156 OUR NATIONAL FORESTS 



needles had turned red would indicate to him that 

 the infestation was more than a year old, since 

 trees attacked in the spring of one year usually do 

 not show the results until the following summer. 

 These two stages are known by the trained ento- 

 mologist as the "yellow-top" and the "red-top" 

 stages respectively. The latter is followed by the 

 "black-top" stage. In this stage, insect infested 

 trees stand out very conspicuously as leafless, gray 

 or black snags, and they tell the story of the work 

 of bark beetles that happened years ago. 



Probably the first external evidence of the attack 

 of a bark beetle upon living trees with normal green 

 foliage, is the presence of pitch tubes upon the 

 outer bark. These are small, reddish-brown (later 

 becoming grayish-white) masses of pitch and saw- 

 dust, which exude from the small cylindrical en- 

 trance made by the adult beetle where it bores 

 through the bark to begin its egg tunnel. Each 

 tube represents the entrance of one or more of these 

 beetles. But we must follow these egg tunnels fur- 

 ther, to learn how the actual damage is done to the 

 tree. As soon as the bark beetle has made its 

 entrance through the bark, it starts to work up 

 through the live bark and cambium of the tree, 



