156 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



its range in the Western States. It also attacks western larch. 

 Normally it confines its attacks to felled, injured, or weakened 

 trees and is not of great importance. At times, however, it be- 

 comes 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 defoliations, or where trees 

 close to logging operations have been attacked by broods developed 

 in slash. In the commercial Douglas-fir region of Oregon and 

 Washington outbreaks are of less frequent occurrence, although 

 the killing 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 beetles, as no pitch tubes are formed. The adults are reddish to 

 dark brown, often black, beetles about y 5 inch long and very 

 similar to other Dendroctonus beetles (p. 131) 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 slightly etch the sapwood. 

 Typical galleries are perpendicular, usually straight or slightly 

 sinuous (fig. 71) and average about a foot in length, though they 

 may range from 6 to 30 inches. The eggs are laid in masses of 

 10 to 36 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. 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 

 congregate, sometimes for rather long periods, beneath the bark. 

 Finally 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 

 overwintering 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 over- 

 wintering larvae do not have time to complete their development 

 before cold weather overtakes them in the fall, and consequently 

 they are obliged to spend another winter in the host tree. One 

 generation of beetles a year is probably the normal rate of de- 

 velopment, but there is considerable overlapping and retardation 

 of broods, somewhat obscuring the demarcation between gen- 

 erations. 



The usual method of direct control is to fell the tree and cut the 

 infested bole into logs, which are then decked and burned. As a 

 large percentage of these insects overwinter as adults and emerge 

 early in spring, fall control is the most effective. 



