They’re flashy, they’re hungry, they’re relentless, and they’re here.
The long-expected, long-dreaded arrival of emerald ash borers in Mankato was officially verified in December. When they finish doing what they do, about one of every six trees in the community will be gone and so will a chunk of the bank accounts of many property owners.
In Indiana, where the infestation already has largely run its course, Purdue University Extension described emerald ash borer as “the most destructive forest pest to enter North America” — one that “has left hundreds of millions of dead ash trees in its wake.”
Jud Scott, a registered consulting arborist in Indianapolis, has plenty of experience with the creature Mankato area residents will get to know in the decade to come.
“It’s the most beautiful little bug you’ve ever seen,” he said of the invasive species from Asia.
And Scott provided a not-so-pretty preview of the decisions facing south-central Minnesotans who have ash trees in their yard, beside their business or in their grove.
“Basically, if you don’t treat ‘em, you’re going to lose ‘em.”
The inevitable arrival
In January 2022, Justin Lundborg still hoped it would be a year or more before EAB was found in Mankato. But Lundborg, a natural resources specialist for the city of Mankato, also knew there was a high probability the pest was already here.
First spotted in Michigan in 2002, hopes of containing it there were quickly dashed. By the end of 2020, the pest had been confirmed in 35 states, including virtually every state east of the Rocky Mountains.
Infected trees had been found in 25 Minnesota counties, leaving Mankato surrounded by quarantined counties to east, west, north and south of Mankato — Steele, Martin, Scott and Brown.
It was 2020 when Mankato crafted an Emerald Ash Borer Management Plan, which warned of the impending devastation to the urban forest.
“With an estimated 17,400 ash trees on public and private properties combined in Mankato — including 2,500 in boulevards, parks and city-managed properties — the city is at risk of losing approximately 17% of its existing tree population in the next 10 years,” the report stated.
In March 2021, Blue Earth County was added to the state’s quarantine list when EAB was found in St. Clair. Mankato began picking its 300 or so most valuable ash trees for possible periodic treatment with insecticide and kicked off a program of cutting down another 200 or so lesser-priority public ash trees each year.
December provided evidence that there was no time to spare. That’s when EAB was confirmed in three trees in two neighborhoods adjacent to downtown — near the intersections of Second and Warren streets and Record and Fulton streets. North Mankato got its bad news in February after the bug was found on a tree on Jefferson Avenue just west of Range Street and on another near the intersection of Monroe Avenue and Center Street.
It’s now the time of the year when EAB larvae will be finishing their feasting beneath the bark of ash trees before emerging as adult beetles starting in May. So that’s when the region will likely find out how far along the infestation really is beyond that handful of confirmed cases.
For owners of residential and business property who don’t know a maple from an oak, it’s time to get educated and figure out if any of the trees they own are in the doomed category. Online resources explain how to identify if a tree is an ash by looking at leaves, branches, buds and bark.
So, the first step is figuring out what that behemoth in the backyard is.
“If it is an ash tree, you have some decisions to make,” Lundborg said.
As sparkly as emerald ash borers are, they’re also small — just a half-inch as adults. And while they make telltale D-shaped holes in the bark of ash trees when they emerge as adults, they generally make their debut high in the canopy of a tree, according to Scott, the Indiana arborist and tree service owner.
“The bug starts at the top and it starts in the smooth wood,” he said. “They spend the winter just decimating (the layer just inside the bark).”
The process continues through a second year, and it will often go unnoticed unless a property owner has binoculars and an idea of the bark damage and other signs to look for. In the third year, the effects become more apparent as the upper canopy thins out and the leaves become a much less-vibrant green.
At that point, even people who are not very observant might see — and hear — evidence of a big problem.
“That second or third year they were infected, we started seeing the woodpeckers going crazy,” Scott said. “I like woodpeckers. I said the one good thing about emerald ash borer, the woodpeckers were well-fed.”
By then, treatment might no longer be an option, because the end is near.
“Fourth year, fifth year, you probably have a dead tree.”
The population of Indianapolis is about 20 times that of Mankato, but the two cities have something in common — an urban forest where an estimated one tree out of six is an ash. And Indy is about 16 years ahead of Mankato when it comes to when the emerald ash borer arrived. The bug, which is believed to first have invaded North America via shipping pallets from Asia, was detected in Marion County, Indiana, in 2006 and really took off around 2010 and 2011.
By 2017, most of the ash trees in Indianapolis were gone.
“There was a period of time when it seemed like all we were doing was taking down ash trees,” Scott said.
His company also provides chemical treatment, which has been effective for those property owners willing to pay the ongoing cost.
“As long as we kept treating, the trees were alive,” he said. “I’d say most of the ashes that weren’t treated are gone.”
A moment to decide
Lundborg is essentially the guy in charge of overseeing Mankato’s city-owned trees, including those 2,000-plus ash trees. So people who know what he does for a living are increasingly asking him what they should do with their property’s ash trees.
“Yes, acquaintances, residents, co-workers. It gets to be kind of a hot topic,” Lundborg said. “The biggest advice I have for anybody with an ash is to kind of take stock of what you have.”
After first verifying the species, it’s time to assess the relative value of the ash trees.
Treatment might make sense for a really vibrant tree, one with room to grow, one that’s providing shade to a home or yard, one with a lot of aesthetic value.
“If it’s healthy and it’s appropriately sized — medium aged, that’s one that might be a candidate for treatment,” he said.
For smaller ash trees, generally under 20 inches in diameter, people can treat their own with store-bought insecticides, Scott said. But people need to be meticulous about using the recommended amount and avoid the more-is-better approach that is “kind of a guy’s mentality,” he said.
“You want to read the label,” Scott said, both to effectively prevent or end an infestation of EAB and to avoid killing beneficial insects such as bees and other pollinators.
For people who don’t want to mess with it or have large trees, a professional is better.
“Call a certified arborist,” he said, noting that chemical treatments are a ripe target for scam operators. “You don’t know what I’m pouring down. I could be pouring water. So definitely a reputable arborist.”
Treatments, which should occur in the spring if possible, will need to be repeated every two years, although the gap could potentially be lengthened in future years. At that point, when the vast majority of ash have succumbed, there will be fewer beetles around to pester the treated trees that remain.
Still, it’s an ongoing expense. One of the first customers who hired Scott to treat a valued ash tree decided, after more than a decade of treatments, to sacrifice the tree anyway: “He said, ‘I’m sick of spending money. Let’s get rid of it.’”
Scott strongly advises against treating trees that are more than 30% infested. And he suggests customers not undertake the expense if a tree doesn’t have room to grow for more than a few years anyway.
Every situation is unique, though. He had one customer who insisted on treating a beloved tree even after Scott said its condition left it unlikely to still be around in five years: “He said, ‘I have terminal cancer. I won’t be here in two years.’”
Looking at it strictly as a financial consideration, Scott said it typically works out to about seven years. That’s roughly how many years it takes for the cost of treatment — which increases with a tree’s size (just like the price of removing a tree) — to match the cost of removal.
Another approach is to treat just long enough for a replacement tree to grow up a bit: “Let’s plant a new tree over here. Let’s treat for a couple of years until the new tree gets established, and then get rid of the ash tree.”
Time is money
For an ash tree that’s not in great condition, is poorly located or is old enough that it’s probably in the twilight of its life anyway, the better choice might be to remove it. And there’s no point in waiting, Lundborg said.
“If you’re going to have it removed, the sooner you remove it, the cheaper it will be.”
In one sense, not even counting inflation, the cost of removing a tree increases over time just because trees grow larger, and larger trees cost more to remove. Hiring a professional to remove an average-sized tree can carry a price tag of $1,000 or more, depending on accessibility, and it can be several thousand for a colossal tree.
But Lundborg’s prediction about the cost of waiting is more about the inflated expense that comes with removing a dead tree.
The branches of ash killed by emerald ash borer are notoriously prone to snapping.
“They become dry and brittle, so they become more dangerous to remove,” Lundborg said. “The price can easily double if they’re in that condition.”
He predicts, too, that once EAB spreads widely across Mankato-North Mankato and other nearby cities, the demand for tree removal or treatment is likely to outstrip the supply.
“It will become difficult finding a company,” he said.
Jacob Schauf, a certified arborist with Carr’s Tree Service, isn’t worried about a shortage of tree contractors when the infestation reaches its peak in Mankato.
After all, the beetles are moving in stages across the continent — and across the state. The arborists and the chainsaws can move with them.
“There are a lot tree services in the area,” Schauf said, adding that Carr’s — which operates throughout much of Minnesota — is regularly adding trained staff to meet the demand of public tree removal and the needs of private property owners.
“Our plant health care unit is really growing rapidly,” he said. “There’s definitely no shortage on our end.”
But he agrees with Lundborg that a dead tree will drive up what a tree service charges for removal.
“They tend to be messier and a little more hazardous,” Schauf said. “You have to handle them with a little more care.”
Failing to remove a tree before it dies also carries the potentially high cost of legal liability if the tree is in a spot where falling branches could cause injury or property damage.
“It all comes down to risk, in my opinion. That’s one of the biggest factors,” Schauf said.
Ash trees in a wooded area in the backyard can more safely be ignored if they’re not in a place where people hang out.
“If there’s a tree in the middle of the woods, you might not ever have to worry about it,” he said. “But is there a big tree next to a trail? Is there a playground? Are you parking a car under that tree?”
Waiting to ask those questions, choosing to worry about it next year, no longer makes a lot of sense for an ash tree owner in the Mankato area, according to Schauf, pointing to the latest updates to the Minnesota Department of Agriculture’s map of where EAB has been documented.
“Really in southern Minnesota, southeastern Minnesota, it’s pretty much everywhere.”
Scott, having watched EAB run roughshod over Indiana, said the borer is anything but boring in its impact. The “most beautiful little bug you’ve ever seen” turns ash trees — a species with wood so tough it’s used for baseball bats and ax handles — into a desiccated thing that will drop large branches on a windless day.
And the beetles are equally adept at eating through the budgets of cities, businesses and homeowners in the process.
The residents of Mankato and North Mankato, having verified the bugs in December and February, might not have more than a year or two before they get to see it all for themselves, Scott said.
He recalled when he first spotted EAB in the northern Indianapolis suburb of Carmel.
“It kind of exploded within three to five years,” he said. “It blows up pretty quick.”
Property owners in towns north and south of Mankato are already getting a sense of that. Two years after EAB was first discovered in St. Clair, the beetles have now been found on 24 trees within or just outside the city limits. And less than eight months after the initial case in the city of Le Sueur, the number of infested trees has grown to 18.
The Department of Agriculture’s online map of verified cases has detail down to the location of each infested tree for people who zoom in on a town and neighborhood.
Michigan State University, supported by the U.S. Forest Service, created the Emerald Ash Borer Network to provide a broad range of information about EAB, including a section specifically for homeowners.
The Minnesota Ag Department also has comprehensive information about the infestation targeted toward homeowners.
The University of Minnesota Extension service has advice on hiring a tree service.
Hennepin County created a guide to help people decide between treatment or removal of a particular ash tree:
Michigan State’s Extension service created an ash tree identification guide that’s so comprehensive the U of M’s Extension service adopted it.
People who believe they have an infected ash tree should contact a tree care professional or the state agriculture department at firstname.lastname@example.org or at 888-545-6684.