Cold enough for ya? Not cold enough to kill tree pests

We all survived the first subzero cold snap of the season by bundling up or staying inside. Unfortunately for Minnesota’s forests, most of the moths, beetles and other pests that make our trees sick during summer probably weathered the cold, too.

Historically, Minnesota could count on its extreme cold to annually exterminate disease-spreading bugs like the European elm bark beetle, which carries Dutch elm disease.

“The colder it gets and the longer the cold lasts, the fewer of those survive, and then fewer of them will be around to transmit the disease from one elm tree to another,” said Lee Frelich, a research associate and director of the University of Minnesota’s Center for Hardwood Ecology.

As the state’s climate warms, more pests are making it through the winter and getting a head start on spring. With several invasive species migrating toward the state and some already settling in, the cold represents a critical line of defense, one that’s gradually going the way of the polar ice caps.The recent cold spell is better than nothing, Frelich said, but it hardly compares to the deep freezes of decades past, when winter temperatures routinely bottomed out at 25 to 35 below zero. Parts of northern Minnesota got that cold over the weekend, but southern Minnesota saw temperatures only around 10 or 15 degrees below zero.

“If this cold weather lasted a couple more weeks, and we had a couple nights of minus 25, I’d expect to see a big decline in the rate of Dutch elm disease,” Frelich said. “As it is now, we had a couple nights where it was minus 17. That’s pretty marginal. It might have a minor impact. You’ve got to hope for the 25 or 30 below if you want to really kill a lot of things.”

Every species has its own range of temperatures in which it can survive, Frelich said, and as the state’s climate changes, it’s opening the door for migrating pests.

One of the bugs causing the most concern is the emerald ash borer, a beetle that’s devastated ash trees in the eastern United States and has recently crept as close as Illinois.

“I wish we knew what the limiting temperature was for the emerald ash borer, which isn’t quite here yet,” Frelich said. “I’ve been trying to figure out whether it’d be too cold here for it, and I recently found out that it lives in Mongolia, so that means it’s probably not cold enough, unfortunately, to control that bug.”

Another, called the mountain pine beetle, has been destroying pine forests to the west in British Columbia. Several days of minus 40 degrees will wipe out the population. It used to have no chance of migrating because temperatures across southern Canada always dropped to 40, 50 or 60 degrees below zero every winter.

“So there was no chance that that bug could start making its way towards Minnesota. But now, all along the southern margin of the boreal forest, a lot of times you can go several winters without getting down to minus 40. So it’ll be interesting to see the monitoring reports and how far they’ve moved next year, whether it was cold enough in British Columbia to stop it from moving more this year, or whether it’s going to take another step toward Minnesota,” Frelich said.

The cold probably slowed the spread of gypsy moths, an invasive pest that eats up the leaves of deciduous trees, which can kill old or weak trees. Another pest that hasn’t gotten here yet is the hemlock woolly adelgid, a bug that’s wiped out at around 15 to 20 degrees below zero.

“As you might imagine, it hasn’t been able to get close to Minnesota yet, but we’re really worried what will happen with global warming if it doesn’t even get down to minus 15 ever again. In a few decades, that bug might be able to make its way here.”

The trees themselves can withstand much colder temperatures than the bugs. Some non-native species can be stunted at 30 or 40 degrees below. Most native species can withstand minus 50 or colder. And spruce in northern Minnesota can withstand between negative 70 and negative 90 degrees. They’re much more likely to be wiped out by warmth, and the pests that come with it.

“The warmer the climate gets and the less frequent these cold outbreaks are,” Frelich said, “the better chance they have of making their way here.”