to 90 eggs on the bark. Eggs hatch in 7 to 10 days and larvae bore in to feed for several weeks. They spend the winter as pre-pupae in the bark, turn into pupae in late April to early May, and adults come out a week or two later.

When to look: Adults come out late May to early June and peak mid-June to early July, so that is when fresh exit holes appear. Check the canopy for top-down dieback while the tree is in leaf. Penn State notes new infestations may not be obvious for up to three years.

Emerald ash borer look-alikes

A few things get mixed up with it:

• Two-lined chestnut borer (Agrilus bilineatus): A close relative that also leaves D-shaped holes, but it attacks oaks, not ash. NC State notes EAB attacks only ash and white fringetree, so check the tree species first.
• Bronze birch borer (Agrilus anxius): Another Agrilus borer with D-shaped exit holes, but it attacks birch. If the tree is not an ash, it is not EAB.

How to control emerald ash borer

Cultural control

• Penn State says removing the tree is the main option, since control choices are limited.
• Save treatment for healthy, high-value, well-placed ash; low-value or unhealthy trees are poor candidates (UW-Madison Extension).
• Trees with more than half the canopy thinned should be removed, not treated (UW-Madison Extension).
• Keep treating protected trees even after nearby ash have died (UW-Madison Extension).

Biological control

• APHIS rears and releases tiny stingless wasps that attack EAB as part of its biological control program.
• Woodpeckers feed on the larvae under the bark.

Chemical control

Extension sources name these active ingredients: imidacloprid as a soil drench or soil injection, dinotefuran as a bark spray on the lower trunk, and emamectin benzoate as a trunk injection by professional applicators. Penn State says imidacloprid goes on in May or early June, and large trees should be treated before they are infested. UW-Madison Extension says most products must be reapplied every year, emamectin benzoate is labeled for two years of protection, soil drench products struggle to protect big trees, and trees with more than 50% canopy thinning should not be treated. Use only products labeled for this use; the label is the law. Large ash should be treated by an ISA Certified Arborist or other licensed applicator. 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

• Buy local or certified heat-treated firewood and don't haul firewood; see our firewood rules guide.
• Check ash trees for the signs above. Penn State says treatment of large trees should begin before they are infested.

When to call a pro

Call an ISA Certified Arborist if you have a healthy ash you want to keep, if a large ash is dying near a house, driveway or power line, or if you want a trunk injection. UW-Madison Extension says trees 20 inches or more across should be treated by a professional arborist.

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

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