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Cold weather could reduce — but not eliminate — ash borer’s spread

January 29, 2019

The bitterly cold weather on the way this week for southern Wisconsin has at least one upside: It’s even more dangerous to a certain invasive insect than to humans.

The emerald ash borer has spread to the majority of Wisconsin counties and become endemic in Dane County since it was first discovered in Michigan in 2002. Its larvae kill ash trees by burrowing into and feeding on the trees’ trunks, cutting off pathways for water and other nutrients the trees need.

But while the larvae can survive temperatures well below zero, they start to die once temperatures inside the trees start to reach about minus 20 Fahrenheit, according to research released in 2014 by the U.S. Forest Service.

“A lot of the emerald ash borers will be killed by the frigid weather,” said Andrea Diss-Torrance, a state Department of Natural Resources forest health entomologist. “But a lot of them won’t be.”

Measurements from the Twin Cities after a January 2014 cold spell that got as low as minus 23 found that between 60 percent and 70 percent of larvae found in log samples were dead.

Madison is expected to be in that same range Tuesday and Wednesday nights, when the National Weather Service on Monday was predicting lows of minus 25 and minus 27, respectively.

Working against a major die off will be the insects’ natural cryoprotectants that shield them from cold weather, the insulation provided by the trees themselves and any sunlight that will warm up the trees during the day, according to Diss-Torrance.

Emerald ash borer was first confirmed in Madison in late 2013 and city officials launched a policy soon after that includes treating some ash trees with insecticide and removing others already too damaged to save and replacing them with different tree varieties.

A partial die-off of the insect can help tree owners who are looking to save ash trees with insecticide treatments, Diss-Torrance said, in that “it will make the pressure on the tree a little less.”

The biggest factor in the survival and spread of the ash borer remains human activity — specifically, moving ash-borer-infested firewood from one area to another. The insect is not able to move very far on its own.

Once the insect had been found in two-thirds of Wisconsin’s 72 counties, the entire state was placed under quarantine in March by the state Department of Agriculture, Trade and Consumer Protection.

A quarantine means businesses handling ash wood, untreated ash products and hardwood firewood have to sign agreements on how they reduce the risk of moving the bug into non-quarantined areas. It also prohibits moving firewood from quarantined to non-quarantined areas.

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