Mountain pine beetle is the most serious insect pest of ponderosa and lodgepole pine forests in Colorado. Outbreaks periodically kill large numbers of pines. Mountain pine beetle populations have been on the rise statewide.
Mountain pine beetles attack only pines greater than seven to eight inches in diameter. The one-year life cycle of the beetle begins in August when adults exit dead trees and fly to attack nearby green trees.
The external symptoms of mountain pine beetle infestation include popcorn-like masses of resin called “pitch tubes” on the trunk, boring dust at the tree base, and patches of missing bark or bark flakes at the tree base as the result of woodpecker predation. In early summer, about nine to ten months after being attacked, the needles of infested pines turn reddish-brown.
Internal symptoms, visible only by removing a portion of bark with a hatchet, 
include the beetle’s distinctive tunnel system, wood stained bluish-gray and the beetles themselves; they are brown and about one-eighth inch long. Mountain pine beetles overwinter in the tunnel system as small white grubs with brown heads.
During outbreaks, infested trees produce enough mature beetles to kill one to three more trees. A community-wide program of prevention and beetle population reduction is necessary in areas of high-value pines.
Prevention involves thinning dense, older pine forests, and application to live valuable trees of the insecticide carbaryl.
Population reduction involves cutting infested trees before beetles exit from them and performing one of the following: solar treating the trunk with clear plastic, debarking or chipping the trunk, or burning or hauling infested trunks to a site at least a mile from pine forests.
For details about prevention or population reduction treatments contact your local office of Colorado State Forest Service or Colorado State Extension, your county Pest Management of Open Space office, or a professional arborist.
For more information, see the following Colorado State University Extension fact sheet(s).
