INSECT ENEMIES OF WESTERN FORESTS 99 
Trees attacked by Dendroctonus beetles can first be distinguished 
by reddish boring dust caught in bark flakes or crevices and around 
the base of the tree, or by pitch tubes that form on the bark at the 
mouth of the entrance tunnels, but in the case of heavily attacked 
or decadent trees pitch tubes are often either missing or are so small 
that they can be seen only from a short distance. Later, discoloration 
of the foliage furnishes a more noticeable evidence of attack. It is 
difficult, however, accurately to correlate the degree of discoloration 
with the status of brood development, as this varies with different 
tree species, regions, and seasons. The most conclusive evidences of 
attack are the egg and larval galleries on the inner surface of the 
bark. These form a pattern which is so characteristic for the work 
of each species that, when considered with locality and host. tree, the 
identification of the species responsible for the attack is relatively 
simple. 
The western pine beetle (Dendroctonus brevicomis Lec.) (20) is 
the most important insect enemy of ponderosa and Coulter pine 
within the range of these trees from Baja California, north into 
Oregon, Washington, Idaho, Montana, and western Canada. Other 
pines may be attacked under exceptional conditions. Normally it 
breeds in a few overmature trees, in windfalls, unhealthy trees, or 
in trees weakened by drought, stand stagnation, or fires. Under 
epidemic conditions it becomes aggressive and kills apparently vig- 
orous trees of all age classes having bark sufficiently thick to protect 
it in its development. Trees under 6 inches in diameter are seldom 
attacked by this beetle nor does it breed in hmbs. The heaviest losses 
of mature merchantable ponderosa pine have resulted from outbreaks 
of this insect in California, Oregon, and Washington. It is less 
important in the more northern limits of its range. Losses as high 
as 50 percent of the timber in 5 years have been recorded, and many 
large blocks of pine timber have been commercially ruined by its 
depredations (fig. 48). 
The adult beetles are about the smallest of the western species of 
Dendroctonus and measure from one-eighth to about one-fifth of an 
inch in length. The larvae found in the outer bark are white, curved, 
and about the size of a grain of rice. Their work is distinguished 
from that of other bark beetles within the same range by the winding 
egg galleries which cross and recross each other, forming a network 
of irregular markings on the inner surface of the bark and on the 
surface of the sapwood (fig. 49). The larvae feed in the inner bark, 
working away from the egg gallery for about half an inch and then 
turn into the outer bark, where their development is completed. 
Flight and attacks start late in spring or early in the summer and 
continue until stopped by cold weather. There are from one to two 
generations annually in the northern part of the range and from 
two and one-half to four generations in the southern portion, where 
its activity continues almost without interruption throughout the 
year. 
Woodpeckers, clerid beetles, and ostomatid beetles are important 
natural enemies of this insect, though its abundance is more often 
determined by climatic influences and the resistance of the host tree. 
Prolonged winter temperatures of —20° F. and lower have been 
found to cause heavy brood mortality. Rapid, vigorous tree growth 
increases host resistance and discourages epidemics. 
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