Ash tree-killing bug inches toward Carlton County

Mar. 19—CARLTON COUNTY — Emerald ash borer was discovered in 2022 in a small area ranging from Cloquet to Jay Cooke State Park but has yet to be confirmed in the rest of the county.

However, with the recent discovery of the destructive pest in neighboring Pine County, local officials are worried the invasive pest could be closing in.

In the past month, the Minnesota Department of Agriculture confirmed emerald ash borer was located in Rock Creek along state Highway 70 in southern Pine County. This northward migration now threatens the vast ash forests of Carlton County, where the black ash tree plays a vital role in the local ecology.

"I've been hoping that we had more time before we began to see emerald ash borer and black ash forests in northern Minnesota," said Mark Abrahamson, plant protection division director for the Minnesota Department of Agriculture.

The emerald ash borer, native to Asia, was discovered in the United States in 2002 and in Minnesota in 2009. It preys on native ash trees by laying larvae that tunnel under the bark and feed on the tree, eventually killing it. Signs of infestation include increased signs of woodpeckers, which feed on emerald ash borer larvae; sprouting from the stump; D-shaped holes; and S-shaped galleries beneath the bark.

In Carlton County, black ash trees play a critical role in the hydrology of northern forests because of their ability to exist in extremely wet conditions, which helps stabilize the water table.

Left unmanaged, the sections of forest that were once dominated by black ash would slowly succumb to a brushland habitat, said Tim Byrns, forestry specialist for the Carlton Soil and Water Conservation District.

"It's a certainty that it's here and it's a certainty that it's going to kill all the black ash trees inevitably," Byrns said.

Eric Otto, forest health specialist for the Department of Natural Resources in Northeastern Minnesota, said this threatens Carlton County and all of northern Minnesota, home to 900 million black ash trees.

The urgency is exacerbated by the fact that the beetle's spread appears to be accelerating.

Though emerald ash borers have devastated the ash tree population across the eastern half of the U.S., it has been slow to overwhelm Minnesota. For the first five years emerald ash borer was in Minnesota, Abrahamson estimates the rate of spread was 20% of what would have been expected elsewhere in the country.

This is partly due to Minnesota's natural advantage over other states: frigid winters. The emerald ash borer, being non-native, has not evolved to endure the arctic cold blasts that define Minnesota winters. When the temperature drops to 30 below zero for a sustained period, it kills the larvae.

However, the warming winters could cause Minnesota to lose its natural edge.

"Once we lose the extreme cold weather, then we rapidly start to lessen our hopes that the (emerald ash borer) is going to be controlled in any way naturally," Byrns said.

The warming winters coincide with a rate of spread that is now comparable to rates seen elsewhere in the country, said Abrahamson, with infected Minnesota counties doubling from 25 to 50 in the last three years alone.

The increased rate of spread marks a change in how the Minnesota Department of Agriculture is instituting quarantines, which regulates the transit of logs, chips, mulch and waste derived from ash trees, and all non-coniferous firewood.

When emerald ash borer was discovered in Carlton County, the spread was still slow enough that the Department of Agriculture felt comfortable limiting the quarantine to only the northeastern portion of the county, Abrahamson said. Now, the rate of spread is so fast that when emerald ash borer is discovered, entire counties go into quarantine.

Full infestation takes a long time — up to 10 years for trees to start showing signs of infestation. Therefore, once an infestation is discovered, it is assumed the scope of the infestation is wider than just the trees showing symptoms.

"It could almost be in every county in Minnesota. It's just very difficult to survey throughout northern Minnesota," Otto said.

In Cloquet, city officials are already dealing with the realities of an emerald ash borer infestation.

The city has removed 32 ash trees identified as infested and is planning to target 150-170 more trees, said Joe Hafner, a technician leading the city's emerald ash borer response.

Unlike neighboring forests, Cloquet's ash trees are largely green ash because of their popularity as a boulevard tree. Hafner believes that the city could have as many as 1,000 ash trees.

Because it was only recently discovered, Hafner believes the emerald ash borer situation is largely manageable. However, the city is trying to stay ahead of what Hafner calls the "death curve" — when the beetle establishes itself, rapidly spreads and kills trees faster than officials can respond.

"If we take a pretty quick proactive approach to try to get after this quickly, it won't be such a drastic die-off of all the trees," Hafner said. "Because at the point where it gets to the top of that death curve, trees will be dying so quickly, it'll be hard for our city crews to deal with them."

If northern Minnesota succumbs to the emerald ash borer, forest managers will need to find a successor that can play the same critical role as the ash tree. Burr oak, red maple, hackberry, peach-leaf willow and swamp white oak have been considered as replacements for Carlton County forests, according to Byrns.

Though the long-term trajectory of ash trees in Minnesota does not look good, there are still hopes that the state can bide enough time to get a handle on emerald ash borer and contain the spread.

"It is generally a depressing story, but I think there are still some glimmers of hope and there are things that can be done to try to make circumstances work better in our favor," Abrahamson said.

Pesticides can be used to treat ash trees. However, these are better used by landowners looking to protect their trees because it is too costly and impractical to implement in wild forests, said Byrns. There are also biological controls such as parasitic wasps, Otto said, which prey on emerald ash borers and have been deployed in Cass County and southeastern Minnesota.

However, the return of northern Minnesota's freezing winters would really slow the insect's spread.

"I'm going to keep some optimism that things may still play out differently in the north, we can get some cold winters," Abrahamson said, "... that will have an impact and at least make things take longer than they would otherwise."