Hungry hordes of tent caterpillars are leaving acres of aspen bare of leaves across much of Colorado, including a swath of the San Juan National Forest from Vallecito to Beaver Meadows northeast of Bayfield.
The Western tent caterpillar, the larval stage of a member of the moth family, also can be found in isolated spots around Durango.
“Most of the damage has been done,” said Roy Mask, an entomologist with the Forest Health Protection Group in the U.S. Forest Service’s Rocky Mountain regional office in Golden. “They are almost ready to pupate (go dormant to later emerge as moths).”
The San Juan, Rio Grande and San Isabel national forests, and to a lesser degree the Gunnison region, were among the areas hardest hit by the infestation, Mask said.
“Infestations, the most severe in 30 years, taper off as you get further north,” Mask said.
Tent caterpillar infestations are cyclical, similar to those of the Western spruce budworm that attacks conifers, he said.
If tent caterpillar attacks don’t occur over several successive years, aspen will refoliate, although their leaves will tend to be smaller, Mask said.
Steve Hartvigsen, a forester in the Pagosa District of the U.S. Forest Service, can’t remember a tent caterpillar infestation since one was concentrated southeast of Pagosa Springs in the mid-1980s.
Bob Thompson, a retired geology and environmental studies professor from Arizona, shares the impression. Thompson, who has spent summers at Vallecito for 25 years, said Thursday this is the first year he’s seen the caterpillar around the reservoir.
“There are literally billions of the wiggly creatures, so many that while walking down an old road from the top of Middle Mountain, one foot would crush 100 worms,” Thompson wrote in an email to The Durango Herald. “It appears that the caterpillars have killed thousands of previously healthy aspen this year.
“Having defoliated the trees, the caterpillars drop to the ground and crawl in search of more leaves,” Thompson said. “Don’t stop for long. When standing still for even a few seconds, you will suddenly see an army of the beasts climbing up your shoes, socks and legs. EEYUCK.”
Ruth Lambert, cultural program director at the San Juan Mountains Association, found a similar scenario June 21 while leading a walking tour of the aspen tree carvings left around Beaver Meadows by early-day shepherds.
“If I stood near a tree to explain something, a caterpillar would drop on me,” Lambert said. “It was kind of creepy, like in a horror flick.”
The caterpillar outbreak isn’t limited to the west side of the Continental Divide.
“It started two years ago here when the caterpillar was limited to isolated areas,” said Mike Blakeman, a spokesman for the Rio Grande National Forest east of Wolf Creek Pass. “Then last year, we had 5- to 40-acre pockets, and this year, we have large expanses all over the forest.”
An entomologist told him, Blakeman said, that natural predators such as bears, birds and wild turkeys tend to keep the caterpillar in check. Also, a virus sometimes brings a quick end to large infestations.
“But you never know when the virus will appear,” Blakeman said.
In the spring, the presence of the Western tent caterpillar is evidenced by platforms of gossamer in the branches of host trees. The larva (caterpillars) feast on the leaves of the trees.
The larvae mature in 30 to 40 days to spin cocoons in trees. Pupation, the time between larval and adult stage, occurs in 12 to 18 days, with moths appearing in late summer.
When first hatched, larvae are about one-eighth inch long and are dark-brown to black in color. Mature larvae are about 2 inches long and vary in color.
In the Southern Rocky Mountains, the host preferred by the caterpillar is the aspen. But caterpillars will feed on cottonwoods, fruit trees, oak and willow. On the Western Slope, the chokecherry also plays host to the caterpillar.
Successive years of defoliation can cause reduced growth and reduced fruit production.
daler@durangoherald.com
