Pine Beetles Likely Survived Recent Cold Snap

By Beacon Staff

GREAT FALLS — Montana’s recent record-breaking cold snap probably didn’t cause widespread mortality in the state’s tree-eating mountain pine beetle population, but it may have killed beetles in localized areas, according to forest health experts.

“It really takes quite a bit to kill those guys,” Diana Six, a forest entomology and pathology professor at the University of Montana, said of the cooked-grain-of-rice-sized insects with big bites.

The insects can stand temperatures as low as 30 below, she said.

Only time will tell whether the cold cut down the bugs in some areas, said Gregg DeNitto, a forest health protection group leader in the U.S. Forest Service’s Region 1 headquarters in Missoula.

Needles on the host trees begin to turn red in the spring following attack.

Especially cold temperatures in some parts of the state, including Great Falls, might have killed beetles, DeNitto said.

“However, probably not enough to make the outbreak go away, just enough to slow it down for a year or two in those specific areas where it still was active,” he said.

In Great Falls, temperatures reached 26 below zero on Dec. 5 and Dec. 6, and minus 33 on Dec. 7, each a record.

Extreme cold temperatures play an important role in controlling mountain pine beetle outbreaks, according to the Montana Department of Natural Resources and Conservation.

But the subzero weather probably was too little, too late to kill large numbers statewide, Six said.

“At this time of the year, the beetles are pretty much cold hardened,” Six said. “They have developed a bunch of antifreeze.”

What’s needed to kill large numbers is cold weather in the fall or spring.

“If you have a cold snap in October or November, that will kill a lot of them because they’re not prepared for the temperatures,” Six said. “But this late in the year, it’s probably not going to knock ’em back that much.”

Six suspects that an early cold snap in October in the Big Hole River area in western Montana killed off a large number of beetles. When she visited the area, she peeled back bark and noted black beetle larvae with their tunnels ending at the same point.

“You see that, you know they all died at the same time,” Six said.

Mountain pine beetles, a native insect, have chewed through pine forests across the western U.S. and Canada, leaving vast stands of dead trees with red needles in their wake.

In Montana, since 2000, the epidemic has spread across 6 million acres, or 23 percent of the state’s 25.6 million acres of forest on private, state and federal lands, DeNitto said.

Outbreaks are cyclical with the beetles attacking older stands of trees, most commonly lodgepole and ponderosa pines. The beetles bore through the outer bark and into the phloem layer of a host tree, where they feed and lay eggs, according to the DNRC.

The feeding activity, combined with a blue-colored fungus they introduce, kills the trees.

It’s a myth one cold winter will kill off mountain pine beetle, Six said. Cold weather, she said, is hard on the beetle, but it is only one factor affecting populations.

“There’s some areas up in high elevation that might see some kill,” Six said. “For the most part, I wouldn’t expect this to do a lot, unfortunately.”

A preponderance of long warm summers with lower moisture, more than the lack of cold, has allowed the beetle to persist, she said.

The insects prefer trees larger than 5 inches in diameter and they’re running out of them in Montana, DeNitto said. As a result, the epidemic is now in decline. A lack of right-sized trees for the beetles to attack, more than cold weather, is slowing the epidemic, he said.

“We can expect to continue to see some tree mortality certainly showing up next year and probably another couple of years,” he said.

Beetle populations still are building in some localized areas, including the southern Bitterroot Valley, the Big Hole River and the Absaroka Mountains. “It’s just a sea of red right now,” Six said of the Big Hole River.

Western pine beetle also is active in the state, Six said.

“It’s actually the western pine beetle that’s actually killing a lot of ponderosa,” Six said.

Western pine beetle is a strong responder to drought-stressed trees, she noted.

What kind of beetle is killing the trees is important to landowners because a mountain pine beetle repellent called verbenone doesn’t repel western pine beetle, Six said.