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.
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.