INSECTS INJURIOUS TO FOREST TREES 515 



annual loss to our forests from insect attacks must amount to 

 many millions of dollars. 



A great part of this injury is due to the work of the bark- 

 beetles, belonging to the family Ipidce. These are small, 

 robust, blunt-headed beetles which burrow into the bark to 

 lay their eggs, and whose larvae burrow out through the live 

 bark in all directions from the egg chambers. Of these bark- 

 beetles the members of the genus Dendroctonus are the largest 

 and the most important. The adult beetles bore into the bark 

 until they reach the cambium where each species makes a 

 characteristic kind of burrow. The larvae also make character- 

 istic chambers in the bark or on the surface of the wood so.that 

 one who is acquainted with their work can always tell from the 

 nature of the borings just which species has been at work on a 

 tree. The members of the genera Ips, Scolytus, and others also 

 frequently do considerable damage. For a long while it was 

 believed that nothing could be done to control the spread of 

 these insects in the forests. But a study of their life- history 

 suggested that many of them might be controlled by cutting 

 out the infested trees and thus destroying the immature 

 stages of the beetles before the adults issue to attack new trees. 

 Experience has shown that this is practicable, and the Bureau 

 of Entomology has agents in the principal forest regions to 

 direct properly the work of cutting wherever the owners care 

 to undertake it. 



