Montana and Wyoming have recorded arctic temperatures during the past week that would seem to make a polar bear shiver, but the cold may have come too late and not stayed long enough to have much of an effect on mountain pine beetle populations that have proven destructive to Rocky Mountain forests.
“It probably is not going to have a significant impact on the mountain pine beetle, which is the major mortality element we are having now,” said Gregg DeNitto, forest health protection group leader for the Forest Service in Missoula. “But it’s very possible there will be some localized impacts, where it has gotten down to 35 or 40 below. It could knock the population back, but we won’t know that for sure until spring when we start looking at some trees.”
The trouble is that by this time of the year, mountain pine beetles have already winterized their systems with a natural antifreeze. So it has to stay really cold — minus 30 to minus 40 degrees — for many days to kill bugs. Even if it does get that cold for an extended period, some beetles are likely to survive if they are in the part of the tree that’s buried under the snow. The snow acts as an insulator, keeping the bugs warmer. Since one beetle can produce lots of eggs, it’s tough to completely wipe out the tree-killing bugs.
Cold weather is actually more lethal to mountain pine beetles in the spring and fall, when their antifreeze levels are lower.
“At that time of year, cooler conditions much warmer than this could have an effect,” said Roy Renkin, a Yellowstone National Park vegetation specialist.
“In 2009 there was an event in October where it got 10 below for five days,” said Wally Macfarlane, a Utah State University researcher. “The following spring we noticed a significant decline in pine beetle populations. One reason was because of the timing.”
Most of Montana’s large pine beetle outbreaks are now concentrated in the west-central portion of the state — areas like the Bitterroot, Big Hole and Wise river regions, DeNitto said. Outbreaks that devastated forests in the Butte, Helena and Anaconda areas are waning, since the beetles have already infested most of the desirable trees.
“They prefer trees with thicker phloem,” the tissue of the tree under the bark that the bugs eat, DiNetto said. “When the trees get smaller in diameter, the phloem is pretty thin.”
In Yellowstone, the forest came under a heavy mountain pine beetle attack that peaked in 1982, with about 1 million acres affected. A second outbreak that peaked around 2009 affected just more than 100,000 acres, Renkin said, mostly in lower elevation lodgepole pines.
Although the latest outbreak was smaller, Renkin said the recent cold snap should help, even if it’s only a little bit.
“It’s acting to kick a dead horse when it’s down,” he said. “This epidemic, here anyway, is on a strong crash, and these kinds of temperatures can add to that.”
A 2012 survey of north-central Wyoming’s forest land showed only 450 acres affected by mountain pine beetle activity. On western Wyoming’s forests, infestation had declined from 335,000 acres in 2011 to 122,000 in 2012. Statewide, the epidemic had declined from 719,000 acres in 2011 to 180,000 in 2012.
According to the Forest Service’s 2012 survey of Montana, more than 660,000 acres of forest was infected by mountain pine beetle, down from more than 1 million in 2011. The intensity of the infestation also declined. The results from the 2013 survey have not yet been calculated.
While pine beetle infestations dropped, western spruce budworm defoliation increased in 2012 to nearly 1.5 million acres in Montana, with the worst of the attacks east of the Continental Divide.
DeNitto said the cold weather won’t affect the spruce budworm, since they overwinter in the egg stage.
Liking the cold
Former forest researcher Jesse Logan, who winters in the remote mountain town of Cooke City to take advantage of the backcountry skiing, said the temperature there has hit minus 30 recently. But he doubts the cold has had much effect on mountain pine beetles.
“I’m happy to see the cold, but I wouldn’t be overly optimistic,” he said.
He’s planning a ski trip onto the Beartooth Plateau in a couple of weeks to investigate how the outbreak is progressing in whitebark pine forests in places like Island Lake that he has been keeping an eye on.
No matter what he finds, Logan said he is happy to see the deeply cold weather, which may sound strange to those who have battled frozen water pipes, high home heating bills and dead car batteries. Yet he’s doubtful the chilly temps have had much impact on mountain pine beetles.
“They’re pretty tough,” he said. “The temperature that will really zap them is minus 40.”