CHEYENNE, Wyo. (AP) — An outbreak of spruce beetles continues to accelerate across hundreds of square miles of new forest in Colorado, although a much larger outbreak of the similar mountain pine beetle continues to slow across Wyoming, Colorado and the Black Hills, according to a new survey by the U.S. Forest Service.

Every year, the Forest Service conducts an aerial survey of forests in Colorado, Wyoming and South Dakota. The 2013 survey shows spruce beetles spreading to ever-larger expanses of new forest in Colorado for a fifth consecutive year.

Spruce beetles infested 338 square miles of previously unaffected Colorado forest last year. The beetles laid claim to 286 square miles of new forest in 2012.

In 2008, spruce beetles were active in about 95 square miles of forest in Colorado. One reason spruce beetles are spreading in Colorado is the state's large numbers of aging, weaker trees of the species they're adapted to infest.

The spruce beetle infestation remains small compared with the havoc wreaked by the mountain pine beetle. Spruce beetles have infested and substantially killed about 2,700 square miles of Wyoming and Colorado forest since the mid-1990s, but that's still only about one-fourth the scope of the mountain pine beetle outbreak.

Spruce beetles haven't been a major problem in the Black Hills. In Wyoming, they've killed large numbers of trees in the Medicine Bow and Sierra Madre ranges in the southern part of the state and remain active in the Absaroka Mountains southeast of Yellowstone.

Spruce beetles were active over about 56 square miles of Wyoming forest last year, a roughly average number compared with infestation rates in the state over the past decade.