Emerald Ash Borer: A Small Pest with Destructive Impacts

EAB Lifecycle Illustration courtesy of Cornell University.

For many years Colorado municipal arborists, tree and landscape professionals and the state and national forest services have had their eyes on the spread of the highly aggressive and ruinous wood boring pest Emerald Ash Borer EAB (Agrilus planipennis) as it wipes out mature and younger ash trees (Fraxinus species) to 1” diameter in both native ash forests and in urban and residential settings.

Since the summer 2002, the Emerald Ash Borer (EAB), a non-native invasive pest of Asia, has killed tens of millions of ash trees in Michigan and since has killed millions of ash trees in 36 states. In September of 2013 EAB was identified in Boulder, Colorado and in 2018 was confirmed in other Colorado communities including Gunbarrel, Lafayette, Longmont, and Lyons. In the fall of 2019, national and state experts confirmed EAB in Larimer County near Berthoud and in June of 2023 was confirmed in Carbondale in Garfield County. Although no one knows exactly how the insect gained admittance to our country, it most likely arrived in solid wood packaging materials that originated from Asia. This could include ash wood used for crating, pallets, or stabilizing cargo in ships.

Lisa Mason, Horticulture Specialist and Entomologist at Colorado State University in Arapahoe County explains that “An unhealthy ash tree doesn’t mean it has EAB. Look for indicators such as distinctive D-shaped exit holes, bark splits and S-shaped tunnels called galleries underneath the bark, gradual canopy thinning and dieback, abnormal shoots of growth, smaller than normal leaves and heavy woodpecker activity”.

If you suspect EAB at your residence or in your community, contact your local extension agent, city forester, or a Colorado State Forest Service field office. See the links below for EAB management and treatment options. Dispose of ash wood safely by double-chipping affected trees to a size of less than an inch square to minimize the survival of larvae or take debris directly to a waste management facility to ensure timely processing and avoid adult emergence from felled logs in the spring.

Ash trees are in the Oleaceae family which includes olives, lilacs, and forsythia to name a few. It is not related to the ornamental Mountain Ash tree (Sorbus aucuparia) that is in the Rose family. EAB does not impact European Mountain Ash trees.

Autumn Purple White Ash Fraxinus americana. Photo by Lisa DiNardo.

European Mountain Ash Sorbus aucuparia. Photo by Lisa DiNardo.

Proper tree identification is an important place to start:

Ash Tree Identification

Emerald Ash Borer Resources:

Emerald Ash Borer Confirmed on Colorado’s Western Slope for First Time
Emerald Ash Borer Information Network
What is “EAB”?
Management Resources for Communities
Insecticides Used to Control Emerald Ash Borer on Residential Shade Trees
Biological Control for Emerald Ash Borer