Mountain Pine Beetle

​The mountain pine beetle (MPB) belongs to a group of forest insects, bark beetles, which are the most destructive insect pests in western coniferous forests.

The MPB is usually present at low levels (endemic) and serves important ecological functions of killing weakened trees and recycling the nutrients they contain. However, outbreaks of this beetle occur every year in some forest of the western United States, causing significant losses of timber. Normally their populations are held in check by natural controls such as insect predators and parasites, insectivorous animals, nematodes and diseases. The health and vigor of the beetles food source, trees, also helps regulate their population. Occasionally, environmental conditions favor large increases in the beetle population, called epidemics, during which natural controls are ineffective and even healthy trees are at risk. Eventually the natural controls combine to cause the insect population to collapse back to endemic levels.

Biology

The MPB primarily attacks lodgepole, ponderosa, sugar and white pines in Nevada, although pinyon pines are also a recorded host. Scotch pine, a common landscape tree, is also highly susceptible to attack.

The beetle spends most of its life under the bark of infested trees, making direct control of the beetle difficult. It has four development stages: egg, larvae, pupae and adult (beetle).

The black or brown adult beetle, about the size of grain of rice , emerges from infested trees during the summer, normally from July to September, and attacks green trees. The beetles bore through the bark and construct a vertical tunnel (egg gallery) up to three feet long between the outer wood and inner bark in which the female lays up to 100 eggs. The eggs hatch in 10 to 14 days into legless, white larvae with brown heads. This stage lasts about 10 months, through the winter. The larvae feed on the inner bark, constructing individual tunnels at right angles to the adult tunnel. They construct oval cells at the ends of their tunnels where they transform into pupae. By July, the pupae have usually turned into adults which then chew an exit hole out through the bark. Soon after, they attack another tree.

The attacking beetles carry fungus on their body which infects the trees. The fungus spreads through water-transporting vessels in the tree, preventing the flow of water to the foliage. The larvae eat the portion of the tree, the inner bark, that transports the food manufactured by the leaves. These impacts, individually or combined, cause the trees death.

A trees defense against bark beetle attack is its ability to produce and exude resin at the point of attack, which “pitches out” the attacking beetle. When a weakened tree cannot produce sufficient resin to expel the beetle, the attack is successful. During epidemics, the number of attacking beetles can be so great, that even a healthy tree cannot repel them all, and it succumbs to the mass attack.

Evidence of Infestation

Infested trees have reddish boring dust around the base of the tree, in bark crevices and onmpb_new_fig1 the tops of branches where they intersect the trunk. Globules of resin called “pitch tubes” may be present on the tree trunk, where the tree attempted to “pitch out” the beetle.

Severely drought stressed trees may not have pitch tubes. Woodpeckers feeding on beetle larvae will peck holes in the bark, which may create piles of bark flakes at the tree base.

Foliage in the trees crown begins to fade, turning from green to yellow and eventually red. This may not happen until the spring following attack. Under the bark, infested trees will have a distinctive pattern of vertical egg galleries an horizontal larvae galleries. A blue-gray color in the sapwood is evidence of the blue stain fungus introduced by the adult beetles.

Control

Once a tree is successfully colonized by bark beetles, it cannot be saved. Infested trees should be cut and removed by early May, before the new generation of beetles emerge. If the tree is kept for firewood, cut wood should be piled in direct sunlight and covered with thick (= 3 mil), clear plastic. Avoid tearing holes in the plastic and bury the edges underground. This will raise temperatures under the plastic up to 160° F, baking the beetles. After 3 months of sunny weather all beetles should be dead. Peeling bark off the firewood is another effective, but difficult, way to kill the beetles.

Improving or maintaining trees health and vigor by thinning, fertilizing, and monthly irrigation is the best method of protecting forest trees. This helps maintain the trees normal defense mechanism against beetles. Avoid thinning and pruning during active flight periods as fresh wounds may attract beetles.

However, as mentioned above, even a healthy trees defenses can be overwhelmed during beetle epidemics. Individual, high value trees can be protected with an annual application of pesticide. The pesticide must be applied before adult beetles attack the tree, by early April. Trees killed by beetles the previous year may still have green foliage the following spring when spraying is done. Look for pitch tubes, boring dust and other signs of infestation before spraying a green tree. Spraying dead trees is expensive and of no value. Contact the Nevada Division of Agriculture for a list of pesticides registered for use against bark beetles on pine. read, understand, and follow instructions on pesticide labels.

Authored by: John Christopherson, Nevada Division of Forestry.