The Mountain Pine Beetle is a native insect that has always been present in our Montana forests. Its numbers have historically been held in check by harsh winters with a week or more of below-zero temperature. Factors predisposing forests to severe outbreaks of beetle infestation are milder winters and more even-aged, denser stands. Lodgepole Pine is the most susceptible species, but Ponderosa Pine and Whitebark Pine are also killed
The beetle normally hits younger trees. The infestation doesn’t typically spread outward to neighboring trees. Instead, beetles that have just emerged fly greater distances to attack other trees. The more limited, “natural”outbreak can create openings in the forest that promote diversity and forest health.
Sign of infestation
The first sign of an infested tree is a “pitch tube” plainly visible on the outside of the bark. The tube is created as the tree attempts to “pitch out” the attacking beetle. If the tree successfully repels the beetle, the tube is milky white. If the beetle’s attack is successful, the pitch tube is darker in color and boring dust may be visible on the tree and on the ground. If there are many pitch tubes on a tree, it is likely to be killed. If very few, it may survive. The beetles kill during their larval stage, as they eat away the nutrient layers under the bark.
The time of year when beetles attack healthy trees is when they emerge from infested, “brood” trees. This is typically in late June or early July, but may be earlier when winters are warmer. The emergent beetle lays eggs under the bark of a new host tree. Eggs hatch, and the feeding larva consume the tree’s nutrient tissues. By the following spring, the feeding larvae may have disrupted the nutrient flow of the tree enough to kill it. The beetle’s life cycle is completed when the larvae transform to adult beetles and emerge in early summer to infest another tree. A dying tree may be noticeable in the spring because of its yellowing needles. After the tree has been killed, all of its needles will be red and it will stand out plainly in the forest. By the time a tree is red, the beetles have emerged and infested other trees.
1. Silviculture: Cut and burn or peel infested trees; thin the forest so trees are less stressed and tree species diversity is promoted. Cutting and “tarping” (covering with plastic tarp) of infested trees is not an effective control. Cutting an infested tree and scoring its bark with a chain saw to dry out the tissue underneath the bark may kill the developing beetles. Cutting an infested tree and peeling off its bark is an effective control.
2. Pheromones: Two hormones are released by beetles. Early in the period during which a tree is being infested, a “happy” (aggregation) hormone is produced. It is attractive to other beetles. Later, an “unhappy”(anti-aggregation) hormone is produced as the tree becomes “full” of infesting beetles. The anti-aggregation hormone “Verbenone” is available commercially at $30 per packet. It can be placed at about 30 foot grids to protect trees in an infested area. Infested, “brood” trees must be removed first. To ensure maximum effectiveness of the hormone, it should be put out as late as possible before the beetles emerge. Mid-June should be good when beetles are expected to emerge in early July. Local sources of hormone packets are Quality Supply, Ace Hardware, and Cenex.