120 MISC. PUBLICATION 273, U. 8S. DEPT. OF AGRICULTURE 
The balsam firs are very susceptible to bark-beetle damage, and. 
in certain years outbreaks sweep through fir stands and kill a high 
percentage of the trees. In such cases if 1s not uncommon to see 
entire hillsides turn red with the discolored foliage of dying trees. 
Since balsam firs are not of great commercial ‘value within our 
western forests, no estimates have been made as to the extent of 
such damage. 
In addition to aggressive tree-killing fir bark beetles, there are 
also a large number of secondary species that breed in dying or 
dead trees, slash, and broken tops. These, under exceptionally fa- 
vorable circumstances, may become destructive to living trees. 
The Douglas fir beetle (Dendroctonus pseudotsugae Hopk. ) is the 
most important bark-beetle enemy of Douglas fir throughout its 
range in the Western States. It also attacks western larch, western 
hemlock, and bigcone spruce. Normally it confines its attacks to 
felled, injured, or weakened trees and is not of great importance. 
At times, however, it becomes aggressive and kills apparently 
healthy, mature trees, singly and in groups, over extensive areas. 
Some serious epidemics have occurred in the Rocky Mountain region, 
particularly where trees were weakened by drought, fires, or detolia- 
tions. In the commercial Douglas fir region of Oregon and Wash- 
ington outbreaks are of less frequent occurrence, although the kill- 
ing of groups of mature trees in second- erowth stands is not 
uncommon. 
Reddish or yellow boring dust caught in bark crevices or around 
the base of trees gives the first evidence of attack by the Douglas fir 
beetle, as no pitch tubes are formed. The adults are reddish to dark 
brown, often black, beetles about one-fifth of an inch long and very 
sunilar to other Dendroctonus beetles (p. 98) except for their reddish 
color and their covering of conspicuous long hairs. These beetles 
work in pairs and construct egg galleries which are mostly in the 
inner bark, though they also shghtly etch the sapwood. Typical 
galleries are perpendicular, usually straight or shghtly sinuous (fig. 
59) and average about a foot in length, though they may range from 
6 to 30 inches. The eggs are laid in masses of from 10 to 36. These 
masses are in grooves at alternate intervals along the sides of the 
gallery. The larval mines diverge from the egg groups and are 
extended through the inner bark close to the wood. They expand 
as the larvae grow, so the completed work from each group of eggs 
is somewhat fan- shaped. The pupal cells, which are constructed 
at the ends of the larval mines, may be exposed when the bark is 
removed from the tree, or they may be concealed in it, depending on 
the thickness of the bark. Im these cells the transformation from 
larvae to pupae and then to new adults takes place. The new 
adults bore away the intervening bark between pupal cells and con- 
gregate, sometimes for rather long periods, beneath the bark. Fi- 
nally they bore through the bark to the surface, emerge, and fly 
to make their attack on other trees. 
Ordinarily the Douglas fir beetle passes the winter in the adult 
stage. although small to mature larvae also may be found. The over- 
wintering adults emerge rather early in the spring, but the delayed 
broods mature and emerge at any time throughout the summer 
months. It is also possible that some of the young overwintering 
