FOREST INSECTS 



71 



and to inhibit the development of extensive outbreaks. Such 

 a conclusion appears to be borne out by experience, except in 



FIG. 32. Work of the Black Hills bark-beetle (Dendroctonus -ponderosce) on the inner 

 surface of the living bark. The vertical galleries are excavated by the parent beetles 

 and the smaller lateral ones are the food burrows of the larvse. The entrance holes 

 (a) are at the lower end of the primary gallery, which is ventilated by other holes, 

 indicated as white spots. (Slightly modified from Hopkins.) 



a few cases, and there the explanation must be sought for 

 along other lines. 



It might appear from such a brief comparison of insect 

 damage to pure coniferous forests (i.e., those which practi- 



