Green Menace: Emerald Ash Borer Could Decimate Denver's Trees

One in six of Denver's roughly 2.2 million trees are ash trees and they're at risk from one of the most voracious insects around -- the emerald ash borer.

Native to Russia, China, Japan and the Korean Peninsula, the beetle has devastated tree populations in 28 states since reaching U.S. shores around 2000. It's not a pest in eastern Asia, but it's wreaked havoc in Europe and North America. It's already invaded Boulder and popped up in Longmont this spring.

Ash trees took root in Denver in the 1970s and '80s, according to City Forester Rob Davis. They do well in the city's poor soil so became an ideal replacement for American elm, which was devastated by Dutch elm disease. Since they grow quickly, they represent more of Denver's tree coverage than other species. But their Achilles heel is an insect that's less than a half-inch long.

To keep ahead of this eastern Asian menace Denver has launched the Be A Smart Ash campaign to protect its roughly 333,000 ash trees. The three-pronged campaign includes an educational website as part of a public outreach effort, aggressive application of a trunk-based insecticide and a tree-planting campaign.

A green menaceOne in six of Denver's roughly 2.2 million trees are ash trees.

"It's very aggressive," says Davis. "On a scale of one to 10, it's a type 10 invasive pest. It kills big ash, little ash, healthy  ash. If the trees aren't treated, the pests will destroy the trees."

The borer is initially hard to detect for foresters and arborists, let alone residents because the larvae bore under the bark, eating away at the layer under the bark that feeds the tree, destroying its ability to feed itself. The first generation of larvae in a new infestation can take up to two years to emerge and become beetles, Davis explains. "When you find the borer it's been there for three to four years."

Even though Longmont arborists just found the beetle, for instance, city officials are now saying the infestation has been there for three to four years. "Then you're four years deep and that bug starts to go exponential," explains Davis. "Every female adult lays 100 eggs, so it's this massive blowup and it can get away from you. It has this spiking curve of exponential mortality."

Knowing the threat, Denver officials went on a tour of other cities already infested with the borer. "We went to Chicago, Milwaukee and Madison," Davis says. "We got a mix of city people, and a council member. It wasn't just forestry people."

On the tour, they clearly saw the devastation wrought by the borer. "We took on this decision that we're not going to say it's here, but we don't know if it is for sure or not."

The adult borer can fly over a mile, but it can travel much farther on strong winds. Davis says they can go even farther when someone cuts an infested tree down and moves it. For all these reasons, it may have already reached Denver.

The ash alternative

To help combat the potential loss the city is planting and offering replacement trees. "We've got funding to try to get a target goal of getting 3,800 street trees planted this year," Davis says. "If any Denver resident goes on to the Be a Smart Ash page, there's a little button that says sign up for a street tree." Users can enter their location, the site will be evaluated for a replacement.

The trees offered purposely include a variety of species. "With the planting program they just don't have to replace an ash tree," Davis says. "You can replace your neighbor's ash tree three doors down by planting a tree in your yard where there's never been one. It's not just replacement trees."

"Diversity is especially important so that we don't end up with another case wherein, similar to the Emerald Ash Borer, a single pest or disease threatens a huge percentage of our community's trees," says Kim Yuan-Farrell, executive director of The Park People, one of the organizations working with the city on the tree replacement campaign.

"The Park People is focusing on getting trees planted along the public right-of-way adjacent to private property," explains Yuan-Farrell. She adds that it could also work with businesses that want support the city forest and help with the Be A Smart Ash effort by adopting planting projects in parks. "Some of those may be parks with a lot of ash trees or parks that just need more trees in general."

The tree-planting effort is aided by the city's tree inventory initiative, which is more than 90 percent complete, according to Davis. That's helped the city and The Park People choose which trees to plant.

"Inspectors visit each address to approve each street tree. They mark the approved locations and recommend a species appropriate for that location," Yuan-Farrell says.

While the organization plants some native trees in parks and in neighborhoods like Western Hackberry and Bigtooth Maple, The Park People plants trees suitable to the climate and setting, she adds. "There are just not a lot of trees native to our area that are good choices for planting in urban areas.

For example, cottonwoods are quite brittle, so they're not ideal in places with lots of people." Instead it plants trees that have habitat value like supporting local wildlife and food webs, says Yuan-Farrell.

Getting ahead of the infestationEmerald ash borers have infested trees in Boulder County..

While the city prohibited the planting of ash in 2004, there are a lot of them out there. To protect as many of the city's 333,000 existing ash trees along streets, the city will treat them with an insecticide. Davis explains that the city will notify residents ahead of time that it plans to treat an ash tree on their property. "It's not a canopy spray," he clarifies. "It's a drilled hole at the base of the tree with a plug. The dose is injected directly into the tree and it protects the tree from the emerald ash borer for three years .

"We will do a third of all the qualified trees in a neighborhood, then another third and finally another third," Davis elaborates. "That puts this pool of treated trees throughout the city's population that can check an infestation a little bit." He explains that he insecticide is deadly to adults feeding on the leaves.

Despite all of these efforts, Davis cautions that the borer could still devastate the city's trees. "No one's been able to stop it and get rid of it," he says. However, all these steps should help mitigate the issue as much as possible while keeping Denver's trees as healthy and as diverse as possible.

Davis offers some advice to help keep the city beautiful and green: "One of the best ways for us to get trees planted is neighbors talking to neighbors. They do it way better than we do. If you're out and about and have neighbors and see empty spots, feel free to pass on the message."