April 11, 2012
Millions of voracious mountain pine beetles that have ravaged Colorado lodgepole and ponderosa pine forests now produce two generations per year and have increased their historical range, according to University of Colorado Boulder researchers.
The beetles were thought to produce only one generation of offspring annually, but a study conducted by CU researchers indicates there could be up to 60 times as many beetles attacking trees in any given year. This increase in the beetle population could be a factor in the scope of the beetle epidemic, which extends from the Sangre de Cristo Mountains in New Mexico to the Yukon Territory near Alaska.
Research for the study was conducted in 2009 and 2010 at CU’s Mountain Research Station about 25 miles west of Boulder.
According to Jeffry Mitton, CU-Boulder professor of ecology and evolutionary biology, adult beetles, two months after being newly laid eggs, were going out and attacking trees. Previously, mountain pine beetles spent a winter as larvae in trees before emerging as adults the following summer.
Warmer high-altitude temperatures in Colorado
Warmer temperatures at higher elevations may be compounding the effects of earlier emerging beetles. In the last 20 years at the Mountain Research Station, mean annual temperatures have become 2.7 degrees Fahrenheit warmer.
Warmer temperatures give the beetle larvae more spring days to grow to adulthood. The number of spring days above freezing temperatures increased by 15.1 in the last two decades, the researchers note, adding that the number of days that were warm enough for the beetles to grow increased by 44 percent since 1970.
Mountain pine beetle expands range
The Mountain Research Station site is located at about 10,000 feet in elevation, 1,000 feet higher than the beetles have historically thrived. The CU study was completed in a site that was thought by the U.S. Forest Service 30 years ago to be climatically unsuitable for the mountain pine beetle.
But in 25 years, the beetles have expanded their range 2,000 feet higher in elevation and 240 miles north in latitude in Canada, Mitton said.
Lodgepole pines at higher elevations tend to have a lower density of resin ducts, which transport resin, the sole defense against beetles. The number of resin ducts in a tree can be a “marker” for whether a tree has a higher or lower resistance to a beetle attack, said Scott Ferrenberg, a CU graduate student and study participant.
The trees at higher elevations had not faced the same intensity of beetle attacks as those at lower elevations until temperatures warmed, and they have not faced pressures of natural selection exerted by attacking beetles.
The CU study was published last month in The American Naturalist.