10 Years with Emerald Ash Borer in Boulder

January 23, 2024

Introduction

The fall of 2023 marks 10 years since emerald ash borer (EAB), an invasive tree pest, was detected in Boulder. EAB has impacted all aspects of Boulder Forestry's urban forest management since its initial detection. Explore the following pages and interactive maps to learn more.

The Urban Forest - This section highlights the importance of Boulder’s tree canopy, illustrates the extent of tree canopy and shows how canopy cover has changed since EAB was detected.
Boulder Forestry - This section details Boulder Forestry’s role in managing public trees.
Emerald Ash Borer Basics - This section gives an overview of emerald ash borer and the challenges associated with managing this invasive pest.
10 Years of Emerald Ash Borer - This section chronicles management decisions and collaborative efforts that have helped Boulder Forestry to slow ash mortality following EAB detection.
Outcomes of the Emerald Ash Borer Invasion - This section summarizes lessons learned and observations of EAB in Colorado.
What Comes Next? - This section explains how you can help to sustain Boulder’s urban tree canopy by planting and GROWING new trees!

EAB has impacted all urban forestry operations and resulted in the removal of thousands of ash trees. However, tree planting and the protection of large public ash trees through EAB management efforts have successfully preserved tree canopy and slowed ash mortality!

The Urban Forest

The urban forest contains all public and private trees within Boulder city limits. Prior to EAB, Boulder had approximately 656,000 trees, with an appraised value of $1.2 billion. Approximately 70,000 of those trees were ash, contributing around 24% of Boulder’s overall tree canopy (2013 USDA Forest Service Metro Denver Urban Forest Assessment Report ).

Canopy Changes Since Emerald Ash Borer Introduction

Overall tree canopy cover has remained relatively stable over the past decade. Based on aerial imagery collected every two years by the National Agricultural Imagery Program (NAIP), canopy cover was 22.2% in 2015 and 21.3% in 2021 ¹.

¹ This is within the +/- 1.3% margin of error based on the methods used to estimate canopy cover.

When looking at smaller divisions within the city, called subcommunities, the picture of urban canopy cover becomes clearer. While citywide canopy cover has remained relatively constant, each subcommunity tells a different story. Subcommunities such as Central Boulder – University Hill, East Boulder and Southeast Boulder have lost significant canopy cover due to EAB and extreme weather events. Other subcommunities, such as North Boulder and Palo Park, have increased canopy cover, due to overall tree growth and tree planting in newer subdivisions.

Explore the subcommunities map to see where canopy has been gained or lost.

The greatest canopy decline occurred in Central Boulder. The greatest canopy increase occurred in North Boulder.

The Importance of Boulder’s Tree Canopy

Boulder’s tree canopy is a legacy of the tree planting and stewardship efforts of previous generations. The urban forest is valued as essential infrastructure because it is intertwined with the natural environment, public health, economic stability, and sustaining a vibrant community. Trees are a component of the city’s nature-based climate solutions because of their role in urban heat reduction and the critical human health benefits they provide.

IIn 2015, an Urban Forest Resource Analysis estimated the quantifiable benefits of Boulder’s urban forest. The analysis showed that the urban forest provides nearly $5.2 million in annual environmental, economic, and social service benefits ($50.39 per capita, an average of $102.48 per tree). On a per tree basis, ash trees provide a higher than average benefit to the community in carbon sequestration and stormwater runoff mitigation because they are long-lived and large statured. Prior to EAB, the benefits of the urban forest attributed to ash trees was higher than their proportion within the total urban forest.

The figure on the right shows the monetary value of the environmental benefits of the urban forest in 2015 in more detail. Boulder’s urban forest provides many additional benefits that cannot be measured in dollar amounts with existing methods, including providing wildlife habitat and foraging opportunities, reducing noise pollution and heat islands, stimulating economic activity, and improvements to public health and safety, community aesthetics, and student performance.

Public Tree Inventory

Public trees are a subset of the urban forest and consist of trees on publicly owned lands and rights-of-way including along streets, in city parks, and at city-owned facilities. Having an up-to-date tree inventory is essential for tracking maintenance activities, costs, and estimating future needs. It allows Boulder Forestry to answer critical questions like, “What impact will the emerald ash borer have in our community?”

In 2015, tree canopy from the public trees (~50,500) covered 651 acres. To replace the public tree resource with trees of similar species, size, and condition would cost approximately $110 million (~$14.6 million from ash). As of 2023, Boulder Forestry maintains 49,629 public trees in overall good condition, including:

36,226 street trees (including 3,137 ash).
13,403 park trees (including 888 ash).

Continue to explore the tree inventory map to learn more about specific trees in the public tree resource.

Boulder Forestry has planted more than 4,700 public trees since EAB was detected in 2013. Additional public trees have been added through private development and city projects. Since many pests and diseases are host specific, species diversity is key to urban forest resilience. Diversity in trees planted also helps mitigate the potential future impacts of climate change. Over the past few decades, Boulder Forestry has continually increased species diversity through tree planting. Annual tree planting depends upon many factors such as budget, staffing, and tree stock availability.

Explore the bar chart to better understand public tree planting and removal over time.

Boulder Forestry has tracked changes in tree diversity over time. See how the percentage of ash trees has changed over the past 30 years. A surprisingly high number of unique species are represented in Boulder’s public tree inventory (317 species in 2023). Since 1995, the number of species in the inventory has nearly doubled and species diversity in the inventory has increased by 88.2%. Although EAB has resulted in the loss of ash trees and a reduction in tree canopy in certain subcommunities in Boulder, the urban forest is becoming more diverse and more resilient.

Boulder Forestry

Boulder Forestry is within the Parks & Recreation Department and is responsible for trees in city parks, around city-owned facilities and in public street rights-of-way. Below are some of Boulder Forestry’s on-going programs and services:

Rotational and Safety Pruning: Street and park trees are pruned for both health and public safety through a systematic pruning rotation (currently a 14-year rotation for street trees and a 9-year rotation for park trees).

Tree Removals: Boulder Forestry surveys all park and street rights-of-way trees annually for tree health and public safety. Dead, diseased, and high-risk trees are removed and replaced when possible.

Tree Risk Assessments: City park and public street right-of-way trees that are greater than 18” in diameter are surveyed on a 5-year rotation to look for structural defects. Trees posing a risk to public safety are either pruned or removed and replaced if the risk cannot be mitigated through pruning.

Tree Planting and Establishment: New trees are planted in city parks and the public street right-of-way each spring as replacements for those lost or through requests from adjacent property owners.

Tree Inventory / Asset Management: Boulder Forestry maintains a tree inventory and tracks all major tree maintenance in an asset management program to monitor costs and assess future needs.

Commercial Tree Program: Boulder Forestry monitors and maintains public trees and the hardscapes around those trees (e.g., tree grates) in commercial areas.

Integrated Pest Management (includes EAB): Trees on both public and private property are monitored through surveys to identify potentially epidemic pest problems. Treatments found to be effective in achieving management goals are implemented for public trees.

Emergency Response: Boulder Forestry coordinates emergency response activities to eliminate tree hazards (e.g., downed trees, large broken hanging branches) due to wind or snowstorms, construction impacts, vehicular accidents, or other emergency events.

Service Requests: Boulder Forestry responds to an average of 730 public tree requests annually from the public and other city workgroups.

Development Review, Tree Protection, and Mitigation: Boulder Forestry reviews development plans and provides construction oversight for private and city projects to ensure compliance with city tree protection codes and Design and Construction Standards. Staff meets with planners, architects and developers to minimize impacts to public trees; when public trees are removed or damaged staff will appraise public trees and assess mitigation which is used to plant new public trees.

Arborist Licensing: Boulder Forestry works cooperatively with Planning and Development Services to license all tree care professionals working on both public and private trees to help ensure safe, quality tree care practices are used.

Urban Wood Utilization: Logs and chips generated from Boulder Forestry operations are brought to the log yard. All woody biomass is diverted from landfills and utilized.

Enforcement of Tree Regulations: Boulder Forestry visually surveys private trees from public property to identify dangerous trees. Per city code, Boulder Forestry requires private property owners to either prune or remove private trees posing a threat to public safety.

Communication, Notification, and Education / Outreach: Boulder Forestry holds annual outreach events and special events, sometimes in collaboration with inter-agency working groups. These, along with educational materials, meetings, city website postings, and mailers allow for regular communication with community members.

Planning Projects (e.g., Urban Forest Strategic Plan): Boulder Forestry participates in city strategic planning to review and assess annual work plans and programs. These efforts ensure progress toward short- and long-term goals.

Staff includes 7 full-time employees and 1-2 temporary employees.

City Forester
Forestry Operations Supervisor
3 Foresters
2 Forestry Field Technicians
1-2 Temporary Forestry Field Technicians

The average annual Boulder Forestry budget is approximately $1.9 million but can vary annually based upon tax revenues, specific planned projects, and mitigation collected. Though Boulder’s 2018 Urban Forestry Strategic Plan identified that for Boulder to invest in the urban canopy at per capita and per tree levels to meet current industry standards, an additional $500,000 is required annually.

Emerald Ash Borer Basics

Emerald ash borer (Agrilus planipennis; EAB) is an invasive insect from east Asia. All ash trees (Fraxinus spp.) are highly susceptible to this deadly threat. EAB was initially detected in Michigan in 2002 and is believed to have arrived in the U.S. in infested shipping material. Since its introduction, EAB has spread throughout much of the United States and Canada ( EAB Network ). The adult beetles can fly short distances (up to 6 miles), but long-distance spread is attributed to the movement of infested wood (e.g., firewood). EAB has killed hundreds of millions of ash trees across the United States and Canada to become the most destructive pest in North American history. Prior to EAB, there were an estimated 7.5 billion ash trees in timberlands and an additional 30-90 million ash in urban areas across the United States. EAB is the most impactful urban forest pest to date due to the number of susceptible host trees in urban and natural landscapes.

For context, Dutch elm disease, an invasive fungal pathogen, threatened 50,000 American elm trees in urban areas over a 40-year period in Colorado and killed an estimated 60% (approximately 30,000 elm trees). EAB threatens an estimated 1.45 million ash trees in the Denver metro area alone.

Photo courtesy of Dan West - CO State Forest Service

The Worst-Case Scenario Pest

In terms of invasive forest pests, EAB may well represent the “worst-case scenario” for both urban foresters and the forests they manage. Read more about why in the diagram to the right.

Signs & Symptoms

It can be hard to know if an ash tree is infested with EAB. Symptoms of EAB such as off colored foliage and crown thinning typically begin in the upper canopy, where EAB attacks first. It can take several years after initial EAB infestation for symptoms to appear within the tree. Larvae feed just under the bark and cut off the flow of water making ash trees killed by EAB brittle. The symptoms of EAB are difficult to distinguish from other stressors that are common to ash trees in Colorado. Once EAB symptoms appear, they can rapidly progress and kill the tree which is an important distinction from other stressors ash trees commonly experience.

Signs of EAB include beetle emergence exit holes on the trunk and branches, bark splits exposing larval feeding galleries, and the presence of adult beetles. Woodpecker activity and damage is common near galleries since they feed on EAB larvae.

To view detailed images of the signs and symptoms of EAB versus other problems ash commonly experience (e.g., other insects and diseases, abiotic issues, and environmental issues), see Diagnosing Emerald Ash Borer and Other Ash Tree Damage: A Diagnostic Field Guide .

10 Years of Emerald Ash Borer

In the fall of 2013, Boulder Forestry staff discovered an EAB infestation within the city. This was the first detection of EAB in Colorado and, at the time, it was the western-most occurrence in North America, 600 miles from the closest known infestation in Kansas City, Kansas ².

Early planning and targeted actions led to the timely detection of EAB in Boulder. Prior to EAB detection, Boulder Forestry prioritized consultations with leading national EAB researchers and local experts in entomology and tree pest management. These experts provided research documents and guidance on EAB management strategies, expected impacts once EAB was detected, and information on how the insect might behave differently in Colorado due to our drier climate and extreme temperature fluctuations. This information helped guide the program and led to an early detection and rapid response.

EAB Detection in Boulder

Boulder Forestry formalized and enacted EAB monitoring and sampling protocols in 2013 due to the EAB detection in Kansas City. Boulder Forestry began to sample all declining or dead public ash trees before removal. In September 2013, EAB was identified in north central Boulder when sampling a dead public ash tree.

² In 2022, EAB was detected in Oregon.

Delimitation Survey

After the initial EAB detection in Boulder, a delimitation survey was conducted to establish the extent of the EAB infestation within city limits. Boulder Forestry collaborated with the Canadian Forest Service, the Colorado Department of Agriculture (CDA), USDA Animal and Plant Health Inspection Service Plant Protection and Quarantine (APHIS), Colorado State University Extension (CSUE), Boulder County, University of Colorado (CU) and urban forestry staff from nine nearby cities to complete the survey. This multi-agency collaboration resulted in the investment of over 1,000 hours from eleven federal, state, county, and city agencies.

To complete the survey, Boulder was divided into a sampling grid containing 38 square mile grid sections. Up to ten public ash trees were sampled in each section, and two branches were removed per tree. Sampling was completed by peeling back the bark on each branch to search for EAB larvae. In total, 384 trees were sampled and 768 branches were peeled. The delimitation survey showed EAB was established in central Boulder. The movement of EAB throughout the city was tracked from 2013 – 2016 using the following detection efforts:

Tree Climbing: Forestry staff inspected symptomatic public ash trees that were identified during visual surveys or through requests from adjacent property owners.

Destructive Sampling: All declining public ash trees removed in EAB non-detect sampling sections were sampled for the presence of EAB.

Branch Sampling: Additional branch sampling was conducted in fall 2014 and spring 2015 in EAB non-detect sampling sections.

Trapping: Several types of EAB detection traps were deployed on an experimental basis throughout Boulder in 2014 and 2015 to detect its presence.

Rotational Pruning: Tree care contractors conducting pruning on Boulder Forestry contracts looked for signs of EAB infestation when working on public ash trees.

Regional Collaboration

Colorado has been recognized internationally for its long history of collaboration amongst urban forest managers. For invasive tree pests, collaborators included agencies at all levels of government and industry partners. The result of the collaborative efforts allowed urban forestry professionals to develop mitigation and management strategies across jurisdictional boundaries, create outreach materials, and coordinate education and outreach efforts to foster community support for managing EAB in Colorado.

In 2009, an interagency team, the Emerging Pests in Colorado (EPIC) Committee, was formed to address a newly emerging disease complex, thousand cankers disease of walnut. EPIC committee members included many of the agency and green industry partners that would later lead the Colorado EAB response. Regular committee meetings provided a setting in which positive, cooperative relationships between stakeholders and responsible authorities were established. It also provided the opportunity for urban forest managers to learn about state and federal pest quarantine scenarios, legal requirements regarding movement of wood and nursery stock, and wood movement patterns. This allowed for a quick, decisive, and unified response to the arrival of EAB.

When EAB was detected in Boulder, USDA Animal and Plant Health Inspection Service Plant Protection and Quarantine (APHIS) instituted a formal emergency response hierarchy called the Incident Command System (ICS) which later transitioned to the Colorado EAB Response Team (CORT). To ensure continued communication and consistent information transfer among agencies, this group included participants from APHIS, USDA Forest Service (USFS), Colorado Department of Agriculture (CDA), Colorado State Forest Service (CSFS), Boulder County, City of Boulder, City of Fort Collins, City of Denver, City of Longmont, University of Colorado (CU), and Colorado State University Extension (CSUE). Highlights of the achievements from both EPIC and CORT include:

Wrote the State EAB Response Plan.
Secured financial and other resources to support local urban forestry programs to support planning efforts.
Created educational resources for invasive pest detection and management and wood disposal options.
Coordinated invasive pest workshops for industry professionals (160 to 200 attendees each year in 2013, 2015, 2017, 2019, 2023).
Managed the EAB Delimitation survey and disseminated information to pertinent agencies.
Coordinated and disseminated information to the public through various channels (news releases, social media, etc.).
Provided EAB updates to the Colorado State Legislature, Metro Mayors Caucus, and Colorado Municipal League.
Created and maintained a centralized website with EAB information (e.g., educational resources, updates concerning distribution and spread and management recommendations).
Developed markets for urban wood utilization.
Held diagnostic workshops for natural resource managers (e.g., EAB identification, sampling and detection protocols) at the Boulder Forestry log yard.
Led van tours of EAB impacted sites across Boulder to educate Front Range green industry personnel and decision makers about the economic, social, and environmental impacts of EAB.

Boulder Forestry Integrated EAB Management

Boulder Forestry received unanimous support of City Council, the Parks and Recreation Advisory Board and the Environmental Advisory Board in 2015 for our EAB Management Strategy.

Boulder Forestry has been successful at achieving their goals for EAB management.

Goal 1: Slow the spread of EAB within Boulder and to other communities. Boulder Forestry follows the city’s Integrated Pest Management Policy and enacts several different control tactics to help manage EAB. Integrated pest management is a decision-making process that considers pest and environmental information to determine if pest control actions are warranted. If actions are warranted, a variety of preventative and curative pest control methods are chosen based on economic and safety considerations.

Goal 2: Stage removals to spread out costs. Ash tree removals have been spread out over multiple years, occurring steadily since 2013 and peaking in 2017.

Goal 3: Preserve significant trees. A number of studies have shown that it is more cost effective to preserve mature ash trees rather than remove and replace them. Boulder Forestry used a set of criteria to determine if a public ash tree is a good candidate for preservation (which is only achieved through the use of pesticides). For an ash tree to be preserved using pesticides, it must meet the following criteria:

Have a diameter greater than or equal to 10”.
Be in good health.
Be situated in a favorable location for long term growth (e.g., considering distance from other trees, access to irrigation).

Goal 4: Maintain urban tree canopy. Canopy cover has been sustained at around 20% through preservation of large ash trees and tree planting. Boulder Forestry prioritized preserving ash in areas with the lowest canopy cover.

Regulatory Control

Quarantine: At the time of its initial detection, EAB was a federally quarantined pest. The Colorado Department of Agriculture (CDA) enacted a quarantine on Boulder County in 2013 to help slow the spread of all life stages of EAB and to allow other municipalities time to prepare for its arrival. Quarantines may be established with a goal to eradicate a pest from a specific location, but because detection is so difficult with EAB, state and federal quarantines were not effective at eradication. The state quarantine was lifted in 2019 when EAB was deregulated by USDA Animal and Plant Health Inspection Service (APHIS). Best management practices, such as not moving raw ash materials outside of infested areas, are still being followed by urban forest managers in the region to minimize further spread.

City Code: Forestry staff survey all city parks and streets to identify diseased and dangerous trees on both public and private property. Currently, city code authorizes Boulder Forestry staff the ability to require private property owners to remove trees posing a threat to public safety. Because ash trees killed by EAB dry out and become brittle quickly, Boulder Forestry requires timely removal of dead ash trees with the potential to threaten public safety or property.

Mechanical Control

Between 2013 and 2022, more than 3,100 public ash trees were removed. From 2013 to 2018, public ash trees that did not meet the criteria for pesticide treatments and fell into one of the following categories were removed and replaced:

High risk trees.
Trees in poor condition or those compromised by other insect pests or exhibiting signs or symptoms of EAB.
Trees with poor placement (i.e., under power lines).
Trees with poor structure (topped or improperly pruned trees, trees with codominant stems) that will mature into a higher maintenance tree.

After 2018 when the EAB progression had slowed, the removal of ash trees by Boulder Forestry was refocused on trees posing a risk to the general public or exhibiting EAB symptoms to preserve existing tree canopy as long as possible. Additional public ash trees are being removed during private development and city projects.

Cultural Control

Utilization and safe disposal of woody biomass from infested trees is essential to EAB management. In Boulder, all EAB infested logs and chips are brought to the Boulder Forestry log yard. Most woody material is ground into mulch but ash wood resulting from EAB infestations has been used in creative ways, including:

Mulch used for all newly planted trees on public property, in the city park system and supplying the Free Mulch pile for resident use (chips are small enough to prevent spread).
Mulch and compost through a partnership with A-1 Organics.
Ash wood repurposed into lumber, art, and furniture in partnership with BLDG 61, Bridge House and ReSource through the TreeOpp Program.
Milled and air-dried wood sold to the public for projects through the partnership with Re-Source.
Whole logs used for nature play installations or projects in city parks or given to local non-profits or schools for projects.
Woody material as a quality heating fuel.
- Chips provided through a collaboration with Boulder County for use in a biomass heating system at the Boulder County Jail.
- Pellets for wood stoves and BBQ grills through a partnership with Confluence Energy.
Planning a pilot program in partnership with the city’s Climate Initiatives Department to utilize wood chips to produce biochar. One use for biochar is as backfill during tree planting to potentially improve survivability of newly planted trees.

Plant Resistance

The Boulder Forestry tree planting program replaces previously removed ash trees to maintain the tree canopy and is focused on increasing species diversity and resilience. Since EAB was detected, Boulder Forestry has planted over 4,300 public trees, an average of 430 trees per year. On average, 36 different species are planted annually. All newly planted trees are resistant to EAB and 90% are large maturing species, which will maximize the environmental, social, and economic services provided by these trees. Planting efforts were also promoted on private property through the nearly 3,000 seedlings given away to Boulder Valley School District elementary students (2016-2018) and the annual Tree Giveaway (2017-2019, 2023) and Tree Sale (2018-2019, 2023) for Boulder residents done in collaboration with the PLAY Boulder Foundation Tree Trust.

Chemical Control

Currently, pesticide applications are the only available option for preserving existing ash trees and to manage EAB on a citywide scale. Treatment is only effective if pesticides are used before the tree has experienced significant damage from EAB larval feeding. Pesticides are an important component of EAB management programs for a number of reasons, including:

Preserve significant ash trees long-term.
Reduce EAB populations and therefore slow the progression of EAB by killing adult beetles before they can reproduce.
Spread tree removal and replacement costs over a longer time period.
Lower risk and public safety concerns associated with large numbers of dying trees.
Slow the loss of urban tree canopy and the subsequent loss of environmental and economic services.

Each of these benefits allow Boulder Forestry to replant and mitigate losses over a longer period of time. Although EAB seems to be progressing more slowly in Colorado, it is still necessary to treat ash trees in Boulder to protect them from EAB.

EAB treatments have been incorporated into routine operations and Boulder Forestry is successfully treating 1,339 (22%) public ash trees on a 3-year rotation through trunk injections of emamectin benzoate. An additional 43 large ash trees in close proximity to creeks or lakes are being treated every two years with azadirachtin. Preserving significant ash trees has required proactive planning and additional funding.

The public ash tree on the left is untreated and heavily infested with EAB while the public ash tree on the right has been treated.

Biological Control

Several predators, pathogens, and host tree defenses prevent EAB from becoming a major pest in its native range in Asia. To assist in EAB management in the United States, the USDA Animal and Plant Health Inspection Service (APHIS) rears EAB biocontrol insects at their Michigan facility and provide these biocontrol agents at no cost to cities, including Boulder. In the post-outbreak phase, local APHIS and Colorado Department of Agriculture (CDA) staff released four different biocontrol insect species in Boulder to suppress EAB populations.

The insect species released in Boulder are tiny, non-stinging parasitic wasps.

The most widespread parasitoid wasp in Boulder is the *Tetrastichus planipennisi*.

The insect species released in Boulder are tiny, non-stinging parasitic wasps, including:

Tetrastichus planipennisi- a larval parasitoid (12,715 released between 2014 and 2019).
Oobius agrili - an egg parasitoid (6,600 released between 2014 and 2019).
Spathius agrili - a larval parasitoid (2,235 released between 2014 and 2019).
Spathius galinae - a larval parasitoid (2,570 released between 2014 and 2019).

Biocontrols will not protect individual ash trees, but these beneficial organisms will further reduce overall EAB populations as part of a comprehensive IPM program.

Explore the map to see the parasitoid release sites. Releases in Boulder are now complete. Areas with large amounts of naturalized ash along waterways are still being targeted for releases in nearby counties as EAB is detected.

Boulder is currently participating in a research study involving EAB biocontrols. The research question centers on the effectiveness of the combination of insecticide use and biocontrol releases. The study is also assessing if chemical treatment will protect ash trees long enough for the parasitoids to build a population that can sufficiently protect larger diameter ash trees.

The Colorado Department of Agriculture (CDA) is monitoring sentinel trees. Sentinel trees are created by removing a 1 foot section of bark from the trunk of an ash tree in the spring to stress it and attract EAB. The trees are later removed, and the bark peeled off to search for the parasitoids. Early findings suggest that several parasitoid species are established, with areas of naturalized ash trees being the most conducive for parasitoid release. Branch sampling has shown a steady increase in parasitism rates since initial releases (from 0 to 13.7%). In areas where wasps were released, parasitism rates are as high as 28% and up to 45% of EAB larvae are consumed by woodpeckers.

Interestingly, woodpeckers are a major biocontrol of EAB larvae in the U.S and Canada. In Colorado, flickers, downy woodpeckers, and hairy woodpeckers prey upon EAB larvae. Although woodpeckers cause damage to individual trees while searching for larvae, they can be effective at lowering overall EAB populations citywide.

Public Service Requests

Since the discovery of EAB, the number of annual public service requests both directly and indirectly related to EAB has increased. While a rise in service requests can be attributed to EAB, some of the increased requests are also due to weather events impacting other tree species. In 2013-2016 the public had questions about EAB control and management, but in more recent years, requests for pruning increased as routine maintenance was deferred due to EAB and severe weather events. This has resulted in even less time available for routine pruning and maintenance needs of other public trees. See the bar chart for more information on the rise in service requests.

Urban Forest Strategic Plan

Boulder Forestry developed the Urban Forest Strategic Plan (UFSP) in 2018 that includes long term management strategies for EAB and other invasive pests. During the development of the UFSP, the community was engaged on many levels (e.g., open houses, community surveys, stakeholder interviews, technical working group, etc.). This allowed for a more comprehensive community-wide discussion, planning, and goal setting around the following topics:

Establishment of a baseline for urban tree canopy and a long-term no-net-loss canopy goal.
Tree diversification goals.
Urban heat island mitigation.
Prioritization of tree planting activities.
Pesticide use guidelines for public trees.

Overwhelmingly, the community supported tree canopy growth over time.

Outreach and Education

Boulder has consistently provided the public with EAB related outreach and education materials. The community has also benefited from materials and events produced by other agencies and organizations involved in the Colorado EAB Response Team (CORT). These state-wide efforts have led to increased awareness and understanding of the impacts and management efforts underway for EAB and encouraged engagement and input from the target community.

City of Boulder – open houses, news releases, website , presentation to HOA groups, Kiss My Ash t-shirts through PLAY Boulder Foundation Tree Trust.
Colorado EAB Response Team (CORT) – EAB trainings and van tours to educate the green industry and decision makers in Denver metro communities.
Boulder County – Watch Your Ash drink coasters delivered to Boulder County brewpubs.
Denver – Be A Smart Ash campaign – bus ads, local news banners, social media, EAB rap song.

State – centralized website , news releases.

Funding, Grants, and Partnerships

Boulder Forestry EAB specific costs were tracked from 2013-2020. EAB management costs including tree planting, ash tree removals, pesticide treatments, wood utilization, and outreach efforts totaled just over $1.67 million for the 8-year period.

After the detection of EAB, Boulder Forestry's annual budget was increased, funded by a dedicated sales tax for the Boulder Parks and Recreation Department. Even with that additional investment, the Urban Forest Strategic Plan identified in 2018 that an additional $500,000 is needed annually to meet increased maintenance needs and community expectations. To supplement programs and events, Boulder Forestry has also relied upon grants and partnerships to supplement programs and events including:

TreeOpp program created through a Knight Foundation Challenge Grant (a wood utilization and skills training program, $200,000).
Xcel Energy Foundation (tree planting grant for public lands through the Colorado Tree Coalition, $6,000).
Arbor Day Foundation - Boulder Tree Recovery Program (provided 285 1-gallon sized trees, annually in 2017-2019).
Partnerships and Agency collaborations such as the Rockefeller Foundation 100 Resilient Cities (supported the 2013 Urban Tree Canopy Analysis), ReSource (lumber sales), Boulder County (biomass material for the Boulder County jail heating system), energy companies (wood pellets).

Boulder Forestry participated in several innovative research projects in 2013-2015 in collaboration with the following organizations and individuals: Canadian Forest Service, USDA Animal and Plant Health Inspection Service (APHIS), Colorado Department of Agriculture (CDA), Colorado State University Extension (CSUE), and local beekeepers.

Outcomes of the Emerald Ash Borer Invasion

After a decade of managing EAB in Colorado, the evidence suggests that EAB is manageable at the local level. The lessons learned in Boulder have helped other communities throughout the state manage EAB once it was discovered in their town, city, or county. Successful EAB management requires a knowledgeable and regionally collaborative forestry program as well as long-term support and commitment from city leaders and the community. EAB management impacts all aspects of urban forestry operations and management actions need to be sustained over time to preserve ash trees as a component of the urban tree canopy.

Lessons Learned in Colorado

Collaboration

In the initial stages of the EAB response, collaboration was especially helpful with detection efforts, developing and implementing best management practices, and creating response plan templates. Collaboration ensured consistent messaging in the development of education and outreach materials, website content updates, and information transfer throughout the state. Coordinated management efforts continue to help communities manage EAB by reducing staff time, streamlining operations, and decreasing costs.

Planning

Proactive planning led to stronger partnerships, better staffed and equipped urban forestry programs, and an integrated approach to EAB management. Establishing a public tree inventory and urban forest canopy analysis allowed urban forestry managers to better assess what was at risk and to identify the necessary resources to protect the urban tree canopy. Information from urban foresters in the Midwest and leading researchers provided important direction on how to manage EAB in a way that meets each community’s needs.

Detection

Boulder Forestry was able to detect EAB relatively quickly due to preemptive and continuous monitoring of symptomatic ash using several techniques (sampling during routine maintenance activities, collaborations with tree care companies, and trapping). Using multiple techniques increased the chances of detection. The delimitation survey was very useful to determine the extent of EAB within Boulder and to pinpoint priority areas for immediate management actions. However, the survey was time-consuming and was accomplished only through multi-agency collaboration.

Management

Preservation of significant ash trees, prompt removal of infested ash trees, planting replacement trees for those lost to EAB, and a long-term push to increase species diversity has been key to maintaining tree canopy. Considering equity and disparities in tree canopy is important when choosing ash trees to preserve. Consistent with the observations of researchers and municipal foresters in the Midwest, pesticides have been effective in Boulder at slowing EAB population growth. This allows for longer term management rather than reacting to a backlog of dead, dying and often hazardous trees. Increased budgets for Boulder Forestry programs were necessary to effectively manage EAB and the associated risks. As more money and staff time was allocated to EAB management (tree removals, planting, treatments, wood disposal, education and outreach efforts, enforcement of hazard tree removals), less resources have been available for other routine maintenance. As a consequence, service levels and deferred maintenance for other trees has fallen short of industry standards and the community’s expectations.

EAB Biology

In Colorado, it appears that EAB is progressing more slowly than in other parts of the country. This is likely due to the relatively low density of naturalized ash in Colorado. Individual infested trees may decline more rapidly because of the arid climate. Though the timing of adult emergence in the spring is consistent with the Midwest, late spring frosts are beneficial in killing large numbers of early adult stage beetles as well as the tender new ash foliage, which is the primary source of food for adult EAB.

Outreach

Extensive outreach and engagement have been key to maintaining support for, and stewardship of, the urban forest. Proactive communication and planning such as up to date information on the Boulder Forestry website regarding pesticide options, replacement tree species and local licensed arborists, helped the city prepare for the EAB related increase in service requests and enforcement. It also allowed residents to take actionable steps to manage EAB on their own properties.

Economics and Partnerships

Through partnerships, Boulder Forestry has been successful in finding creative ways to decrease costs and generate funds for EAB management efforts. Fostering non-profit organizations such as the PLAY Boulder Foundation Tree Trust has helped the program support tree planting and care on private property, where a significant portion of ash trees and approximately 75% of the urban tree canopy are located.

For more information on the lessons learned, please visit this website:

City of Boulder : Emerald Ash Borer and Other Pests

What Comes Next?

Boulder Forestry continues to ask questions about, and support future research on, EAB. Current questions include:

How should the city prioritize tree planting efforts and grow more trees to address historic inequities, mitigate for climate change, reduce urban heat, and improve the community’s resilience?
Is EAB behaving differently in Boulder or has early detection and consistent, proactive management prevented the exponential ash tree mortality rate seen in the Midwest?
Because EAB seems to be progressing more slowly in Colorado and the biocontrols have been successfully established, can Boulder Forestry move from a 3-year to a 4-year pesticide treatment cycle without losing significant public ash trees?

Other Invasive Species

Boulder Forestry conducts periodic risk assessments for high priority tree insect and disease pests. Pests are evaluated for the likelihood of introduction into Boulder to prepare monitoring protocols and responses to those threats. Monitoring activities include trapping for insects under evaluation, visual surveys, and destructive sampling at the time of tree removal.

Boulder Forestry has a reputation for finding new tree pests. Boulder Forestry was the first to discover and help identify thousand cankers disease of walnut, drippy blight of red oak, and Turkish filbert leaf spotting, and the first in the western U.S. to detect EAB. Boulder Forestry is a national leader in continuous monitoring for pests and for a robust IPM program to support pest response and management.

Image to the right is an example of drippy blight on a red oak at Central Park.

You can help grow our Boulder urban tree canopy by…

Being on the Lookout for Invasive Pests!

Early detection and rapid response are key pieces of invasive species management. The earlier a new pest is detected; the more management options are available. We can use your help! Please be on the lookout for the tree pests shown below. For more information click these links:

If you think you have found an invasive pest, contact:

Boulder Forestry Program phone: 303-441-4406 email: AlexanderK@bouldercolorado.gov Inquire Boulder request: https://bouldercolorado.gov/report-issue
Colorado Department of Agriculture (CDA) phone: (888) 248-5535 email: CAPS.program@state.co.us
USDA Animal and Plant Health Inspection Service (APHIS) phone: (303) 371-3355 email: Adda.Capella@usda.gov Online reporting form: APHIS

Preserve Existing Trees and Grow New Trees!

It takes a long time to grow a tree! Some trees that comprise the urban forest are over 100 years old. Tree canopy goals cannot be achieved through tree planting efforts alone. Existing trees, on both public and private property, must be preserved and cared for to mitigate against the heat and public health threats due to a changing climate, and to provide continued environmental, social, economic and health benefits to the community.

Tree Care for Existing Trees

Join the PLAY Boulder Foundation Tree Trust and become a Tree Tender to support tree planting, maintenance and preservation efforts on private property.

Tree Trust

Learn more about proper planting and tree care techniques.

Tree Planting, Care, and Watering

Email the Boulder Forestry team if you are treating a public ash tree adjacent to your home.

Treating a Public Ash Tree Form

Contact a City of Boulder licensed certified arborist to help you with your tree care needs.

Arborists and Tree Contractors

Growing New Trees

Don’t just plant a new tree, grow a new tree! Newly planted trees require care, especially for the first few years, to ensure they get established in our harsh urban environment. Proper planting, ongoing watering, and the maintenance of an organic mulch ring help ensure the survival of new trees.

Request a Tree