Last summer, a grazing permittee who runs cattle in the Lake Canyon area of the Dolores Ranger District noticed something disturbing: stands of ponderosa pines with red needles, high on a mesa top. The red needles are a sign of trees that are close to death.
When officials with the Gunnison Service Center, a regional forest-health center in Colorado, flew over the area as part of an annual checkup they also noticed the afflicted ponderosas. They sent a map and a description of the site – which is approximately 7 miles north of Bradfield bridge and about 1 mile east of the Dolores River on the northwest portion of the Glade – to personnel with the Forest Service’s Dolores Ranger District. Mark Krabath, supervisory forester for the district, was disturbed by what he saw.
“We had some information from 2012 that showed very little bark-beetle infestation, but we just got the results from 2013 and it had ramped up quite a bit,” he said.
Krabath asked an entomologist with the Gunnison Service Center to check out the site in person, and they confirmed that the cause of the ponderosa die-off was an infestation of the round-headed pine beetle. What was particularly disturbing is the fact that the beetle is not normally present in Colorado, but plagues trees in warmer states such as New Mexico and Arizona.
“He was somewhat surprised,” Krabath said of the entomologist. “I can’t say for certain this is the first time the beetle has been in Colorado, but it’s the first time in his recent memory.”
Since then, district officials have been studying the site and obtaining better photo imagery to ascertain the extent of the infestation, which they now estimate at 300 to 400 acres, with a few isolated trees a half-mile away also infested.
The round-headed pine beetle (Dendroctonus adjunctus Bland ford) has ravaged large stands of ponderosa in southern New Mexico over the last 30 years, according to a pamphlet from the U.S. Department of Agriculture, and has also afflicted trees in Arizona and Nevada as well as Mexico. The tiny insects prefer ponderosa but have also attacked limber pine in the United States and several other types of pine in Mexico, according to the pamphlet.
The beetle has a one-year life cycle in the United States (in Mexico it may spawn two generations a year). In late fall they bore through the bark of uninfected trees and lay eggs; the larvae emerge in spring and begin feeding on the soft inner bark, slowly killing half or more of the infected trees. They pupate in the summer, then emerge through the bark and fly on to new trees.
Once the beetles are present in a stand of ponderosa, they are difficult to stop.
The average distance they fly is just a quartermile, Krabath said, “but if they get the right winds they could go up to a mile. So it’s kind of like trying to stop a wildfire.”
At present there are pockets of infested trees as far as a half-mile outside the main infestation, he said.
Trees stressed by drought and overcrowding are especially vulnerable. The round-headed pine beetle has infested trees ranging from 5 to 25 inches in diameter at breast height and seems to prefer mature trees, he said.
The larvae can be killed by winter temperatures of 25 degrees below zero, he said, “but typically we haven’t been getting those cold winters.”
The situation has many parallels with that of the spruce bark beetle that has decimated trees around Colorado and the West, notably on Wolf Creek Pass in the local region. “There are too many trees, a lack of fire, and when the temperatures rise and we get little moisture, it creates the right conditions for the beetle,” Krabath said.
The ideal method for treating an infestation is to thin the trees around the area, both to make it more difficult for the beetles to reach new trees and to make the remaining trees stronger. Logging would do the job, but the market for small-diameter ponderosa is virtually non-existent.
“If I could do some aggressive timber-sale work, that would be the best option,” Krabath said. “But we do not have much timber industry locally that even wants the wood, so the timber-sale option is not in our toolbox.
“If I had some biomass industry here or somebody that would take the wood for a nominal amount I’d be happy, but we don’t. So we’re looking at some mastication with hydroaxe, which would chop up some of the infected trees and reduce some of the stand density of the green trees so the rest have more vigor and health.”
However, treatment with the hydroaxe costs $300 or $400 per acre or more, according to Krabath.
Prescribed burning is another option, he said, but not an ideal one, as some research indicates it just stresses the healthy trees and makes them more vulnerable to the beetle. Prescribed fire could be used after a beetle kill to reduce fuels.
However, forest officials are hoping that natural barriers will prevent the round-headed pine beetle from spreading too far. The ponderosa-pine zone in the Lake Canyon area is bounded by piñon pines and Gambel oak to the north and the Dolores River Canyon to the west.
“The entomologist said this beetle is at the border of its northernmost range, so it may not like it here for too long,” Krabath said.
The beetles have already flown this fall and infested new green trees, Krabath added in an email, and will take wing again next fall, so treatment will need to happen this summer to be effective. Planning has started on the treatment project and public comments will be solicited this winter.