INSECT ENEMIES OF WESTERN FORESTS I79 



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. 



Figure 87. — Spreading calcium arsenate dust from ;ni airiiluiu^ 111 (uiMiiiy 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. [Mimeographed.] 



