Management Strategies For the Mountain Pine Beetle

What Is the Mountain Pine Beetle?

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.

What Can Be Done To Manage This Insect?

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 attacked 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 successfully colonized, the infestation continues to spread out from this initial point.

There are several natural regulating factors of mountain pine beetle. Woodpeckers, nematodes (a very small roundworm) 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 against colonization by "pitching out" the beetles as they bore into the tree. However, during outbreak conditions these natural regular factors are not as effective.

How can I protect the one or two ponderosa pines on my residential lot?

Protection for trees that are not infested: Homeowners may want to consider treating large diameter or newly planted ponderosa pine trees with a preventive spray. Contact the division, or your local commercial applicator, to find out more about preventive spraying. Be sure that whatever formulation is chosen is labeled for mountain pine beetle control. These specific formulations contain additives that "hold" the pesticide on the bark longer. The pesticide should be applied from ground level to about 35 feet or where the trunk is less that 5 inches, whichever occurs lower. Spray until the entire circumference to the appropriate height is thoroughly moistened to the point of run-off. It is strongly recommended that homeowners contact a professional service to provide this treatment to be sure that the tree is thoroughly covered. This treatment must be applied before June, the earliest date the adult beetles begin to fly. These products will provide one season of protection so they must be reapplied every year that outbreak conditions persist.

Protecting the surrounding trees from an infested one: If one or two nearby trees (within 1/4- to 1/2-mile) are successfully attacked, these trees must be treated to prevent the infestation from spreading to the surrounding trees.

The green infested trees should be removed and treated prior to beetle flight that occurs in late June or July. These infested trees can be burned. If the infested tree is felled before the beetle pupates, about the middle of May, the tree can also be either chipped or debarked and left on site. Once the beetle has pupated debarking or chipping are not effective strategies for eliminating the insect.

Probably the most reasonable treatment that leaves the trees on site is cutting the infested tree down and then cutting the log into 2 foot lengths. The cut pieces should not be stacked and should be left exposed to as much sun as possible. This will cause drying of the inner bark tissue and beetle death. The part of the log that is in contact with the ground will remain somewhat protected and will produce some beetles. Generally, the amount surviving on the underneaeth side is relatively low, 10-20% of the total. If there is concern about killing these beetles the logs can be rotated at least once after cutting to ensure all sides of the log dry. Treatments should be done as early as possible to allow for maximum drying time.

If the infested trees are felled between late summer and the next spring and they are not debarked, the best action is to fell and burn the trees as soon as possible. Another option, although not as effective, is through a solar treatment. The tree is cut into 4 foot lengths (the shorter the better) to a stem diameter of 5 inches. Smaller material, such as found near the top of the tree or branches, generally is not infested. The wood should be stacked no more than two logs high in a sunny location, preferably a south-facing slope, and covered with 6 mil, clear plastic. The plastic must be well-sealed at ground level. The logs should be soaked with water before covering. The logs will become moldy within a few weeks. This will aid in killing the beetles but should not greatly reduce the fuelwood value of the wood. The wood should be left covered for at least two warm months. The solar method should be avoided if at all possible as it may only be 50 percent effective. The adults that do emerge from the logs can easily chew through the plastic and attack other trees. Spraying infested trees, whether standing or felled, to kill emerging beetles is not effective. Girdling infested trees is not 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 1-800-275-4955.

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.

How can I protect my forestland?

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 increased air-movement through the stand disrupting the beetle's flight, and dispersing the beetle's pheremones . A professional forester can help you determine the overall condition of your forested land and provide you with management advice.

*The use of trade names is for reader convenience and does not imply product endorsement.

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