Ash Tree After Being Infected with the Emerald Ash Borer.

Ash Tree After Being Infected with the Emerald Ash Borer. (Photo: Shutterstock).

New to North America in 2002 and found in Boulder in 2013, emerald ash borer is a nonnative insect that established itself and began infesting and killing ash trees in the county. With no natural predators to keep it in check, its spread ensures the loss of true ash trees (genus Fraxinus) that are left untreated. Mountain ash (genus Sorbus) is not susceptible to EAB.

The quarantine put in place in 2013 into 2019 to block the movement of firewood and other hardwood across Boulder County borders was intended to slow EAB’s spread and give other areas time to prepare for its inevitable arrival, not to contain the invasive species.

It worked as intended. EAB is still in Colorado, but with proper management, damage to ash trees can be controlled, and in some cases prevented.

Recognizing EAB

The adult beetle is a bright metallic-green-colored insect, about 10 to 13mm long. They emerge from trees during May and June by cutting their way through the bark, leaving a small exit wound shaped like the letter D. Larvae are white with a series of bell-shaped segments, and cause distinctive S-shaped damage called a “gallery” to the inner bark and phloem of ash trees.

Signs and symptoms of infestation

You’ll notice the canopy of your ash tree gradually thinning and worsening from year to year. In comparison, sudden dieback may be caused by extreme weather events or other acute wounds.

Your tree will start suckering. The tree tries to shoot new growth wherever it can, usually at the base of the tree or on the main branches.

Early fall color may appear in mid-to-late summer while surrounding ash trees remain green. Because early fall color can also be a sign of drought, check surrounding conditions to rule it out.

The leaves on all or part of the tree appear smaller and lighter in color, and cracking or splitting bark shows S-shaped galleries You may also notice a mottled appearance, caused by woodpecker activity as they remove bark in search of larvae.

Controlling EAB

There are four systemic-type insecticides used to control EAB, each with its own application method. They are imidacloprid, dinotefuran, ememectin benzoate and azadirachtin. They are sold under various trade names, and many must be applied by a licensed professional. Consult a licensed arborist for available control options in your situation. Treatment must be continued for the life of the tree.

CSU Extension Fact sheet 5.626 (Insecticides Used to Control Emerald Ash Borer on Residential Shade Trees) covers basic information about the insect and provides detailed control options, including availability and application.

The Colorado State Forest Service took the lead on education surrounding EAB after the quarantine was dropped. Check out their website at for more information.

National EAB Awareness Week is May 22-28. Start thinking now about how you can manage EAB on your property — untreated ash trees, if not infected already, will be — it’s just a matter of time.

For more information on this and other topics, visit or contact your local CSU Extension Office.

By Patty Rhodes, Colorado State University Extension. Patty is a Colorado State University Extension Colorado Master Gardener.