Ten years in, the emerald ash borer breaches the Western Slope as it continues its steady — but slow — spread in Colorado

CARBONDALE — Carl Meinecke sidles up to an ash tree and wraps a tape measure around the trunk, gathering both circumference and diameter in one move.

“Diameter is 17.9 inches,” Carbondale’s town arborist tells colleague and public landscape technician, Allison Uri.

While Uri logs the number into her phone, Meinecke stands back, looks up and inspects the tree’s crown. No obvious signs of damage — no thinning, no dead spots, no noticeably small leaves.

But all is not well in Carbondale.

While this particular green ash on Crystal Bridge Drive may appear healthy, the emerald ash borer — an invasive wood-boring beetle native to East Asia that has decimated tens of millions of ash trees across 36 states over the last two decades — has come to this Roaring Fork Valley town of 6,500 situated between Glenwood Springs and Aspen. The discovery, confirmed this month by state officials, is the first sign that the borer has migrated to Colorado’s Western Slope, most likely by hitching a ride on firewood transported from the Front Range.

The grim news from Colorado’s high country comes on the eve of the 10th anniversary of the first detection of the ash-devouring pest in the state — Boulder was ground zero in September 2013 — which sparked an avalanche of doomsday headlines about the future of Colorado’s urban tree canopy.

In a paper published around the time the beetle was first found in Colorado, entomologists declared the emerald ash borer “the most destructive and economically costly forest insect to ever invade North America.” Approximately 15% of the trees in the state’s urban forest are ash, according to Colorado State University, with nearly 1.5 million casting shade in metro Denver.

“I was maybe a little shocked — I wanted to not believe it,” Meinecke said as pointed to several ash tree stumps in front of the post office where the first signs of the beetle emerged in Carbondale last month. “I was hoping I wasn’t seeing what I thought I was seeing.”

Meinecke and his small team are trying to get ahead of the nascent scourge before it explodes across the 400 ash trees on town land, taking lessons from more than a dozen Front Range cities and counties on what they’ve done over the last decade to control the spread of the ash borer — be it pesticide treatments, removal of susceptible trees or even deployment of tiny parasitic wasps known to snack on the beetles’ larvae.

“Over 10 years in Boulder, unfortunately, we have lost thousands of ash across the city,” said City Forester Kathleen Alexander. “The good news is the EAB (emerald ash borer) is manageable. But you have to plan for it.”

Unlike in the Midwest and on the East Coast, where whole swaths of forest dominated by ash were denuded in short order, Colorado’s infestation has been more laggard and contained. It has to do with the state’s drier and more unforgiving climate and with the fact that ash trees are not as plentiful — and ash forests not as expansive and contiguous — as back east.

Dan West, forest entomologist with the Colorado State Forest Service, acknowledged infestation in the state “is a little slower moving.”

“Ten years later, I anticipated a few more communities to have been confirmed by this stage,” he said.

But slow doesn’t mean gone. Less than two weeks ago, Littleton became the latest Colorado community to find the borer in its ash trees. The Arapahoe County suburb represents the Front Range’s southernmost infiltration of the beetle, which up to now has largely taken root in urban canopies north of Denver — from Fort Collins to Longmont and Thornton to Arvada.

West said the situation can be managed if taken seriously — but that starts with taking the emerald ash borer itself seriously.

“They’re incredibly adaptive,” he said.

Slow but steady spread

Native to China, Mongolia, Japan, the Korean peninsula and far eastern stretches of Russia, the emerald ash borer was first detected in the United States in the Detroit area in 2002. According to the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, it most likely came to the U.S. on wooden pallets and packing materials used in cargo shipping.

The half-inch-long, dark green beetles lay eggs on the bark of ash as they feed on the tree. After hatching, the larvae tunnel into the bark and create S-shaped “galleries” that eventually disrupt the tree’s nutrient-carrying network, girdling and killing the tree.

Ash trees in North America have no natural defenses against the pest and the borer has few natural predators to control its numbers.

“This pest kills stressed and healthy trees and is so aggressive that ash trees may die within two years after they become infested,” the Colorado State Forest Service says on its website. “It is possible for EAB to infest an ash tree for up to four years before visible signs of decline in the tree occur.”

That’s why Thornton began treating its ash trees before the beetle was even discovered in the city, said Craig Gress, Thornton’s arborist. Confirmation of its presence came just over a year ago when Gress noticed an ash tree in decline in the parking lot of a city park.

He felled the tree and found a dead adult borer. He also spotted the distinctive D-shaped tunneling holes the adult beetles make. Then Gress peeled back the bark.

“I found the tunneling galleries that the larvae do underneath the bark,” he said.

Thornton has about a thousand ash trees on city property. Gress said the city hopes to treat 30% to 40% of its ash inventory, focusing on the most viable and healthy trees, known in the field as “high value” specimens.

“We’ve been going through that mitigation to treat our largest and most healthy trees,” he said.

Likewise, Westminster began planning for the emerald ash borer shortly after Boulder got hit, despite the Colorado Department of Agriculture placing a quarantine around Boulder County to prohibit the movement of all untreated ash plants and plant parts out of the infested area. The ash borer can fly about a mile or so from its birth tree in a year, and if that’s not good enough to make it to new wood, humans do the heavy lifting by moving infested firewood across long distances.

City Forester Brian McCoy said Westminster has been treating ash trees on city property for six years, even though detection of the beetle in the city didn’t happen until the fall of 2019. This past spring, the city conducted a right-of-way inventory and found 3,174 ash trees worth saving.

But local governments can only do so much — the vast majority of ash are on private property, over which municipalities have little control. In Westminster, there are nearly 70,000 ash in total, which works out to one in seven trees in the city. The city launched a cheekily named campaign — Save Your Ash — to encourage homeowners to assess the health of their ash trees.

“This can be mitigated with the proper treatments and it doesn’t have to be like it was on the East Coast,” McCoy said.

Treatments can be expensive for homeowners, depending on how many trees there are on the property and how big they are. For a medium size tree 14 inches in diameter, the cost can run from $150 to $250 on average. Treatments, which take the form of soil drenches, trunk sprays or direct injections into the trunk, typically last for two years.

Michael Sundberg, district manager for the southeast office of Denver Davey, a tree care company, said removing a dead or dying ash tree is even costlier. Davey has treated thousands of ash trees on the Front Range over the last decade, while also removing hundreds.

“There will still be some tough decisions to make whether to pay for treating a tree long term or removing or replacing it depending on the tree’s inherent value to a property,” Sundberg said. “Treating all of their ash trees could be too costly for a given budget, but also having a lot of ash trees die and need to be cut down… could bust the budget too.”

If they can afford it, Sundberg said, “homeowners will have good management tools to preserve their ash trees as long as they know they have ash trees on their property and get in front of it.”

An additional factor when considering whether to invest in treatment, Fort Collins City Forester Kendra Boot said, is the fact that ash trees punch above their weight. In her city, she said there are half a million trees, of which 60,000 to 70,000 are ash. But ash comprises a third of the urban canopy due to its propensity to grow big crowns and live for a long time.

Boot knew trouble was on the way in the spring of 2020, when the emerald ash borer was found just over the city line in unincorporated Larimer County. Fort Collins began proactively injecting its ash trees the following summer, splitting the city into three zones to get to all 7,400 ash trees on city property protected in a staggered fashion.

“We’re really trying to choose the big trees, 12 inches in diameter or bigger,” Boot said.

But she acknowledged the beetle has likely been in and around Fort Collins for several years. Just last month, the emerald ash borer was finally documented within the city itself, Boot said.

“If you drive by now, it’s pretty dramatic how many trees have died,” she said of an ash cluster at the northern end of the city where the borer was first found three years ago. “We’ll really start to see trees die here next year.”

Multifaceted attack

For the clearest illustration of the borer’s potential trajectory over time, Boulder is ground zero. According to the city’s 2023 State of the Urban Forest Health report, the city has removed 3,315 infested trees, treated 1,339 trees and planted more than 4,000 new trees to replace the canopy lost to the emerald ash borer.

Boulder has also implemented biocontrols in the form of tiny stingless wasps — called parasitoids. Because the emerald ash borer doesn’t have much in the way of natural predators here, scientists with the U.S. Department of Agriculture traveled to China and Russia after the detection of the beetle in Michigan two decades ago and brought back some weapons.

Four species of stingless wasps — three of which attack borer larvae and one of which targets the beetles’ eggs — were brought to the United States. The agency conducted tests to make sure the wasps, which are the size of mosquitoes, wouldn’t attack other insects before releasing them.

Eight million wasps have been released across 30 states, starting with Michigan in 2007. A recent study conducted in that state and several northeastern states showed that the wasps kill 20% to 80% of emerald ash borer in ash trees that are up to 8 inches in diameter.

In Boulder, three species of parasitoid wasps were deployed. Alexander, the city forester, said the wasps have killed about 28% of the larvae.