In northern Minnesota, researchers and foresters prepare for emerald ash borer invasion

A person uses a draw shaver to strip the bark
Rob Venette, director of the Minnesota Invasive Terrestrial Plants and Pests Center at the University of Minnesota and research biologist with the USDA Forest Service, uses a draw shaver to strip the bark from an ash tree that was showing signs of being impacted by larvae of the emerald ash borer on March 4 near Remer.
Derek Montgomery for MPR News

Since it was first discovered in a St. Paul neighborhood in 2009, the invasive emerald ash borer has slowly munched its way northwestward across the state, and has now reached the doorstep of the largest stretch of ash forest in North America.

That has researchers scrambling to learn more about how the invasive pest is faring in Minnesota’s quickly warming winters, and preparing for what might follow an ash borer invasion in the state’s northern forests.

Larvae from an emerald ash borer
The path the larvae carves through the tree is called a gallery.
Derek Montgomery for MPR News

Last fall, a crew from the Minnesota Department of Agriculture spotted the tell-tale signs of an emerald ash borer, or EAB, infestation in the Chippewa National Forest, next to Big Rice Lake near the small town of Remer.

Earlier this month, U.S. Forest Service biologist Rob Venette pointed out black ash trees with their bark split open. Their bark was pockmarked with woodpecker holes — the birds eat emerald ash borer larvae.

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He used a curved draw knife to carefully peel back the bark of an infected tree, revealing smooth tunnels, about the width of a pencil, carved in S-shaped patterns known as “galleries.”

A stand of trees near Big Rice Lake
A stand of trees near Big Rice Lake shows signs of the arrival of the emerald ash borer.
Derek Montgomery for MPR News

“What’s really distinctive about these is they have this serpentine pattern. So they’re feeding back and forth,” explained Venette, who also directs the Minnesota Invasive Terrestrial Plants and Pests Center at the University of Minnesota.

All around this small grove he points out infected trees. Within three years, all will likely be dead. Hundreds of the insect larvae will target a single tree, tunneling under the bark and feeding on the part of the tree that moves nutrients up and down the trunk.

“So by cutting off that vascular system, the tree can’t survive,” said Venette. “Once emerald ash borer shows up in a tree, if no action is taken, that’s pretty much a death sentence for the tree.”

The lines in this ash tree
The lines in this ash tree left by the larvae of the emerald ash borer are called galleries and are seen on an ash tree.
Derek Montgomery for MPR News

Venette uses a smaller knife to pry out creamy white worm-like emerald ash borer larvae he finds, and places them into small tubes. He’ll take them back to the lab to study how well they tolerate extremely cold temperatures.

Studies of EAB in the Twin Cities found that temperatures of negative 30 degrees Fahrenheit will kill 90 percent of the insect.

“And so far, our initial data say that the insects here are similarly tolerant to the cold, which means that this northward movement is not emerald ash borer adapting to Minnesota winters,” Venette said.

Larvae from an emerald ash borer
Larvae from an emerald ash borer in the hand of Rob Venette, director of the Minnesota Invasive Terrestrial Plants and Pests Center at the University of Minnesota and research biologist with the USDA Forest Service.
Derek Montgomery for MPR News

That’s good news. The bad news is that Minnesota’s winters, which have warmed significantly in recent years due to climate change, are not getting consistently cold enough to prevent emerald ash borer from moving north.

“And so all of this work is really meant to help us forecast how far north would we expect emerald ash borer to move,” said Venette. “And once it gets there, how much damage might it cause? And how quickly might those effects be realized?”

‘Smorgasbord of ash’

Those are important questions, because this spot sits on the edge of a vast expanse of an estimated one billion black ash trees that stretches across north central Minnesota.

“What I tell people is EAB has never seen anything like this yet,” said Brian Palik, a research ecologist with the U.S. Forest Service Northern Research Station in Grand Rapids who has studied the insect for the past two decades.

“It’s going to be overwhelmed by the smorgasbord of ash that it sees.”

And if EAB does in fact wipe out this black ash forest over the next several years, scientists say there’s a real concern that the forest won’t grow back. That’s because black ash are uniquely adapted to grow in wetlands. They function like giant straws, and suck water out of the soil.

“So if you remove all that ash in one fell swoop, you’re cutting off that hydrologic pump, and the wetlands get even wetter, to the point that tree regeneration becomes difficult,” Palik explained.

Simulated emerald ash borer attack
U.S. Forest Service Researcher Brian Palik explained how a black ash tree on the Chippewa National Forest in north-central Minnesota was ''girdled'' to simulate being killed by an emerald ash borer.
Dan Kraker | MPR News

What’s likely to grow back instead are cattails, or shrubs, including the invasive buckthorn. And that would mean a loss of important services that forests provide, including carbon storage and wildlife habitat, across 1.2 million acres. It could become an entirely different ecosystem.

For the past several years the U.S. Forest Service has planted other species of trees to see what might be able to replace ash — trees that could also could survive in a warmer climate.

Most promising is swamp white oak, Palik said, which is native to southeastern Minnesota. Seedlings planted about a decade ago now stand 15 feet tall.

“It’s a wetland species. It is projected to have a habitat that’s suitable with climate change. So that’s what we’re pointing people to,” Palik said.

It’s an example of what’s known as assisted migration — planting tree species from a different location as a proactive approach to climate change.

The Wisconsin Department of Natural Resources and the Fond du Lac Band of Lake Superior Chippewa have planted swamp white oak in black ash forests, Palik said. The Minnesota DNR has planted it on an experimental basis.

It would require a massive effort to replant different tree species across 1.2 million acres of northern Minnesota forest. Palik said agencies will likely need to prioritize the most important places to maintain forests.

“It’s not a lost cause,” he said. “The urgency is even higher than it was now that we know it’s right on the doorstep of this resource across the north.”

A stand of trees near Big Rice Lake
A stand of trees near Big Rice Lake that shows signs of the arrival of the emerald ash borer.
Derek Montgomery for MPR News

Black ash is also an important species culturally to many Ojibwe bands across northern Minnesota and Wisconsin. It’s used, for example, to make baskets and lacrosse sticks.

“There’s a threat now that those those cultural practices may disappear,” said Leech Lake Band of Ojibwe forestry director Keith Karnes.

Karnes’ department has worked with other agencies to plant different tree species on about a thousand acres of ash stands. But he said there’s only so much the Leech Lake Band can do. His department only has three staffers.

He said different agencies need to come together to save as much forest as possible.

“You’re going to lose the ash,” he said. “But try not to lose the forest. The work can be done. But it’s got to get done on a grand scale. And it’s got to get done quick.”