Insect-infested trees exhibit different symptoms
By Olivia Lueckemeyer
For many nearby trees, what may look like insect infestation is nothing more than the shedding of old needles, a routine part of the organism’s annual growth cycle. And while some of the characteristics might mimic insect damage, there are ways to determine the difference.
Forester for the Gunnison Field Office of the Colorado State Forest Service Sam Pankratz says the easiest way to tell whether a tree is infested or shedding is to locate where the brown needles occur on the branch. If located on the lower portion of the crown or close to the trunk, natural shedding is most likely the culprit. Soon-to-be shed needles typically turn yellow or orange first, then a reddish or brown color before dropping off.
“These are non-productive needles for the tree, and are generally shaded and at the interior,” Pankratz explained.
In Gunnison County, Englemann spruce, Douglas fir, subalpine fir and blue spruce are the tree species that shed their needles in September and October. Other species, such as certain types of pine, will hold on until snow on the branches encourages shedding to begin.
“I would imagine that for the next month or so we will continue to see older needles that are dying,” Pankratz said.
Trees infested by insects, such as the spruce beetle, will exhibit signs of disease, such as pitch tubes—popcorn-shaped globs of sap found on the trunk of the tree—or evidence of woodpecker damage, as the birds attempt to access insect larvae. Another indication of insect infestation is that newly infested trees are usually located by dead trees, since beetles tend to move from one available host tree to the next.
“There are caveats to that, though, since bark beetles are lazy fliers,” Pankratz said. “We’ve seen strong winds carry them over mountain ranges.”
In the high country, needles on bark beetle-infested trees will fade simultaneously throughout the crown, and normally this won’t occur until the tree comes out of dormancy in the late winter or early spring. Tree mortality in the case of bark beetles also moves with the predominant wind, which in this area occurs in a northeasterly projection.
Gunnison County is still ground zero for the spruce beetle epidemic, and according to Pankratz, the best way to manage the problem is to plant diverse species.
“Bark beetles are specific to one tree species and they like mature, older trees, so the more you can plant of varying species, the better,” he said.