Host:  Ponderosa pine

Figure 101. Egg gallery of western pine beetle on ponderosa pine. Note that galleries have a sinuous, crisscrossing pattern.

Symptoms/Signs:  Larval feeding in the inner bark and adult mating and egg laying creates mazelike galleries. Entrance holes, reddish dust in bark crevices, and pitch tubes are indicators of attack. Exit holes are created when the mature beetle leaves the host. Trees, which have been successfully attacked by beetles, are most noticeable when the foliage begins to fade from green to yellow to red to gray. Also, infested trees often exhibit signs of woodpecker feeding activity in the bottom half of the bole. Needles fall off approximately 1 year after the initial attack.

Figure 102. Cross section of bark showing western pine beetle larvae in outer bark.

Biology:  Adult beetles usually begin flight and attack of suitable host trees in late spring or early summer and continue until cold weather begins. Females produce from one to three broods a year. Eggs are laid in niches along galleries. When the eggs hatch, the larvae feed in the inner bark working away from the egg gallery.

Effects:  The direct effect of successful attack is tree mortality. Usually, the beetles breed in and kill scattered, overmature, slow growing, decadent, or diseased trees and trees weakened by stand stagnation, lightning, fire, or mechanical injury. This tree mortality may be considered part of the normal ecological process of succession during which a forest matures and replaces itself. Epidemics can affect ponderosa pine ecosystems by reducing the density and size distribution of its host, and altering species composition and stand structure.

Similar Insects and Diseases:  Other bark beetles attacking ponderosa pine. Western pine beetle egg galleries can be distinguished from other beetles by their maze-like appearance.

References:  18, 24

Figure 103. Pitch tubes indicating western pine beetle attack are frequently small in size (<1/4 inch, 6.4 mm).

Figure 104. Woodpecker activity ("flaking off of bark") on ponderosa pine indicating that western pine beetle larvae are in the outer bark.