The bark beetle has also spread into Wyoming, state and federal officials said Monday. Colorado's mature pine forests are expected to be dead within five years, the Denver Post reported.

Jeff Jahnke, the Colorado state forester, blamed the infestation on a combination of drought and warm winters. The beetle is native to Colorado but in the past temperatures have fallen in the winter to 40 below, cold enough to kill the beetles and end outbreaks.

Foresters say that the eventual outcome may be a diverse and healthier forest.

"One of the things that is going on in Colorado is our forests are ready to regenerate," said Rick Cables, a regional forester with the U.S. Forest Service. "They are old. And if you look at the many different species -- aspen, spruce, lodgepole -- they all have one thing in common. They are old."

In the short run, however, mountainsides covered with dead and dying pine trees are a fire hazard and could hurt Colorado's tourist industry.