Denver City Forester Luke Killoran was in his office when he got the news on his cell phone. It was June 2025, and during routine inspections of trees in the Congress Park neighborhood of Denver, one of his inspectors had finally found the needle in the haystack – or more precisely, the infested tree in the vast urban forest.
“We knew EAB was coming and that it was going to be here any day,” Killoran said. Denver had been preparing for the arrival of the deadly and invasive emerald ash borer (Agrilus planipennis) ever since it was discovered in Boulder in 2013. EAB attacks and kills all true ash species native to North America, and Denver has over 30,000 ash trees.
The infested ash in Congress Park showed all the classic symptoms of EAB: D-shaped exit holes in the bark, dieback on the crown, S-shaped galleries and larvae under the bark, and dead metallic green adults, some stuck halfway out of the tree. Before becoming Denver’s city forester, Killoran was the forestry supervisor in Lakewood when EAB was first detected there in 2024, so he knew this was likely the real deal. But there are several EAB lookalikes in Colorado that attack ash trees, so Denver forestry staff gathered samples and drove them to the C.P. Gillette Museum of Arthropod Diversity at Colorado State University, where Dr. Chuck Harp confirmed them as EAB.
After the official confirmation, “we did a deeper dive in Congress Park and sure enough, it popped positive around the corner and then around the corner and then it was like, this is it,” Killoran said.
The CSFS maintains a map of Colorado municipalities with EAB detections to track its spread. Over the years, as more cities and towns were added, several large gaps existed on the map, even though it was expected that EAB was present in more cities. EAB can live in an ash tree for several years before the tree shows signs of infestation. It builds populations over those years before a crescendo occurs, when the canopy thins and conditions are prime for adults to emerge.
Denver, Aurora, Golden, Berthoud, Edgewater and Wheat Ridge all had initial detections of EAB in 2025, the most in a single year since EAB was first found in Boulder, filling in big gaps in the map.
“It’s not really surprising to find it in these locations. We’ve all assumed it was in Denver for many years,” said Kathleen Alexander, Boulder city forester. “It’s very hard to detect EAB especially here in Colorado because we have so many other abiotic and biotic problems that can cause symptoms that look like EAB. … By the time you find EAB and detect it based on those symptoms, it’s already spread, so it’s to that point where it’s becoming a lot more obvious.”
About 15% of the trees in Colorado’s urban forests are ash, making EAB a major threat to trees in cities and towns statewide. As this insect spreads across Colorado, it has the potential to reduce the tree canopy for communities and the benefits that trees provide, including clean air and water, lower energy costs, wildlife habitat, noise reduction, economic value, shade, beauty and places to recreate.
Alexander has been with the City of Boulder for 29 years. She was there when EAB was first detected in Colorado and watched its spread from the epicenter in Boulder. She credits planning and a proactive response to EAB for helping to hinder its spread in Colorado initially.
Before it was detected, partners with the Emerging Pests in Colorado (EPIC) group drafted a state EAB response plan, knowing the devastation it had caused in urban forests in the Midwest and the many visitors who travel to Colorado with the potential to bring infested ash wood.
At the time of detection in 2013, the nearest known infestation was in Kansas City about 600 miles away, but a human can drive the insect hundreds of miles to a new place – much farther than the few miles an adult can fly on its own to find and infest a new tree.
Boulder monitored its ash for EAB and was ready when it arrived. “The earlier you detect a pest, the more management options you have available to you,” Alexander said.
After the detection, Boulder implemented an aggressive strategy to curb EAB’s spread by reducing populations and its food source. This included the following actions:
Other nearby cities enacted similar management efforts, and diverse partners established the EAB Response Team to coordinate education and outreach. In addition, the Colorado Department of Agriculture – and then the federal Animal and Plant Health Inspection Service – established a quarantine around Boulder County to prevent and reduce the spread of ash wood. (The quarantine was repealed effective Dec. 30, 2019.)
All of this speaks to the cooperative nature in Colorado that is present in so many aspects of natural resource management. “We certainly did our part [in Boulder],” Alexander said, “but it was that collaborative effort. It was everyone working together, coming together, because we knew that this had the potential to be really bad after what we had seen in the Midwest.”
Along with effective management, the dry, arid conditions in Colorado may impact the spread of EAB, along with cold snaps in the spring. These conditions make it hard for ash trees to grow in Colorado, putting them in perpetual defense mode, but they also make it hard for EAB to survive and thrive as well. Picture the EAB half-out of the bark, dead on emergence like the ones Killoran and his staff found in Lakewood and Denver.
In addition, while ash trees are prevalent in many Colorado communities, they are not native to areas of Colorado where EAB is currently present and suitable habitat for EAB is fragmented. Ash trees were planted in cities and towns and have become naturalized in localized areas over time. But there are no forests of ash that connect communities to each other, making it more challenging for EAB to spread naturally.
All those factors – management actions, climate conditions and fragmented habitat – can play a role in the ability of EAB to infest new trees. Humans, however, play a more direct role in the spread of EAB across Colorado by moving infested wood.
EAB showed up north of Fort Collins in 2020, seven years after first being detected in the state, many miles from the closest known detection in Colorado at the time. A decade after that initial detection in Colorado, EAB was found in Carbondale in 2023 on the other side of the Continental Divide, far from the northern Front Range where all other detections of EAB in Colorado exist.
With people being such a key factor in the spread and management of EAB, both Alexander and Killoran cited ongoing education as a critical component for addressing EAB in Colorado. More residents and arborists should know how to detect EAB and the options for managing the insect, especially with many people moving to Colorado and not knowing about EAB. “Continuing that outreach is super important; that’s not just a box you check,” Alexander said.
Communication among municipal foresters to share information and support is also valuable, with EPIC, the Colorado Tree Coalition and urban forestry councils being useful networks in Colorado. When faced with detections in their cities, Alexander and Killoran said they reached out to peers in Colorado and abroad for advice and support. “Collaborate. You’re not alone in this,” Alexander said. “When we first detected EAB in Boulder, we knew exactly who to call and exactly what was going to happen after that because we already had this great network.”
“Before I found it in Lakewood, before I found it in Denver, I talked to a ton of other foresters,” Killoran said. This spring, he said Denver is ramping up its efforts to educate residents about EAB and their options for managing their ash, as well as the many benefits of trees. Denver also continues to implement its successful Be A Smart Ash program, which plants FREE trees of different species and removes and treats ash trees in public rights-of-way with the support of residents.
Killoran was ready for that detection of EAB in Denver last year thanks to effective planning and preparation. “There is this bittersweet sense of relief that OK, now we can stop searching,” he said. “Now we can breathe and move forward with the next threat or pest and prepare for it.”
Semiochemicals can be important tools to monitor and control bark beetle populations as part of an integrated pest management strategy.
The CSFS used aerial survey and FIA data to examine ponderosa pine-dominant forests affected by an outbreak of mountain pine beetles in a nine-county area of the Front Range.
The Colorado State Forest Service and its partners conducted a vast majority of their forest management work in high-priority areas of the state and greatly increased the acres of forests they managed and enhanced annually from 2020 through 2024.
