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Pine Beetles
A tree attacked by pine beetles exudes sap under pressure in an attempt to expel the bugs. (Courtesy photo)

GREAT FALLS — The acreage of pine forest in Montana infested with the mountain pine beetle more than doubled in 2009, but a forester says the epidemic is losing steam in some areas.

Pine beetles infested 1.2 million acres of forest in 2008 and 2.7 million acres in 2009, based on aerial surveys.

Gregg DeNitto, group leader for forest health protection for the U.S. Forest Service's Region 1 in Missoula, said elevated levels of pine beetle are likely for five to seven more years.

DeNitto says unless the pine beetles are killed off by some significant weather event, like an extremely cold winter, the outbreak will continue until most of the susceptible trees in the area have been killed.

Most of the outbreak has occurred in forests near Butte, Anaconda and Helena.

Aerial surveys last year show that pine beetles are attacking with less intensity in those areas. Even though those areas still have the most mortality, fewer trees being killed per acre, DeNitto said.

The reason for the slowdown is that the beetles are either running out of food or trees, he said.

The Beaver-Head Deer Lodge National Forest still has the most acreage of infested trees, with 1 million acres observed in 2009, compared with 739,023 acres in 2008. The infested acreage in the Helena National Forest was 523,797 in 2009, up from 261,860 acres in 2008.

There were large increases in the acres of beetle-infested trees west of Anaconda in the Big Hole River area and in the Lewis and Clark and Gallatin national forests, DeNitto said.

In the Lewis and Clark National Forest, acres of infestation were listed at 354,208 in 2009, up from 50,046 in 2008, while the Gallatin National Forest saw an increase in infested acreage from 133,208 in 2008 to 374,150 in 2009.