CU researchers document accelerated breeding by tree-killing insects
By Bob Berwyn
SUMMIT COUNTY — In one of the clearest signs yet that global warming is having a direct impact on the environment and economy of the Colorado high country, a trio of University of Colorado researchers say they’ve shown that mountain pine beetles have responded to climate change by speeding up their breeding cycle.
Because of the extra annual generation of beetles, there could be up to 60 times as many beetles attacking trees in any given year, their study found. And in response to warmer temperatures at high elevations, pine beetles also are better able to survive and attack trees that haven’t previously developed defenses.
That puts other high-elevation pine species, including limber, whitebark and even the iconic bristlecone pines at risk, according to CU graduate student Scott Ferrenberg, who launched the high elevation pine beetle study.
“They have not yet reached their maximum development rate. They have the potential to develop even faster,” Ferrenberg said, explaining why the current epidemic is unprecedented. “Nobody really understood what climate was going to mean in this context,” he added.
“We followed them through the summer, and we saw something that had never been seen before,” said CU ecology and evolutionary biology professor Jeff Mitton, describing the study conducted at CU’s Mountain Research Station, about 25 mile west of Boulder. “Adults that were newly laid eggs two months before were going out and attacking trees.”
Essentially, springtime temperatures have warmed so much that larvae are able to reach maturity within a couple of months – by May in some cases. Those beetles then go off and attack new trees, laying more eggs that hatch the following spring, Ferrenberg explained.
That means two generations of beetles instead of just one within 12 months, and that doesn’t just double the population — it can increase the numbers exponentially.
In ideal conditions, each female lays 60 eggs, so if all those eggs hatch in May and grow up, each of those beetles, in turn, could fly off and hatch eggs in new trees, for a total of 3,600 new bugs the following spring.
The effects seem particularly pronounced at higher elevations, where warmer temperatures have facilitated beetle attacks. In the last two decades at the Mountain Research Station, mean annual temperatures were 2.7 degrees Fahrenheit warmer than they were in the previous two decades.
Warmer temperatures gave 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, Mitton and Ferrenberg report. Also, the number of days that were warm enough for the beetles to grow increased by 44 percent since 1970.
The Mountain Research Station site is about 10,000 feet in elevation, 1,000 feet higher than the beetles have historically thrived. In their study, Mitton and Ferrenberg emphasize this anomaly.
“While our study is limited in area, it was completed in a site that was characterized as climatically unsuitable for (mountain pine beetle) development by the U.S. Forest Service only three decades ago,” they write.
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.
This exponential increase in the beetle population might help to explain the scope of the current beetle epidemic, which is the largest in history and extends from the Sangre de Cristo Mountains in New Mexico to the Yukon Territory near Alaska.
The trees at higher elevations are probably more susceptible to attacks because they haven’t evolved with the same defense mechanisms as trees at lower elevations, which have a higher density of resin tubes, which enable healthy trees to repel the insects.
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, Ferrenberg said.
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 trees in that area are somewhat naive in their response,” Ferrenberg said.
That’s also what could make other high-elevation pine species susceptible to the beetles, Ferrenberg concluded.
The findings are being being published this month in The American Naturalist.