Invasive Animal: Emerald Ash Borer

Why is emerald ash borer a problem?

Emerald ash borer (EAB) is an invasive wood-boring beetle that kills ash trees by feeding on the tissues under the bark that transport water and nutrients for the tree. It will kill native ash trees of any species, size, age, or state of health. A tree that has been attacked by EAB can die within two to four years.

Emerald ash borer
Photo of emerald ask borer by David Cappaert.

It’s estimated that more than 50 million ash trees are dead or dying in the Midwest because of this insect. Wisconsin’s woods alone contain more than 770 million ash trees, or nearly 7 percent of all the trees in the state!

Ash trees are easy to identify by their “compound” leaves: that is, look for trees with many leaflets on each leaf stalk.

How do I recognize it?

This metallic-green beetle is native to East Asia and was accidentally imported to the United States. It has only recently been found in Wisconsin.

The symptoms of EAB are general, so it’s important to look for several of the following signs and symptoms to determine if you have it in your woods.


Ash borer galleries under bark
An ash tree with its bark stripped away, showing the S-shaped galleries created by emerald ash borer larvae.
  • Larvae: Larvae are cream-colored, slightly flattened, and have pincher-like appendages at the end of their bodies. Mature larvae reach one-inch in length and are found feeding beneath the bark.
  • S-shaped larval galleries: Larvae wind back and forth as they feed under the tree bark, creating S-shaped “galleries” that are packed with sawdust.
  • D-shaped emergence holes: As adult insects emerge from under the bark, they create a D-shaped emergence hole about 1/8-inch across.
  • Adults: Adult beetles are metallic green in color, and are 3/8- to 1/2-inch in length and 1/16-inch wide. Adults have flat backs and rounded undersides.


  • Tree crown dieback: Larval feeding disrupts nutrient and water flow to the upper canopy, resulting in leaf loss and crown dieback. Dieback usually begins at the top of the tree, where EAB prefers to attack first.
  • Unusual branching: Stressed trees will attempt to grow new branches and leaves where they still can. These branches and leaves appear just below where the larvae are feeding, including possibly on the main stem of the tree.
  • Bark splits: Callus tissue that develops around larval galleries causes vertical splits in the bark.
  • Woodpecker feeding: Woodpeckers feed on EAB larvae under the bark. Feeding is typically evident higher in the tree where EAB prefers to be.

How do I prevent it?

EAB spreads primarily through the transport of infected wood, most often firewood. Firewood should therefore be harvested in the same area where it will be used. If you’re traveling, purchase firewood at your destination.