THE GENUS DENDROCTONUS. 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 S 
