The age of the Ash Borers

Arvada infestation proves that emerald pests are here to stay

Scott Taylor
Posted 7/1/20

After years of municipal efforts to fence in the Ash tree killing Emerald Ash Borers and keep them from expanding out of Boulder County, the inevitable has arrived. They’re here. “I guess the …

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The age of the Ash Borers

Arvada infestation proves that emerald pests are here to stay


After years of municipal efforts to fence in the Ash tree killing Emerald Ash Borers and keep them from expanding out of Boulder County, the inevitable has arrived.

They’re here.

“I guess the message I’d like to tell people is that if they’ve been holding back and waiting to do the treatment on their ash trees because it’s not quite here yet, well, we do believe that it’s here now,” said Westminster City Forester John Kasza.

Westminster crews first found the emerald pests infesting the trees in the parking lot outside the Willow Run Shopping Center at the end of August 2019.

Now they’ve been found just to the south, in neighbor Arvada. Craig Hillegass, Arvada’s city forester, said the bugs were discovered on Ash trees on private property on the Southeastern portion of the city, in the neighborhood surrounding Homestead Park.

Ben Irwin, Arvada’s public information officer, said they’ve been getting ready for this moment.

“We finished up a management plan for this earlier this year, even before they discovered the outbreak,” Irwin said. “We are doing an inventory of all of our trees, looking at their condition. Some will be treated, based on their location and their value to the community. Others may be replaced slowly, over time.”

Westminster’s Kasza said he’s not sure that the bugs have spread elsewhere in between, but he’s pretty confident they have.

“We have not had any confirmed cases besides the one up north,” he said. “But it’s hard to detect emerald Ash Borers at the early stages of the infestation. It’s usually been in the tree for a year or two before we find it.”

So far, the cities of Northglenn and Thornton have not reported infestations of the green bugs.

U.S. pest since 2002

Emerald Ash Borers are a non-native wood-boring beetle responsible for the death and decline of tens of millions of ash trees in the United States and Canada. As a non-native insect, it lacks predators to keep it in check.

Colorado’s municipal forestry departments have been preparing for the bugs for years. The pest was first discovered in the U.S. in Michigan in 2002 and has been detected in Boulder, Gunbarrel, Longmont and Lafayette since 2013. The state forest service began a quarantine around Boulder County in 2013 to help prevent human-assisted spread of the pest.

But the borers broke quarantine two summers ago when pests were found near 136th Avenue and Main Street in Broomfield on August 2018. State state agriculture officials said they didn’t know if it spread there naturally — the insect can spread a half-mile each year on their own — or if it was brought in by people accidentally, on firewood or infected ash bark used to decorate yards and gardens.

And the bugs could find plenty of places to spread. Westminster has about 69,000 ash trees in the city limits — about 15 percent of the city’s total tree population. About 1,700 of those trees are under the city’s control, the rest on private land.

“We’re doing what we can about our trees,” Westminster’s Kasza said. “We’re trying to spread the word to everyone else.”

Arvada claims about 97,000 Ash trees within the city limits, and about 1,500 on public property.

Hillegass, Arvada’s city forester, said the his crews efforts now are on saving high value trees. Those include special trees planted as memorials and some in certain areas.

“We’re going to treat about 50% of the trees,” he said. “If you look at them now, most of our Ash trees are in pretty rough shape anyway. They had a tough winter and late freezes, so it’s not cost effective for us to treat the ones in really bad shape or the ones smaller than ten inches around.”

Holes in the bark

Property owners with ash trees should be on the lookout for thinning of leaves in the upper tree canopy, 1/8-inch D-shaped holes on the bark and vertical bark splitting with winding S-shaped tunnels underneath. People that find suspicious trees should report them by calling the Colorado Department of Agriculture at 1-888-248-5535 or filling out the EAB Report Form at

Kasza said the green colored pests are not the only problems Front Range ash trees have right now. A native Ash borer, the lilac variety, does kill some tree limbs but does not ultimately kill the tree.

“Late frosts hurt those trees as well,” Kasza said. “It’s been a difficult year for all of our trees.”

The best treatment is emamectin benzoate, an insecticide that was invented in 2001. For a simple treatment, tree owners can put the poison on the ground around the trees, where it will get absorbed into the trees trunk or they can drill into the trunk itself or inject the pesticide.

Kasza said injection tends to be more expensive and requires a professional to do it right, but it’s safer for the environment.

“If it’s in the soil, it can be absorbed into other plants, and then it kills beneficial insects like bees,” Kasza said. “We’d rather have people inject it right into the trunk, if they can.”

Both Kasza and Hillegass said that’s what their crews are doing.

“And then, we figure each treatment should last us about two years,” Hillegass said.

Green sashes

Kasza said the city is trying to get the word out about the bugs and the effect they can have on the urban forest. They’ve started wrapping select Ash trees in Westminster’s Forest with green plastic sashes to show how big a threat an infestation could be.

“We just chose to put them out, or help make the public aware of what’s going on, that the emerald Ash borers have been found in Westminster and Arvada,” Kasza said. “If they have nice Ash trees that they’re proud of, now is the time to act.”


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