Emerald Ash Borer Update

If you live on the Front Range and have ash trees, it’s time to think about taking action to maintain your landscape.

An adult emerald ash borer

An adult emerald ash borer

Since the Emerald Ash Borer (EAB), an invasive, non-native pest with few local predators, was first identified in Boulder two Septembers ago, alarms have raised about the future of 12 to 15 percent of the Front Range tree population, most of which may be affected by the pest in the next half-decade.

We’ve seen the precedent of destruction that this small metallic-green pest can cause, with 21 other states dealing with the loss of over 50 million ash trees since 2002, and in some situations the lack of tree diversity in communities has left them shade-less and barren.

The EAB does not discriminate on the types of North American ash trees it chooses to infest; whether it’s a green, white, black or blue ash, it’s a target.

Homeowners should start with identifying ash specimens on their property, a task that sometimes takes the help of a professional arborist.

The tricky part can be identifying whether or not a tree is infested, because symptoms can take 2 to 4 years to manifest after the pest initially bores into the tree (arborists can also help with this). Since the EAB often starts boring at the top of the tree, homeowners can go out with binoculars to look for “D” shaped bore holes in the trunk,as well as damage to the canopy (30 to 50 percent canopy defoliation = past point of no return). Some other key signs to look for are:

  • dead branches and crown thinning

    EAB with characteristic damage and nickel for size comparison

    EAB with characteristic damage and nickel for size comparison

  • lots of sprouting
  • “S” shaped tunnels under the tree’s bark
  • “D” shaped exit holes on the bark’s surface where adults passed through
  • the presence of woodpeckers, which peck the bark of the tree to find the EAB larvae
  • overall decline in health of the tree

Perhaps even more tricky for homeowners, however, is to decide which action to take for their ash trees. After ashes are identified, it’s time to evaluate their significance. Have they grown well? Do they look healthy? How big and old are they, and what type of shade do they provide?

In the case of established ash trees that are “past the point of no return” (see above), as well as younger, less-established ash trees, the City of Boulder and CSU Extension Service recommend cutting them down and replacing them with other native species that are resilient to both pest invaders and environmental stress. Below is a list of suitable replacements, recommended by City of Boulder Parks and Recreation.

Large maturing trees for ash replacement in Boulder

(Acer saccharum) Sugar Maple – “Green Mountain”

(Acer tataricum) Sugar Maple – “Legacy”

(Aesculus hippocastanum) Common Horsechestnut

(Aesculus octandra flava) Yellow Buckeye

(Catalpa speciosa) Western Catalpa

(Celtic occidentalis) Common Hackberry

(Gleditsia triacanthos v. inermis) Honeylocust

(Gymnocladus dioicus) Kentucky Coffeetree

(Platanus x acerifolia) London Planetree

(Quercus alba) White Oak

(Quercus bicolor) Swamp White Oak

(Quercus robur) English Oak

(Tilia americana) American Linden

(Ulmus japonica x U. wilsoniana) ‘Accolade’ Elm

Carol O’Meara of the Colorado State University Cooperative Extension said in a recent article that planting another young sapling as an ash replacement now can encourage its small roots to spread quickly into the soil and get established, so the tree will thrive and one take the place of its ash predecessor.

For the existing ash trees that a homeowner just can’t part with, CSU master gardeners say that systemic insecticides can be effective in warding off borers, or killing them within the tree (hopefully before they’ve sustained irreversible damage). The master gardeners did, however, have a caveat for systemics in light of the growing movement to make consumers aware of the effects of these chemicals on the surrounding environment, and especially their impacts on pollinators.

“Many systemics will stay active in a tree for up to two years,” said CSU Master Gardener Elsbeth Diehl. “That includes the leaves, which in the fall will drop into the soil and release the toxin into the nitrogen cycle, so neighboring plants may absorb the residual poison from the soil.”

Diehl and her colleague Liz Abbott, who’s another master gardener in the CSU program, said that making the decision to protect a tree with insecticides will be a continual one, requiring a re-treatment every other year, probably for the foreseeable future.

They did recommend alternative treatments, like trunk sprays, which work their way out of the tree’s system by the time their leaves fall, and therefore stay out of the soil’s nitrogen cycle.

Another thing to keep in mind, the master gardeners added, is that there are other pests affecting ash trees that are not necessarily the EAB, namely the Lilac Borer, which enters into the lower portions of tree bark and makes rounder bore holes (as opposed to “D” shaped ones).

If there’s a moral to this story, it’s that planting a variety of tree species in your yard to enhance your property and surrounding neighborhood is good thing. Whatever the route you choose, remember there are resources that can help guide you along the way, like CSU, the City of Boulder, or your favorite hardware store!

Make sure and check out the Boulder County Free Tree Give-away on Saturday, April 25th, from 9am-1pm, where you can receive replacement trees if you have ash trees on your property, but only while supplies last! The two locations for this event are:

Boulder County Clerk and Recorder’s Office

1750 33rd Street, Boulder


The Boulder County Fairgrounds

9595 Nelson Road, Longmont