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Polar Vortex Could Hit Tree-Damaging Insects Hard

Feb. 1, 2019, 11:01 AM

The bitter cold that’s settled in across the Midwest isn’t just bad news for humans—it’s also killing numerous insects that harm forests and crops.

The extreme temperatures that moved in this week may kill a high percentage of emerald ash borer larvae, according to entomology and forestry specialists. Since its discovery in 2002, the invasive beetle has killed an estimated 50 to 100 million ash trees in the U.S. and Canada, according to the U.S. Forest Service.

In some northern states we could see about 80-percent mortality,” expected when temperatures dip to minus-20 degrees Fahrenheit, said Rob Venette, a research scientist with the Forest Service in St. Paul, Minn.

The polar vortex “will be a big benefit for people trying to manage this infestation,” he said.

Early on Jan. 31, Minneapolis was -23 degrees Fahrenheit and Chicago was at -21, according to the National Weather Service.

While a large number of ash borers won’t make it through the winter, “it’s not going to eliminate all the bugs everywhere, said Lee Frelich, director the University of Minnesota Center for Forest Ecology.

Frelich said that in places where temperatures get cold enough, as much as 75 percent to 90 percent of the population could be killed. Those levels are enough that it would take the tiny green beetles several more years to build back up.

The caveat, he said, is that it has to be really cold: “It has to be what we call ‘Minnesota cold,’ not Michigan cold or Ohio cold.”

Numerous Factors

And it was “Minnesota cold” in many cities this week. Still, the temperatures required to kill the emerald ash borer depends on a number of other factors.

“The temperatures on the weather networks are not what the insects experience under the bark on trees, and wind chill doesn’t affect them at all,” said Brent Sinclair, a biologist specializing in cold-weather insects at Western University in Ontario.

According to Sinclair’s research, the emerald ash borer can generally withstand temperatures well below zero, but once it gets down to minus-30 Fahrenheit, 90 percent of individuals will die.

“They’re actually very cold-tolerant,” he said, “but in the early stages of infestation, when they live high up in trees, or on smaller branches, they’re more susceptible to cold.”

According to Sinclair, chemical management for the emerald ash borer has proven effective in protecting smaller numbers of ash trees, but it’s not an economically sustainable solution for protecting large numbers of trees or forests.

“It can cost upwards of $100 per tree,” he said, “and takes a lot more work than spraying because you have to drill into the tree and insert the insecticide under the bark.”

Giving Trees ‘Several More Years’

The ash borer has now reached as far north as Winnipeg, Manitoba. The city started battling the insects even before discovering the first beetle last year.

City officials estimates the ash borer could cost Winnipeg as much as $90 million over 10 years and devastate the tree canopy, which includes more than 350,000 green ashes.

“But we’ve had several of these big polar freezes in recent years, and each one buys trees several more years,” said Lee Frelich, director of the University of Minnesota’s Center for Forest Ecology. “That, combined with treatment, could be enough to manage some urban areas.”

Frelich said new research suggests that extreme cold events might impact other invasive species even more, such as the Asian long-horned beetle and the hemlock woolly adelgid.

Besides firewood, ash was once used commonly in things such as tool handles and baseball bats, but that might be changing as well.

“We’ve accepted the fact that the [emerald ash borer] is killing off ash and we need to move to other species of wood to make Louisville Slugger bats,” said Rich Redman, a spokesman for Hillerich & Bradsby Co., which makes Louisville Slugger bats.

“Maple currently makes up about 75 percent of our wood bats, birch about 10 percent, ash makes up the rest,” he said.

To contact the reporter on this story: Adam Allington in Washington at

To contact the editors responsible for this story: Gregory Henderson at; Steven Gibb at; Chuck McCutcheon at