How Denver Is Preparing to Battle a Catastrophic Tree-Killing Pest

DENVER — The Emerald Ash Borer is a bullet-shaped, glimmering metallic green, half-inch-long jewel beetle—a tiny beautiful horror that over the last 17 years has plowed through hundreds of millions of ash trees in the United States, felling roadside giants at a furious clip, threatening urban air quality and water management and straining local government budgets.

Sometime in 2011, the borer landed in Boulder, Colorado, making its first appearance in a Western state after leaving a trail of destruction across the Midwest and Northeast. It jumped from Boulder ash tree to ash tree. Then it landed 20 miles north in Longmont, another vulnerable, unprepared small city. It was like the bug was gathering strength, like it was preparing to launch its main regional attack 30 miles south on the rough 1.45 million ash trees in the Denver metro area that dot residential property, city parks, public rights of way and school grounds.

So far, no sightings of the borer have been reported in Denver—but that doesn’t mean it’s not already there. Typically, the bug has settled into a city years before it is detected.  

“All of the experts agree it’s just a matter of time,” said Rob Davis, forester for the city and county of Denver. “We decided it was better to do something than to do nothing and just wait for the bug to arrive.”

Denver launched an ambitious effort to draw up a plan of action that included fact-finding missions to cities already wrestling with the ash borer plague and a tree census conducted via satellite paired with university research on the spread of the pest. The effort culminated in a planned five-year public awareness campaign that launched in May 2016 and, a year later, is being touted as a wild success.

The sassy “Be a Smart Ash” campaign is deadly serious, data-driven and boldly entertaining.

“I’m beautiful, I’m nasty, and I’m coming to kill your ash tree,” sings Denver’s Jonny 5 of the Flobots in the music video commissioned for the campaign. He is videoed wandering around the city in a bug suit, making trouble and dropping rhymed informational tidbits.

I do it all with no consequence,

cuz I have no natural controls on this continent…

Consider me an invasive seminal explorer.

You know what my name is—Emerald Ash Borer.

The campaign also includes a website, informational videos, an interactive map, tree tags and bus advertisements.

Davis said public awareness on the issue is key because there is an enormous number of ash trees in the region located on private property. Of the 330,000 ash trees in Denver County, 300,000  live in people’s yards, and the cost of trying to deal with those trees—either to save them or to cut them down and haul them away—would be much more than the city and county could take on.

“We just couldn’t wrap our minds around it,” Davis said. “Providing direct help to residents would be extremely difficult. The problem would quickly become so big. The costs would be enormous. In fact, that was one of the things we thought after visiting Chicago and Milwaukee and Madison. We thought that we had the opportunity to do a better job maybe than they did of getting out in front of the problem, so we wouldn’t be stuck all at once dealing with the dead trees.”

Davis said that he measures the success of the campaign by how many contacts the city makes with the public.

“People know the difference between a german shepherd and a golden retriever, you know?” he said. “Well, that’s what we need now, that awareness about what kind of trees you’re looking at. The fact is, we can only plant 3,000 new street trees in a year, so we need members of the public to help offset the loss that’s coming. That’s how we’ll limit the amount of injury and property damage and the loss of canopy. We have to get people saving money now to take down and replace their trees.”    

Denver is treating its public-property ash trees in waves using TREE-äge brand pesticide, which staffers inject into tree trunks every two years, and will keep injecting until the bug is gone, either of its own volition because untreated ash trees have all been eaten or because perhaps, as sometimes happens, some other animal has arrived to eat it or otherwise chase it away. The pesticide treatment is 90 percent effective if done right. It’s costly—about $5 for every inch of tree—but much less costly than losing the trees.

As the infestation took hold in Boulder, Davis took a tour of the city. One block in the neighborhood where the University of Colorado flagship campus is located showed early signs of the coming devastation. Davis noted the license plates of cars parked all around—Illinois, Wisconsin, Missouri, Kansas—states already ravished by the bug.

“These were the cars of students in rental properties,” he said. “I don’t know, people are weird about wood. Maybe they brought a bunch of firewood with them from home.”

Time is key in the fight against the ash borer, said Davis, and Denver is trying to take best advantage of the time it has right now.

“City processes are slow,” he said. “You have to put aside money in the budget. You have to map and target trees. You have to decide which ones to take down and which ones to try to save — a hundred-year-old tree in a park, trees along the streets, on school grounds. You have to get a program for replacing them with new and different kinds of trees before the urban canopy is diminished. Trees are about an investment of time. Starting over is painful.”  

Scientists believe the ash borer arrived in Michigan at the turn of the century on shipping crates carted from Asia through the Great Lakes. The bug kills all variety of ash trees—some 16 varieties. In its larval stage, the borer munches its way through the spongy wood just under the bark of the tree, leaving telltale “galleries” or snaking trails before emerging fully formed through D-shaped holes it nibbles through the bark to make its getaway.   

“Toronto wasn’t proactive,” Davis said. “When the time came, there just weren’t enough chainsaws in the city to take down the trees.”

Indeed, the ash borer works fast. Mature ash trees reach 60 feet into the sky and spread their branches 40 feet wide. Green ash trees typically live 175 years. White ash trees live 300 years. But the ash borer kills all of them in two years. Roughly a decade after the bug is first spotted, it will have wiped out an entire local ash tree population. Suddenly, tens of thousands of trees fall victim at once. They swiftly begin losing their ability to move water to their limbs. They become brittle and break easily. Enormous branches stream down to the ground. Whole trees split and topple. Cars and homes suffer damage, and humans suffer health effects — and not just from falling branches. Over the years, the U.S. Forest Service has documented increased mortality rates in ash-borer-affected communities tied to increased heart and lung illnesses.

The theory is that the massive ash tree die-offs remove a major urban air-filtering system. The Forest Service values the pollution management work performed by urban trees in the U.S. at $3.8 billion. The Pinchot Institute for Conservation estimated in 2011 that trees in Washington, D.C., removed an amount of nitrogen dioxide equivalent to taking 274,000 cars off the roads. Those kinds of statistics provide a dark backdrop to the fact that now-imperiled ash trees have long been one of the most popular urban trees in the country.

For decades, cities have planted durable green, white and velvet ash trees along roadways. In 2013, the Forest Service reported that, by 2023, Chicagoland likely will have lost all of its 13 million ash trees—18 percent of Chicago metro-area trees. As of 2016, the ash borer had been tracked through 27 states. The bug’s arrival in Colorado marks what may be the beginning of another new chapter in the borer horror story: Ash trees are most abundant in the country’s eastern forests, but the forests of the southwest are home to greater ash tree diversity. At least eight ash species grow in the region between Texas and California.

“Denver has time to choose how to handle this,” Davis said. “We decided we didn’t want the bug to choose. We wanted to manage the bug. We didn’t want the bug to manage us.”