INSECT ENEMIES OF WESTERN FORESTS 179 
bution of dust is extremely difficult to obtain, owing to slight wind 
movements and irregular topography, and in order that dosages on the 
foliage may be lethal, a great deal more dust must be applied than 
would ordinarily be required in the ground-dusting of small trees. 
FiGURD 87.—Spreading calcium arsenate dust from an airplane in dusting operations 
against the hemlock looper in Washington. 
Furthermore, the question as to the effect of heavy applications of 
arsenicals to large forest areas has not been fully answered. In 
its present stage of development airplane dusting cannot be recom- 
mended for general use in the control of forest defoliators. 
BARK-BEETLE CONTROL 
A tree in which bark beetles have successfully established them- 
selves cannot be saved, and the best that can be done is to destroy 
the infesting insects before they are able to emerge and attack other 
trees (50). Bark-beetle broods can be destroyed by several methods 
of artificial control,® the method and time of application varying for 
different species and different regions. ‘Though bark-beetle out- 
breaks can be reduced, these insects cannot be exterminated, so con- 
trol measures must not be considered as a panacea or cure for all 
time. The results of a sucecssful bark-beetle control project may 
last for years or they may be of extremely short duration. 
In justifying the application of measures for the control of bark 
beetles, such factors as the value and merchantability of the timber, 
the destruction of the forest cover in its relation to watershed pro- 
tection, the creation of fire hazards, and the danger of the epidemic 
spreading into more valuable stands of timber must be considered. 
The proper evaluation of these factors, balanced against the cost 
of the operation, will determine the economic justification of the 
project. However, it is difficult to foresee the extent of the probable 
8 KEEN, F. P. MANUAL OF BARKBEETLE CONTROL IN WESTERN PINE FORESTS. U.S. Dept. 
Agr., Forest Serv. 58 pp., illus. 1927. [Muimeographed. ] 
