The mountain pine beetle (Dendroctonus ponderosae) is a small insect that lives most of its life in the inner bark of pine trees. The adult beetles are black to rusty brown and about 1/4 inch in length. They fly from infested trees to new host trees in late June or July. Once they have located a favorable host the adults tunnel beneath the bark to lay eggs. After the eggs hatch the young, known as larvae, feed within the tree until the following spring when they pupate, a resting stage, for several weeks before becoming adults. The adults emerge from the now dead host and seek a new tree to begin the cycle again.
The beetles can colonize trees in large numbers. The tunneling beneath the bark by the adult beetles and their larvae harms the tree by disrupting the movement of food, produced by the needles, to the roots. The adult beetles also can carry a blue-stain fungus from tree to tree. This fungus stops the movement of water from the roots to the needles. The combination of these two factors results in the tree's death.
More information regarding identification and biology can be found in the bulletin The Mountain Pine Beetle in the Black Hills.
During periods between outbreaks the most susceptible trees are those injured by natural events – most commonly those with the tops snapped by the wind or struck by lightning – or trees previously unsuccessfully colonized by mountain pine beetles. Fire and human-related disturbances such as skidder damage appear to be relatively unimportant.
During outbreak periods all ponderosa pines trees are candidates for attack. Generally when several trees in a group are colonized, the infestation continues to spread out from this initial point.
There are several natural regulating factors of mountain pine beetle. Woodpeckers, nematodes (roundworms) and several other insect predators and parasites often keep the low population in check. Extremely cold winter temperatures at specific times can also result in larvae mortality. Healthy trees can also defend themselves by “pitching out” the beetles as they bore into the tree. However, during outbreak conditions these natural regular factors are not as effective.
Burning within the Black Hills Forest Fire Protection District requires a permit. Permits are available from your local Wildland Fire Suppression office. Offices are located in Hot Springs (605.745.5820), Lead (605.584.2300) and Rapid City (605.393.8017); or by calling 800.275.4955.
If the infestation is in an urban development it is far better for homeowners to manage the bark beetle in a coordinated manner by following the recommendation listed under managing forest stands.
The most effective defense against the mountain pine beetle is maintaining well-managed tree stands. The most susceptible stands are those with trees more than eight inches in diameter and a basal area greater than 120 square feet.
As the average diameter and density decreases, the risk of mountain pine beetle attack also decreases. Adults may select trees as small as one-inch in diameter for attack but will not reproduce in them. Attacks in four to six inch trees are common during outbreak conditions and they can complete their life cycles within trees of this size.
The primary focus trees, the ones initially attacked and from which the infestation spreads, are usually greater than 10-inches in diameter. Greater importance, however, should be placed on the density of the stand. Crowded trees, those in stands with a basal area exceeding 120 square feet, are much more susceptible to attack due to two reasons.
First, the trees are competing with one another for water, nutrients and light and are generally not growing as vigorously as more open-grown trees. Second, the lower light intensities and cooler temperatures found in dense stands influence the attack behavior of the insect. Stands that have a basal area less than 100 square feet are much less susceptible to attack due to the more open light conditions and individual health of the trees.
A professional forester can help you determine the overall condition of your forested land and provide you with management advice.