THE GENUS DENDROCTONTJS. 97 



dence is found in the fading foliage which begins to change to sorrel 

 tops in May and June and to red tops in July and August. The finding 

 of these conditions within the region occupied by this species will 

 indicate destructive work by barkbeetles, but positive evidence of 

 the presence of this species can only be determined by cutting into the 

 bark and finding the characteristic galleries and mines occupied by 

 authentically identified beetles. As a rule, the broods have left the 

 trees by the time the leaves are all dead, and sometimes before the 

 leaves have changed from yellow or sorrel to red. Exceptions are 

 frequently found when but one side or the top of a tree is killed the 

 first year and the remaining living bark is infested with broods of the 

 next. It is safe to conclude, however, that after the leaves are all 

 dead and brown, very few living examples of this species will remain 

 in the bark. 



EFFECTS ON COMMERCIAL VALUE OF THE WOOD. 



Owing to the thick sapwood of the western yellow pine, the com- 

 mercial value is reduced for certain purposes by a bluing condition, 

 which affects it soon after the trees are infested by the beetles in 

 August and September and long before the leaves begin to fade. 

 The heartwood of large trees is not usually reduced in value for several 

 years after the trees die, provided they do not suffer from subsequent 

 injury by storm, fire, wood-boring insects, or premature decay. If 

 left standing, however, with the bark on, until the branches and tops 

 begin to fall, the loss from decay may be complete. On the other 

 hand, if the bark be removed from the trunks of the standing trees, 

 the heartwood will remain sound for many years longer. 



The danger, however, of the total destruction of the dead timber 

 by forest fires is so great that in order to insure against such losses, 

 and at the same time destroy the broods of insects, the insect-killed 

 timber should be utilized before the insects emerge. 



FAVORABLE AND UNFAVORABLE CONDITIONS FOR THE BEETLE. 



Favorable conditions for the multiplication and destructive work 

 of this species are found in somewhat isolated forests with a predomi-' 

 nance of large mature timber. Unfavorable conditions for destruc- 

 tive outbreaks will be found in forests, isolated or not, which are kept 

 under a system of forest management or regulations which provide 

 for the utilization of the mature timber and the barking of trees 

 injured by lightning or dying from any cause, before the broods of 

 Insects develop in the bark and emerge. 

 89535— Bull. 83, pt. 1—09 8 



