j^20 MISC. PUBLICATION 273, U. S. 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 it is 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 

 westi^-n 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 

 liemlock, 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 defolia- 

 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-growth 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 

 similar 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 ^gg galleries which are mostly in the 

 inner bark, though they also slightly etch the sapwoocl. Typical 

 galleries are perpendicular, usually straight or slightly 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 ^gg 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. In 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. 



Ordinarih^ 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 



