Helena’s recent wave of brutally cold temperatures probably isn’t doing much to beat back the mountain pine beetle infestation that’s killing hundreds of thousands of evergreen trees around here.

Entomologist Ken Gibson with the U.S. Forest Service in Missoula said low temperatures around 35 degrees below zero are needed for several days running before they’ll knock back the beetles.

The wind chill factor doesn’t count.

“You can find some places where the beetle larvae dies, but it can survive some pretty cold temperatures,” Gibson said on Wednesday. “You need 35 below zero for three or four days in a row, and to have it not get very warm during the day — say, around zero — to have a significant impact on the population.

“And that whole wind chill factor is more of a phenomenon that affects human beings. It doesn’t apply much to trees or the bark beetles.”

Helena’s low temperatures have “only” been in the minus 17 to 25 range since Sunday, with last night’s low forecast for a relatively balmy 1 above zero.

Mountain pine beetles are an important part of forest ecology, taking down weak or dying trees, and providing natural thinning of a forest. But in today’s conditions, when trees are stressed from drought and/or competition for water and sunlight, plus above-average summer and winter temperatures, the beetles’ numbers have risen to epidemic proportions.

The pine beetle outbreak has been particularly intense in the Helena and Beaverhead/Deerlodge national forests, killing trees on more than 118,000 acres on the 1 million-acre Helena National Forest last year alone.

They kill trees by eating the inner bark and cutting off the tree’s nutrition path. In addition, the beetles carry a blue fungus that clogs the trees’ water transportation system.

Experts say the large numbers of bark beetles are here to stay until all their food is gone — in effect, the trees — or the area experiences sustained cold temperatures.

While neither has happened yet, Gibson offers a little bit of hope for those discouraged by seeing acres after acres of dead red trees.

“It’s probably been colder up higher, so there may be some pockets of places where the temperatures will cut back on the mortality,” Gibson said. “Plus, the coldest temperatures usually are here in January and February, so if this is any harbinger of things to come, it’s possible.”

Still, Rich Prewitt with the National Weather Service said that in the 30 or so years he’s been here, he hasn’t experienced a week of 35 below zero temperatures. It can get that cold, he noted, but it typically a one-day event.

The forecast calls for a bit of a warming trend, he added, with a storm expected today to drop 4 to 6 inches in the valleys. That will be followed by lows of 25 below Friday and Saturday night.

“But then it starts warming up again to 5 to 10 degrees below zero,” Prewitt said.

And once the children get out of school, Prewitt is calling the temperatures “teachers’ revenge” with highs in the single digits for

Christmas.

He added that they’ll be keeping a close eye on the weather to ensure Santa Claus can make his required rounds while in the Treasure State.

“We already have his radar path tracked, and will keep tabs on him so his sleigh doesn’t ice up,” Prewitt said.

Reporter Eve Byron: 447-4076 or eve.byron@helenair.com