For the afternoon session of Rocky Mountain National Park BioBlitz 2012, my son and I learned about mountain pine beetles from Dave Leatherman, retired Colorado State Forest Service entomologist. We met in an old ponderosa savanna near the Lawn Lake Alluvial Fan.
Mountain pine beetles have been in the news for the past few years as they decimate pine forests across the Rocky Mountains. As we walked, Dave explained why. The beetles bore under the bark and attack mostly lodgepole, but also ponderosa, bristlecone and limber pine trees. The trees need to be more than five years old because the beetles need thick bark to protect them in deep winter freezes.
The beetles use pheromones, or chemical attractants, to attract mates to the chosen tree. If there are enough beetles, they can jump to piñon pine or other conifers. In many areas, whole mountainsides are covered with rust-colored dead pine trees which, as they dry out, become perfect fuel for forest fires.
You won’t actually see mountain pine beetles unless you happen to notice them on the few days of late summer that they come out of one tree to fly to another, or you start pulling bark off of infested trees, as Dave did. Their entire lifecycle is lived underneath the bark of their dying victims.

What you might notice are pink or yellow popcorn-like globs of sap stuck beneath pinprick holes in the bark of the tree. At the base of the tree, you’ll find sawdust. These signs are exactly what they look like: holes, called pitch tubes, where the female beetle burrowed in, the boring dust falling to the ground beneath the tube, and pitch or resin that the tree has oozed out in an attempt to pitch the beetles out (drown the beetle).
The beetles carry bluestain fungus into the trees on their bodies. We used to think that the bluestain fungus was what actually killed the trees. But instead, it seems that the bluestain fungus takes up nutrients from the tree and concentrates them. The beetles need those nutrients to survive. The beetles burrow through both the inner bark of the tree and the fungus. As the beetles eat, they cut the tubes the tree needs to move sap. Dave said that within three days after the first beetle arrives, they have done enough damage to the tubes that the host tree is doomed.
Because they live under the bark, spraying the trees with insecticide doesn’t kill the mountain pine beetles. Once tucked into an older tree, it takes five days of -30 degree weather to kill the beetles.
There are always mountain pine beetles in pine forests; they are a natural part of the forest ecosystem. In true predator fashion, in a healthy forest they attack trees that are weak or sick. But beetle numbers explode when pines get crowded, as they have over the last one hundred years.
Mountain pine beetles have killed over 3.6 million acres of pine in Colorado and southeastern Wyoming. The epidemic will probably end when there are no more lodgepoles older than five years to infect – an expected 90% of the lodgepoles will die. As the beetles run out of lodgepoles, ponderosa and limber pines are attacked as well.
But as the lodgepoles die, woodpeckers and other predators will feast on the beetles, and owls will have more nesting sites. As grasses and shrubs sprout beneath the dead trees, grazers like deer and elk will have more to eat. And in five years, as aspen move into the open space, the mountains will be dazzling in their autumn colors.