Sub-zero temps kill emerald ash borer

You probably didn’t jump for joy when sub-zero air burned your cheeks this morning. And with good reason: these temperatures, ranging from -20° F in the metro to -40° F in northern Minnesota, pose a danger to us. But there is a silver lining: we can expect this cold to kill a significant portion of emerald ash borers (EAB) in the state.

Emerald ash borer larvae overwinter under the bark of ash trees. Their larvae use one of Mother Nature’s more fascinating mechanisms to supercool: they generate chemicals to prevent their tiny bodies from freezing at the normal freezing point. It’s similar to the effect of salty compounds on winter roads. Still, they can—and do—freeze when temperatures reach the extreme lows we’ve seen across Minnesota in the past few days.

“When temps get to -30° F, 70-90 percent of larvae may be frozen,” says Rob Venette, a USDA Forest Service research biologist. “[But] the precise relationship between cold and EAB mortality changes a bit from year to year.”

The last Polar Vortex, in January 2014, offers a good example. The air temperature in the Twin Cities fell to -23° F that year. 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, Venette says.

So, this year’s Polar Vortex is good news for ash in Minnesota!

In case you’re wondering: the extreme cold doesn’t have much of an effect on Minnesota’s native forest pests. They are well adapted to our cold winter temperatures. The eastern larch beetle, for example, can survive down to -56° F as larvae and -43° F as adults. In addition, many adult larch beetles overwinter under the snow line, which insulates them against low air temperatures. The eastern larch beetle has wiped out hundreds of thousands of acres of tamarack in Minnesota over nearly two decades, but it will take more than cold to stop it.

While the cold won’t do much for our tamarack, it will help our ash. So, the next time you step outdoors and feel your eyelashes freeze, remember your suffering is not in vain.