TRAVERSE CITY, Mich. (AP) -- A federal report says hungry insects are a leading threat to the nation's forests, a problem worsened by drought and a milder climate.

In a report obtained Wednesday by The Associated Press, the U.S. Forest Service says the amount of forestland with large-scale tree deaths caused by insects jumped to 37 million acres between 2003 and 2007. That's a three-fold jump from the previous five years, in which 12 million acres died from insect attacks. Millions more have perished since then.

Officials say several factors have made forests more susceptible, including dry weather that has weakened trees. Milder winters also have boosted the survival rates tree-eating bugs.

Despite the threats, the report says overall U.S. forest acreage has remained stable at about 751 million acres over the past 50 years.