Recent sub-zero temperatures are expected to kill a significant portion of emerald ash borers in the state, according to the Minnesota Department of Natural Resources.

Emerald ash borer larvae spend winter under the bark of ash trees. Their larvae generate chemicals to prevent their bodies from freezing at the normal freezing point. However, they can—and do—freeze when temperatures reach the extreme lows seen across Minnesota in the past few days.

“When temps get to 30 below zero Fahrenheit, 70-90 percent of larvae may be frozen,” said Rob Venette, a USDA Forest Service research biologist. “[But] the precise relationship between cold and emerald ash borer mortality changes a bit from year to year.”

The last polar vortex, in January 2014, offered a good example, when air temperature in the Twin Cities fell to 23 below zero Fahrenheit. Samples taken from cut logs and standing trees that had been outdoors during the winter showed 60-70 percent of larvae had been killed in most locations, according to Venette.

The effect of this week’s cold temperatures on emerald ash borer larvae population has yet to be seen.