Emerald ash borer has led to removal of 7,000 ash trees in Eau Claire

May 5—EAU CLAIRE — A tiny green insect is making a giant-sized impact on Eau Claire's landscape.

The emerald ash borer was first spotted in the city in December 2017 in a dying ash tree near Water Street on the UW-Eau Claire campus.

Eau Claire forestry supervisor Matthew Staudenmaier knew the discovery, which officials had been dreading but expecting, meant the city's tree population was in for a massive shakeup.

Scientific models at the time showed the infestation of metallic green beetles would lead to mass die-off of ash trees in about five years.

"Now you can see many trees are heavily infested," Staudenmaier said. "We're at that point now where most ash trees that still exist on boulevards or in people's backyards or in parks, if they haven't been treated, will show real signs of emerald ash borer."

The city's preemptive approach to the ash borer's arrival has involved mostly removing and replacing ash trees, which made up about 35% of Eau Claire's forest cover when the program began in 2013.

As a result, city crews have removed nearly 7,000 ash trees since that time, trimming the population from 9,800 to about 3,000, Stoudenmaier said.

A check this week of the city's updated inventory showed ash now account for 12% of the more than 28,000 trees managed by the city in boulevards, parks and public spaces.

"We're steadily taking them down," Stoudenmaier said. "We've been mostly doing it by neighborhood, and I think we're just about through every neighborhood in the city in the first round."

In the initial cycle, the city identified ash trees in boulevards and notified property owners of their presence and gave them the option of having the trees removed at no cost.

Some homeowners chose to have them taken down, some chose to treat them and others didn't respond or chose to do nothing.

"Now we're seeing that many of those ash trees still standing are dying," Stoudenmaier said, noting that at some point city crews will have to remove trees that reach the point where they become a safety hazard.

Evidence of infested ash can be found all over town.

"Anywhere you see blanched bark and bark that looks like it's falling off, you can pretty much be sure those are ash trees and that woodpeckers are going after emerald ash borer larvae in the tree," Stoudenmaier said.

Growing diversity

As the city replaces ash trees, it is working toward greater diversity in the urban tree population in the hope of keeping Eau Claire green when insects, diseases or other threats target specific kinds of trees.

The goal is not to have any species account for more than 10% of the urban forest so any future disease can't damage the appearance of neighborhoods as much as emerald ash borer.

"If just one of 10 trees is affected, then the damage is not nearly as palpable as when an entire street is decimated," Stoudenmaier said, adding that some formerly tree-lined streets in Eau Claire lost all their mature trees to the disease. "We're trying to get ourselves to a place where our community is more resilient to the next big thing."

However, reaching the 10% mark will take a long time given the large number of maples in Eau Claire.

Maples, by far the most common trees in Eau Claire's public spaces, comprise 33% of the city's canopy. But with no current threat to maples, city officials don't plan to cut them down just to meet the goal. Instead, plans call for thinning their numbers through attrition and by limiting any planting of more maples.

All other trees already are below that 10% target in the city. Among the leading varieties are linden at 9%, oak at 7%, elm at 5% and hackberry and locusts at 3%.

Backyard beetles

The pesky emerald ash borer, of course, pays no attention to which trees are on public or private property, so the infestation problem is likely at least as bad on privately owned land where city workers aren't monitoring the health of trees, Staudenmaier said.

"If people haven't been treating their ash trees regularly, it's not a matter of if, but of how soon, they will die," he said.

Ultimately, the pest is expected to cause more than 99% mortality of ornamental and forest ash species, according to the state Department of Natural Resources.

"It's very sad. Ash trees are one of my favorite trees," said Margaret Murphy, a UW Extension horticulture outreach specialist serving Eau Claire, Chippewa and Dunn counties.

Murphy said she occasionally gets calls from Chippewa Valley residents concerned about ash trees that look stressed. If it is determined that the problem is caused by the ash borer, Murphy said she talks to people about their options: having the trees removed, treating them or doing nothing immediately and thus likely just putting off removal.

While annual insecticide treatment can be effective at keeping the bugs at bay, it can be costly, so homeowners should consider whether the tree's value outweighs the continuing financial cost, Murphy said.

Ash borer populations can increase rapidly, as each female beetle lays between 40 and 200 eggs, the DNR reports. Larvae then tunnel beneath the bark for one or two summers. The tunneling creates squiggly channels that cut off a tree's water movement and lead to its death.

Woodpeckers often remove an infested tree's outer bark to eat the ash borer larvae. Ash that already have endured heavy woodpecker damage can't be saved.

In a cruel twist of fate, Murphy noted, many people across the country planted ash trees to replace elm trees lost to Dutch elm disease, a fungal infection spread by bark beetles that decimated elm populations throughout much of Europe and North America in the 20th century.

The ash borer, which is native to China, is believed to have entered the U.S. on packing material, first showing up in 2002 in Michigan. The bug, which can emerge from wood for up to two years after a tree is cut down, was first seen in Wisconsin during 2008 in Ozaukee County.

Its persistence, Murphy said, serves as reminder that the DNR urges people not to move firewood long distances. Instead, the agency advises folks to obtain firewood close to where it will be burned or use certified pest-free firewood.