Sorry, but the cold likely won't kill off invasive insects

LANSING — Bugs know how to stay warm.

The infamous invasive emerald ash borers creeping their way through Midwest forests likely will survive the cold snap hitting Michigan this week, scientists said. It's unclear whether the weather will be bitter cold for long enough to kill many ash borers or other invasive insects.

What is the ash borer?

The ash borer — a beetle that has killed millions of ash trees since arriving in Michigan and making its way across the country — burrows into insulating tree bark. Brown marmorated stink bugs, also invaders, like to spend the winter indoors. Ticks, not invasive but becoming more widespread in a changing climate, spend their winters in leaf litter or soil.

Bugs also know how to protect against cold snaps, Michigan State University Entomologist Howard Russell said. They concentrate sugars in their bodies that act as an "anti-freeze" to help them get through the winter.

"I think for the most part, insects won't be affected," he said. "They're pretty cold-hardy."

And even if some don't make it, insect populations rebound fast since they can have dozens of offspring every year.

"It doesn’t take long for the population to rebound even if you have some pretty good mortality," MSU Entomologist Deborah McCullough said. 

Creepy crawlies perhaps are more cold-hardy than humans, who are in danger of getting frostbite within minutes of exposure to what National Weather Service meteorologists are calling "a very dangerous and life-threatening arctic blast" threatening the upper Midwest this week. Wind chills could bring temperatures well below zero degrees.

In Michigan, the temperatures likely won't get cold enough to kill the ash borers. In Minnesota, where the weather is consistently colder and the polar vortex delivered a wallop, the story appears different, Lee Frelich, director of the University of Minnesota's Center for Forest Ecology, said. 

"The colder it gets, the bigger the proportion of insects are killed," he said. "You'll get a little bit of mortality around 10 or 15 below zero, and some substantial mortality at 20 below, and 30 below zero you’re talking about a very large majority of the insects that would be killed."

But wind chill doesn't count, Frelich said. So although it felt like 27 below zero in mid-Michigan on late Wednesday morning, the actual temperature was closer to 5 below.

In central Minnesota, however, the actual temperature reached minus 27 at that time, well within Frelich's range for substantial ash borer mortality.

More: Battle of the Ash Borer

Insects more likely to survive in short cold stretch

Although extremely cold, this snap is expected to be a short one. Mid-Michigan temperatures will hit the mid-30s by the weekend, forecasts predict.

A short cold stretch means insects are more likely to survive, Russell said.

But as many warnings the meteorologists offer, their forecasts don't always show the temperatures bugs are battling.

When the mercury says it's zero degrees, the ambient temperature surrounding part of a tree could be 3 degrees, and at another part could be 5 degrees, McCullough said. Ambient temperature depends on things like sunlight, snow cover, wind cover and height.

"You would be surprised at how much variability there can be on one tree," McCullough said.

That keeps entomologists from declaring a certain low temperature as a pest's deadly threshold.

While that makes mortality hard to predict, it doesn't mean bugs are immune to extreme cold temperatures. Gypsy moth egg masses are susceptible to cold, Russell said, and McCullough pointed to the invasive hemlock woolly adelgid that died around Holland last year when the thermometers hit 6 below zero.

Tip from the ash borer: Burrow to stay warm

Another complication is some insects' habit of burrowing into trees and soil, which insulate them from the outside air. Emerald ash borers do that, and they are doing just fine in very cold places like Michigan's Upper Peninsula and northern Minnesota, McCullough said.

"You may see some mortality there, but wood and bark are really good insulators," McCullough said.

So is soil and leaf litter, where ticks make their winter home, MSU associate professor Jean Tsao said in an email. They can survive bitter cold.

"What really helps them survive the extreme cold, is a nice, blanket of snow, which insulates them from the cold air above," Tsao wrote.

Those ticks may be happy to hear the National Weather Service's recent prediction: Another 5 inches of snow for the Lansing area.

Support stories like these. Find our subscription deals and offers here.

Contact Carol Thompson at (517) 377-1018 or ckthompson@lsj.com. Follow her on Twitter @thompsoncarolk.