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DENVER – Tree-killing bark beetles decimated 550,000 acres of forests in Colorado and Wyoming last year, bringing the total area ravaged by the insects in both states to 4 million acres since 1996, the U.S. Forest Service said on Sunday.

"The significance is that the trajectory is moving north and east into more visible and populated areas," Janelle Smith, spokeswoman for the U.S. Forest Service, told Reuters.

Federal and state foresters just released their annual aerial survey of impacted lands across the two Rocky Mountain states. The main culprit is the mountain pine beetle responsible for infesting 400,000 acres in Colorado and southern Wyoming.

The burrowing insects are moving into ponderosa pine forests from lodgepole pine stands along the Continental Divide, Smith said.

The spruce beetle, more active in southern Colorado, attacked an additional 150,000 acres in 2010, the report noted.

The 4-million-acre combined tally accounts for less than a quarter of the estimated 17.5 million acres of trees attacked by bark beetles across the interior American West as a whole, including Idaho, Montana, Utah and New Mexico, since the 1990s, the Forest Service said.

The problem is even more extensive in Canada, where nearly 40 million acres of forest in the provinces of Alberta and British Columbia have been infested.

The beetles bore into tree bark, where their larvae feed on the trees, choking off their ability to thrive. Aged forests weakened by drought, overcrowding and fire are most susceptible.

Bark beetle outbreaks are common in the Rocky Mountains, Smith said, but there is little that can be done to stop the infestation once the beetles attack. Sustained frigid temperatures will kill the insects, but prolonged warmer winters have allowed the insects to proliferate.

Additionally, mountain pine beetle activity is at "epidemic levels" in the Black Hills National Forest in South Dakota and in northeastern Wyoming. In those areas, the infestation doubled in size between 2009 and 2010, according to the survey.

Trees killed by the beetles can fall on power lines, provide fuel for wildfires, and pose a hazard to human activity.

"We were extremely aggressive in 2010 with our efforts to remove trees killed by the bark beetle to reduce the risk of falling trees to forest visitors and employees," said Tony Dixon, the U.S. Forest Service's Rocky Mountain region forester.