The deep freeze that engulfed Southwest Colorado in recent weeks was cold, no doubt. Whether it's enough to make a dent in bark beetle populations remains to be seen.
"I think it will. Some years the cold is so intense it kills a certain percentage. But it's impossible to determine the extent, or which localities," said Tom Eager, an entomologist with the U.S. Forest Service in Gunnison. "Coming up with hard, fast data will be difficult until it warms up. The real story will be told in the springtime when we can measure how the beetles responded."
"I'd say it's OK to be mildly optimistic," he added.
But any effects are likely to be isolated, not sweeping. For the cold to wreak significant havoc on beetle larvae in mid-winter, "temperatures of at least 30 degrees below zero must be sustained over at least five days", according to a Colorado State University report.
The insects are resilient survivors.
Tree bark and snow offer a base layer of insulation from icy winds. Diet does the rest. Contrary to their name, bark beetles feast not on bark itself but on a sugary underlayer called phloem.
"As the weather cools down in the fall, (the larvae) do this neat biochemistry trick. They convert the sugar into an alcohol (glycerol) that functions as antifreeze," Eager said. "They reverse the mechanism in spring, flipping those alcohol molecules back into sugar and they're back in business."
A 2011 aerial reconnaissance survey by the Forest Service showed that bark beetles have blighted more than 4 million acres of Colorado woodland since the present outbreak began in 1996. For the most part, beetle infestations are confined to higher elevations north and east of Montezuma County. Wide swathes of mature spruce trees over Wolf Creek Pass, within the Rio Grande National Forest, have dried to a crisp and turned a sickly reddish shade.
Near the northern state border with Wyoming, the beetles have also made inroads. Lodgepole and ponderosa pine landscapes in places like Longmont, Greeley and Fort Collins are decimated.
"We've been lucky here. The conditions are right (for an outbreak)," said Mark Krabath, supervisory forester for the San Juan National Forest's Dolores District. "The forest is full of overstocked pine stands, where the trees are all the same age - about 80 years old. (Loggers) harvested the forest pretty intensively in the 1920s and we've not seen much cutting or wildfire since. It's gotten pretty crowded."
Unlike the coasts or Great Lakes - where invasive pests like the emerald ash borer or Asian long-horned beetle have emerged - Colorado's beetle species are native, Eagan said, and periodic outbreaks are a "naturally occurring event".
"It's almost a symbiotic relationship. Clearing forests paves the way for the next generation of trees. It's not all bad. Beetles are closely intertwined with the life cycle of trees," he said.
Still, scientists wonder if the symbiotic relationship will turn one-sided, or if it already has. Climate change models show a warmer and drier Colorado by 2050, and such conditions make it easier for beetles to proliferate: reproductive cycles are sped up, cold-induced mortality declines, and drought-weakened trees are less able to resist invasion, according to Barbara Bentz of the Western Bark Beetle Research Group.
lukeg@cortezjournal.com