THE GENUS DENDROCTONUS. 



45 



out into the outer corky bark to pupate and transform to adults. 

 After the adults are fully matured, and the proper time has come for 

 them to emerge, they bore out of the bark (fig. 8) and fly to other 

 trees, there to start a new at- 

 tack. While very few observa- 

 tions have been made on the 

 habits of flight, it is probable 

 that the beetles swarm during 

 the evening and at night. 



ECONOMIC FEATURES. 



While it appears that this 

 species prefers to attack weak- 

 ened and felled trees, or isolat- 

 ed healthy ones, it often attacks 

 large numbers of healthy trees 

 and causes extensive depreda- 

 tions. It is especially destruc- 

 tive to the western yellow pine 

 (see fig. 10) in central Idaho, 

 and in the mountains and 

 higher valleys of eastern Wash- 

 ington, Oregon, and California. 

 It must therefore be classed 

 among the important primary 

 enemies of the pines within its 

 range. As a rule, the largest 

 and best trees are attacked, 

 and the winding egg galleries 

 beneath the bark serve to com- 

 pletely girdle them and cause 

 their death before the broods 

 have developed and emerged. 



EVIDENCES OF ATTACK. 



The external evidence of at- 

 tack by this species on healthy 

 trees is the presence of pitch 

 tubes (figs. 8, 9) at the entrance 

 of the galleries, or reddish bor- 

 ings lodged in the flakes of 

 bark on the trunk and around 

 its base. Trees attacked in 



Fig. 9.— Work of the western pine beetle: Pitch tubes 

 on bark of tree. (From Webb.) 



