STEVE LEWIS/Durango Herald file
The widespread cold in Southwest Colorado in recent weeks has been penetrating enough to shrivel people, but probably hasn’t fazed the spruce bark beetle, foresters say.
It requires days, maybe weeks, of minus 20, minus 30 or minus 40 degrees to affect the bark beetle, which has ravaged broad stands of Engelmann spruce in many areas, including the Weminuche Wilderness north of Durango and Wolf Creek Pass to the east.
“The colder it is and the longer it’s cold will bring more beetle mortality,” said Kent Grant, the district forester in Durango for the Colorado State Forest Service. “It would be nice to think it could happen, but don’t bank on it.”
Steve Hartvigsen, a forester with the U.S. Forest Service’s Pagosa Ranger District, put it this way:
“The recent cold may have knocked the beetle back a little. But temperatures haven’t been cold enough, long enough or widespread enough to have a real effect.”
The spruce bark beetle is included in a genus of the androgynous beetles that includes the mountain pine beetle, the western pine beetle and the Douglas-fir beetle.
The beetles often live in downed trees but migrate to large-diameter, standing trees when their population increases.
Spruce beetles usually have a two-year life cycle, although beetles at low elevations may live one year and beetles on cool, well-shaded north slopes can have a three-year life, a U.S. Forest Service report on forest insects and disease says.
Even prolonged cold could have limited effect on the beetle which, through a metabolic process, produces its own antifreeze from ethylene glycol, Hartvigsen said.
What’s more, Grant said, snow, which can be 4 to 6 feet deep at higher elevations, paradoxically offers insulation for the beetle, which makes its home in the cambium and phloem layers of the lower trunk.
“That’s where the sugars are,” Grant said.
Absent any killing cold, disease could decimate the beetle. The insect also could conceivably eat itself out of a home, Grant said.
Increased moisture could make trees healthier, which would allow them to manufacture more pitch to eject invading beetles, Grant said.
The spruce bark beetle population is such that the insect is attacking smaller trees, which it generally shuns, Hartvigsen said.
The eastern San Juan Mountains may have lost 90 percent of some spruce stands to the bark beetle, he said.
In its 2012 report (the 2013 report isn’t out yet), the Colorado State Forest Service said the mountain pine beetle continued to be the most damaging forest pest in 2011.
But it added that in 2007 it identified the spruce pine beetle as the next challenge.
In 2012, the spruce beetle continued to affect mature Engelmann spruce forests in many areas of the state, the report said. “The largest outbreak, in the San Juan Mountains and upper Rio Grande Basin, has spread north and now poses a threat to spruce forests on adjacent private lands.
“Most of the mature spruce trees already have been killed in the Weminuche Wilderness, but active infestations continue in younger stands and in krummholz (stunted timber) at the edge of the timberline,” the report said. “Other bark beetles that continued to damage Colorado forests in 2011 include the Douglas-fir beetle, which killed many trees in the Sangre de Cristo and San Juan mountains.”