Tree deaths caused by insect infestation and disease in the western U.S. declined significantly last year, largely because mountain pine beetles have devoured so many
forests that they are running out of food, according to a report by the U.S. Forest Service. Researchers reported that about 6.4 million acres of forest died nationally in 2011, compared with 9.2 million acres in 2010 and a peak mortality of 11.8 million acres in 2009. Scientists say about 60 percent of the mortality was caused by one pest, the mountain pine beetle, a native insect that has decimated lodgepole and ponderosa pine forests across western North America because warmer winters are not killing off beetle larvae. While the researchers say a critical factor in the decline has been a reduced number of available lodgepoles, they say ponderosa pine and high-elevation white bark pine remain at risk. The greatest forest mortality was reported in Colorado, Montana, Idaho, and Wyoming. “Native insects and diseases run in cycles, and right now we are grateful the trend is downward,” said U.S. Forest Service Chief Tom Tidwell. He added, however, that forests still face significant threats, including from climate change and new invasive species.