DENVER - Colorado's fast-spreading infestation of bark beetles has killed 1.5 million acres of lodgepole pines and could largely wipe out the state's lodgepole forests in three to five years, state and federal forestry officials said Monday.

The infestation was first detected in 1996 and grew by half-million acres last year.

"This is an unprecedented event," said Rick Cables, Rocky Mountain regional forester for the U.S. Forest Service.

Officials said the infestation was concentrated in five northern Colorado counties straddling the Continental Divide and has reached southern Wyoming.

Boulder and Larimer counties, both east of the divide, had a 1,500 percent increase in the number of acres taken over by the beetles last year, officials said. While spruce and aspen trees are also affected by the infestation, it's the tall, slender lodgepoles that have been hardest hit.

The infestation can leave entire mountainsides red with dead and dying trees. The devastation is apparent along some stretches of mountain highways.

Forest officials attributed the spread of the beetle to warm winters and drought. Susan Gray, group leader for forest health management with the U.S. Forest Service Rocky Mountain Region, said only 20-below-zero temperatures for a sustained period can kill the beetles.

"Unfortunately, it hasn't been cold enough long enough," she said.