Frigid, but not too cold for bark beetles

Baby, it’s cold outside.

Cold enough, by chance, to kill bark beetles?

Don’t count on it, says Bob Cain, an entomologist with the U.S. Forest Service’s Rocky Mountain Region.

This week’s frigid spell might give Coloradans a bit of a warm feeling inside if they could take comfort in knowing it was icing the destructive beetles. But that would be underestimating the toughness of these tiny creatures, which have infested 4 million acres of pine trees in Colorado and Wyoming, turning huge swaths of forest rust-red.

Unfortunately, Cain said, the beetles become more cold-hardy during the depths of winter. They spend the winter in a larval stage and build up in their bodies a high level of glycol, which is like the antifreeze in your car.

“This (cold) snap in midwinter is well within the normal range for what the beetles would have evolved with in the Rocky Mountains,” Cain said.

Although some parts of the state have been reporting temperatures around minus-30, it would take perhaps a week of temperatures that low to kill the beetles, Cain said.

The exact amount of time required and the amount of time each day where temperatures would need to be that low are unknown, he said. A lot of the research on the subject has been done in petri dishes. In the field, there would be other factors in play, such as the amount of snow cover, which helps keep beetles warm in tree trunks near the ground, and what tree species is involved. It all comes down to how long it would take cold temperatures to get through the insulating bark.

It’s also important to bear in mind that temperatures such as those being reported this week usually are recorded in valleys, which, because of inversions,  can have colder weather than on forest slopes, Cain said.

Temperatures such as this week’s have occurred in past winters without being cold enough to kill the beetles, he said.

Temperatures as low as minus-50 would be more likely to take out beetles quickly, Cain said. Such temperatures helped bring an end to a massive bark beetle outbreak in the White River National Forest in the 1950s.

Beetles are more vulnerable to temperatures like this week’s in the late fall and early spring, when they’re not so cold-hardy, he said.

By the way, Cain said, pine trees would survive just fine in the kinds of temperatures it would take to kill beetles this time of year. The pines’ northern ranges far exceed those of bark beetles, Cain noted.

“It would be nice to get one of those minus-50s just to see it all happen,” he said. “I think we’ll just have to keep waiting for a little bit colder.”


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