For 22 years, Linda Livingston watched three ash trees grow in her yard in Broomfield.
“We knew that they were at considerable risk,” Livingston said. “You look at the beauty of those trees and how long it took them to grow, and you really don’t want anything to happen to them.”
When emerald ash borers were first detected in Boulder County in 2013, she enlisted the help of a tree care service to determine if her trees were at risk.
Livingston learned that her trees weren’t infested, so she began a series of pesticide treatments to preemptively deter the destructive pests.
“We’re all for doing whatever we can, hoping it works and we can maintain them,” she said. “They’re majestic. We’ve really cared for them.”
The level of commitment to protecting ash trees varies in Boulder County, said Dave Walker, who owns Walker Tree Care.
Pesticide treatments, which are priced based on the diameter of the trunk, can cost up to $18 per inch. Some homeowners weigh the cost of treating their ash trees against the cost of having them removed, he said.
“Some people have really valuable trees,” Walker said. “If you aren’t particularly attached to it, and you’re willing to take it down, then that might be a better option because it could be more economically feasible for a lot of people.”
And the pests don’t disappear after one round of pesticide treatments, he said. Ash trees need to be treated every two years in perpetuity, and not everyone can commit to a lifelong fight against emerald ash borers.
“If you have an old ash tree, a nice ash tree, it’s going to be a liability for you for years to come if you want to keep that tree,” Walker said.
Slowing the spread
Boulder County’s ash trees have been under siege by emerald ash borers since the invasive beetles were first detected in the city of Boulder in Sept. 2013.
These pests spread through the transportation of infested firewood or nursery stock.
Ash trees comprise 12 percent of Boulder County’s public tree inventory. The loss of such an abundant tree species threatens the ecology of the entire county, experts say.
“With recent floods, if we lose 12 percent of our urban canopy, our water runoff issues will get severe,” said Tom Read, who works in the forestry section of Boulder’s Parks and Recreation Department.
Urban tree canopies reduce runoff during rain events by intercepting rainwater before it reaches the ground. Ash trees also provide more shade and live longer than other species.
County officials recommend homeowners develop a plan to manage infestations and care for ash trees, but they offer few services for those trying to do so.
“We don’t intervene on private property at all,” said Brett Stadsvold, the emerald ash borer coordinator for Boulder County Parks and Open Spaces. “In terms of controlling the spread from the county level, it’s pretty difficult. I think it’s an effort that everyone has to be involved in — public agencies as well as private property owners.”
For homeowners, he said, the most important step in managing emerald ash borers is detection. These beetles spread quickly from tree to tree. Stadsvold said by the time ash trees start showing symptoms, the borers have moved on to other trees.
Emerald ash borers haven’t yet been detected outside of the city of Boulder, but Stadsvold they’ve likely spread to other areas in Boulder County.
Method of attack
The symptoms of emerald ash borer infestations are sudden and severe.
Starting midway up the crown, the beetles lay eggs beneath the bark of ash trees. The larvae from these eggs then feed off the layer of tissue that controls the flow of water and nutrients in the tree.
This causes the canopy to wither, and the tree gradually dies from the top down, Read said. The definitive sign of an emerald ash borer is a D-shaped exit hole that’s about one-eighth of an inch in diameter.
The lifespan of a tree once emerald ash borers have moved in varies depending on the tree’s size. Emerald ash borers can destroy small trees at a faster rate than full-sized trees, Read said, because they usually attack the trunk and the crown simultaneously.
Treating ash trees
Boulder County officials have attempted to curb the number of emerald ash borer infestations by applying pesticide treatments to ash trees on public land.
Following the detection of emerald ash borers in the city of Boulder, a group of specialists called the Colorado Emerald Ash Borer Response Team began mapping infestations in the city of Boulder.
The group initially detected emerald ash borers in the center of town, but in the past two years, the pests have spread throughout city.
“If you have an ash in the city of Boulder, it’s at risk,” Read said.
They treated 1,500 of the 6,000 ash trees on public property in Boulder. The 4,500 trees that didn’t receive pesticide treatments will have to be removed at a projected cost of $1.65 million, said Boulder City Forester Kathleen Alexander.
“We want to slow down that progression because of the economics and also because we don’t want to lose all of those environmental, social services that the canopy is producing as well,” she said. “The longer period of the time that we can spread out the progression of emerald ash borer, the less it’s going to cost us on an annual basis.”
In 2014 and 2015, the city of Boulder removed 192 declining ash on public property. Officials plan to remove an additional 50 this winter. Alexander said the ash trees that were removed are being replaced with species that have fewer pest issues.
In 2014, the city of Boulder began a three-year cycle of pesticide treatments that wraps up in spring 2016. The goal was to slow down the spread of emerald ash borers and reduce the annual cost of tree removal.
The city of Boulder also partnered with the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service to study how parasitic wasps impact the spread of emerald ash borers.
Researchers from the federal government released parasitic wasps in heavily infested areas of Boulder for the first time in Sept. 2014. The wasps are host specific, which means they only feed on emerald ash borers.
“If these wasps can really be established and control EAB populations long term, we may not have to use pesticides as much,” Alexander said.
Emerald ash borers are native to China, where Alexander said ash trees have more natural resistance because of the parasitic wasps. The wasps lay eggs on the borer larvae, which then become food for the larval wasps.
Emerald ash borers are regulated on the federal and state level. Under the Pest Control Act, the federal government instituted a quarantine that prevents travelers from carrying ash wood across state lines.
State officials enacted a similar quarantine that prohibits the movement of ash wood between counties.
“The wood is host material, and it’s risky,” said Laura Pottorff, the state’s quarantine manager. “If you think about how emerald ash borer got to Boulder, it was more than likely in wood.”
Those who attempt to move firewood face a fine of $1,000 per violation, but Pottorff said enforcement varies.
“We have a choice as to how we define a violation,” she said. “It could be per piece of wood that leaves. Or nursery stock trees can’t leave the quarantine area. In that case, it would be $1,000 per tree.”
It’s important to monitor the movement of infested ash wood because emerald ash borers spread quickly. Pottorff said Colorado is a hot spot for their dissemination because visitors often bring their own firewood on camping trips.
Students and tourists, she said, are likely the reason for the high concentrations of emerald ash borers in Boulder.
“We can’t prove it, but it would appear that the wood that started this may have been brought in by someone with connections from the Midwest,” Pottorff said.
“When you have part of the state that’s attractive to folks from other parts of the U.S. that have pest problems, then the pests tend to follow people,” she said. “That’s just the way it goes.”