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A benefit of extreme cold: It kills the dreaded emerald ash borer

The emerald ash borer is a small, metallic green beetle native to Asia that is about 1/2 inch long and 1/8 inch wide. Special to The Forum1 / 2
Extreme cold temperatures of 20 below or 30 below prove fatal to the invasive emerald ash borer, which first was detected in Minnesota in 2009. The insect has been found as close to Fargo-Moorhead as northwest of the Twin Cities and in Winnipeg, but so far has not been found in northwest Minnesota or North Dakota. Forum News Service file photo2 / 2

MOORHEAD — Here’s a warm thought for ash tree admirers as the region finds itself coping with an arctic deep freeze: Temperatures that plunge far below zero can be fatal for emerald ash borers — mortal enemies of the popular trees.

Researchers in Minnesota have established that when temperatures drop to around 20 below zero, about half of emerald ash borers, which burrow into ash trees during their larval stage, are killed. The mortality rate can reach 90 percent when the temperature reaches 30 below.

There’s a good chance the current cold snap — with widespread temperatures colder than 20 below and even 30 below — has killed a significant portion of the emerald ash borer population in Minnesota, entomologists and biologists said Wednesday, Jan. 30.

So far, no ash borers have been confirmed in the area surrounding Fargo-Moorhead — but they have been found as close as northwest of the Twin Cities in Wright County, Minnesota, and in Winnipeg.

In Minneapolis, the official low temperature reached 28 below zero early Wednesday morning, and was 20 below or colder for hours.

“That’s definitely going to have negative effects on the emerald ash borer,” said Jeff Hahn, a University of Minnesota extension entomologist in St. Paul.

To have a lasting effect in controlling the ash borer, an invasive species from Asia that first was discovered in Minnesota in 2009, temperatures would have to drop to extreme cold levels regularly, Hahn said.

“If we were to get these temperatures every year, that would help to limit them,” he said. “That’s not to say you’ll never have them, but that certainly helps with the emerald ash borer.”

Further north, in Fargo-Moorhead, hitting those extreme cold levels more frequently might help to restrain the population, if it reaches the area, said Rob Venette, a research biologist with the U.S. Forest Service in St. Paul.

“I think Fargo is really pushing the limits of our research,” said Venette, who's a Fargo native.

In Fargo, Wednesday’s official low temperature was minus 33, and temperatures stayed 20 below or colder most of Tuesday and Wednesday.

“It’s definitely going to help ash trees,” said Venette, referring to the widespread polar cold air mass. “This is going to set back emerald ash borer populations, and in some places quite a bit.”

The tree bark provides some insulation for the ash borer larvae, enabling some to survive. Also, some pockets of relative warmth in areas that are sheltered could enable some to survive the intense cold, Venette said.

“Wind chill doesn’t matter for the emerald ash borer,” he said.

The ash borers in Winnipeg are believed to have gotten there through shipments of firewood or other wood imported from infested areas, rather than through natural migration, Hahn said.

In Minnesota, the progression of the ash borer has been more limited than in some other states, likely because the invasive species was detected at an early stage of the infestation, Venette said.

“We found it earlier than many,” he said. The emerald ash borer was first detected in Michigan in 2002.

Since then, there has been an aggressive response to limit the spread, he said.

Still, Venette added, “We really hate to say that an area is out of the woods.”

Patrick Springer

Patrick Springer first joined the reporting staff of The Forum in 1985. He can be reached by calling 701-241-5522. Have a comment to share about a story? Letters to the editor should include author’s name, address and phone number. Generally, letters should be no longer than 250 words. All letters are subject to editing. Send to

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