Although thousands of evergreen trees are beginning to display dying yellow or brown needles, most are simply going through a natural shedding process and are not being damaged by bark beetles, insects or disease, according to the Colorado State Forest Service.
Colorado evergreens shed their older, interior needles as part of an annual growth cycle. Trees stressed because of drought or root damage may shed more needles. Soon-to-be shed needles typically yellow first then turn a reddish-orange or brown color before dropping off.
Ponderosa and lodgepole pines, as well as some spruce and fir species, are most commonly reported to be shedding needles in September and October.
Needles on a beetle-infested tree typically change color throughout the entire tree (as opposed to only the older needles in healthy trees), initially starting with an off-shade of green and turning to reddish-orange by the next summer. Bark beetle-infested trees will show other signs of attack, such as popcorn-shaped pitch tubes on the trunk, boring dust resembling fine sawdust collecting in bark crevices and at the base of the tree and regular woodpecker activity.
For more information, contact local Colorado State Forest Service field offices or visit www.csfs.colostate.edu/forest-management.