FOREST INSECTS 69 



live at the expense of trees of almost every description. Some 

 of the larger ones, belonging to the genus Dendroctonus 

 (Fig, ol), attack perfectly robust, healthy trees, but the ma- 

 jority live in weakened, moribund, or dead trees. Those of the 

 first type commonly appear in the form of invasions or epi- 

 demics, of which many have been noticed in our own coun- 

 try, from time to time. The others find great opportimities 



Fig. 31. A bark-beetle (Dendroctonus monticoUe); adult beetle, larva and pupa. (After Swaine.) 



for multiplication following sporadic outbreaks of other in- 

 sects like those mentioned above, and may complete the ruin 

 of considerable areas of forests that might eventually have 

 recovered from the effects of partial, or even complete defolia- 

 tion. 



Fortunately the bark-beetles that kill healthy trees are 

 few in number, but they are rather widespread in our own 

 country, and our coniferous forests have suffered greatly from 

 them in numerous instances. Destruction in these cases also, 

 has usually been due to sudden outbreaks of limited duration. 

 Thus, at several times during the last century, spruces in the 

 northeastern states have been killed over rather definite areas 



