By Daniel Wallis

DENVER, March 24 (Reuters) - Forests in Western U.S. states that have been ravaged by mountain pine beetles are no more likely to be consumed by wildfires than forests unaffected by the insects, a new study by scientists in Colorado has found.

Warmer than usual winters in recent years have allowed the tree-killing beetles to survive the cold months and leave behind stands of dry wood that experts had feared could help fuel early season wildfires.

But the peer-reviewed study by researchers from the University of Colorado at Boulder found that activity by the so-called bark beetles had a "negligible" affect on the area burned during three peak years of wildfires since 2002.

"Although infestation and fire activity both independently increased in conjunction with recent warming, our results demonstrate that the annual area burned in the western United States has not increased in direct response to bark beetle activity," the scientists wrote.

The paper, to be published this week in the Proceedings of the National Academy of the Sciences, said the beetles have killed pine trees across some 71,000 square kilometers of forest in the U.S. West since the mid-1990s.

Researchers have widely debated whether infestations contribute to flammability and an increased risk of wildfires first due to the reduced moisture in affected trees, and then by adding to forest floor fuels when they shed their needles.

"The assumption that outbreaks raise fire risk is driving far-reaching policy decisions involving expenditures of hundreds of millions of dollars," the authors of the new paper wrote.

They said their 18-month study, however, found no effect on the subsequent area burned. "Therefore, policy discussions should focus on societal adaptation to the effects increasingly important driving factor: climate warming," they wrote.

Nevertheless, the researchers noted, the effects of beetle outbreaks on other measures of fire behavior, including most importantly on the intensity of blazes, may be important "and clearly warrant caution from a fire-fighter safety perspective." (Reporting by Daniel Wallis; Editing by Sandra Maler)