Pine Beetle - A Disturbance in the Forest

One of the first species of tree a visitor is likely to encounter upon entering the Bighorn National Forest is ponderosa pine. This stately conifer is widely distributed throughout the west, and grows in nearly pure stands along the eastern flanks of the Bighorn Mountains.

Mountain pine beetle is a widespread and important insect in various pine forests throughout the Rocky Mountains. Adult beetles attack trees in July and August, when they bore through the bark into the phloem region, or inner bark. The females build a network of pathways in the phloem, called galleries, where they lay their eggs. In addition to depositing eggs, the beetles carry a fungi that causes a blue-gray staining of the wood. The combined action of the beetle galleries and the bluestain fungi inhibits the flow of water in the tree. The eggs quickly hatch, and the larvae overwinter in the trees, then mature and fly from their home tree the following summer to commence another cycle.

An excellent opportunity to witness ponderosa pine forest dynamics lies a short distance from Dayton, Wyoming. Driving west on Highway 14, the climb up the mountain begins as the short and mid-grasses of the Great Plains fade in the rearview mirror and are quickly replaced by ponderosa pine forest.

photo of beetle kill












The Ponderosa pine forest in the Sand Turn area is becoming a "taste treat" for the beetles, as the forests are the proper age, size and density for their spread.

Ponderosa forests share a common bond with other forest types in that they grow and change all the time. Fire, insects, and diseases are natural disturbances that change age, distribution, and density of forest communities. Historically, fire was the most significant factor to influence the Bighorn Mountains. Pine stands like those west of Dayton were kept open and park-like by the occurrence of frequent, low-intensity surface fires (and occasional crown fires) that ran up the mountain from the plains every few decades. Clad in a thick, scaly armor, the bark of mature ponderosa pines is ideally suited to withstand the recurrent ground fires that swept through the area and cleared the understory to create an open, widely scattered, pine forest.

With the advent of fire suppression around the turn of the century, the patchy ground fires that created openings and variety in the forest composition were snuffed out, and forests have become more densely stocked. As a result, insects, such as mountain pine beetle, play an increasingly active role in altering forest structure.

Trees that are under stress due to old age, drought, crowding, and diseases are most susceptible to an attack, but will often mount their own defense against an invasion; the presence of white or reddish-brown pitch tubes on the trunk indicate an attempt to drown beetles with resin as they bore into the inner bark. Despite these efforts, mountain pine beetles are often highly successful.

This picture shows mountain pine beetle at work in the ponderosa forest near Sand Turn. A number of these patches have become evident over the past few years. The trees in the center of this patch were killed several years ago, and are surrounded by red-needled trees that were killed in the last year or two. This is a typical spread pattern of the beetles.

Although an adult mountain pine beetle measures a scant 1/8 to 1/3 inch long, the impact they have on their environment is sometimes anything but small. Outbreaks have killed more than 1,000,000 acres of forest when they reach epidemic levels. This native insect has always been present in the Bighorns, but has only periodically reached population levels that kill large areas of forest.

Treatment of infestations is aimed at harvesting and burning, or chemically spraying, bug infested trees. Synthetic beetle attractants are also available to manipulate small outbreaks. Typically, management actions are not taken in remote or wild areas of national forests, as dead trees provide nesting sites for birds and small mammals, and the natural openings add habitat diversity with the increased grass, forb, and shrub growth.

During the current Forest Plan Revision process, evaluation of current and potential insect outbreaks and other ecological occurrences will be given to aid in planning for the future management of the Forest. The Revision will set goals and objectives for areas of the Forest where beetle management will be appropriate or inappropriate. For example, in timber production emphasis areas, timber harvest would be appropriate to reduce the beetle risk by thinning stands to a lower density. On the other hand, beetle outbreaks may be allowed to run their course in areas allocated to biological and species diversity emphasis.

The mosaic of beetle-killed trees interspersed with live trees in the pine forests of the Bighorns is a reminder that forests change, with or without human interventions.