26 



SOME INSECTS INJURIOUS TO FORESTS. 



the beetles usually settle upon a few trees close together and crawl 

 about upon the bark from near the base to about two-thirds of the 

 distance to the tree's top, seeking suitable places for entrance. Crev- 

 ices in the bark are favorite places with them for this purpose. The 

 female appears to bore the entrance hole in the bark, and may or 

 may not be closely followed by her mate. In some cases where gal- 

 bries had just been started, females were found alone, that is, one 

 female to a single gallery. In others, the female was followed by the 

 male. As the first incision is made into the living inner bark, the tree 

 begins exuding pitch to cover the wound made by the intruding 

 beetle. This pitch or resin collects at the mouth of the entrance hole 



in the form usually known as 

 a pitch tube (figs. 11 and 12, 

 c). Where the attacking 

 force of beetles is small, the 

 efforts of the tree to heal 

 these wounds not infrequent- 

 ly succeed, the flow of pitch 

 being so great as to overcome 

 and suffocate the beetles. In 

 such cases the dead beetles 

 may be found in the pitch 

 masses after the tree has re- 

 covered. Where the attack- 

 ing force is large, however, 

 the flow of pitch does not se- 

 riously hinder the beetles. 

 After completing the egg lay- 

 ing, the parent adults remain 

 for some time in the galleries 

 and excavate irregular 

 branching burrows toward 

 the end farthest from the en- 

 trance, where they remain 

 until they die. 



After successfully effecting their entrance into the bark, th? females 

 excavate, through the inner layer of bark, winding, irregular gakeries, 

 which run into and cross each other many times (fig. 8). The eggs 

 are laid at the sides of the gallery, each in a little niche hollowed out 

 to receive it and packed in with the borings made in excavating the 

 gallery. 



Almost immediately after hatching the larva begins feeding upon 

 the cambium surrounding the niche in which it hatched. For a few 

 days it remains in the cambium, then bores out toward the outer 

 bark. As it progresses, it is at the same time growing, and this 

 growth is indicated by its constantly widening mine or burrow, which 



Fig. 12. — The western pine-destroying barkbeetle (Den 

 droctonus brevicomis): a, hibernating or transformation 

 cell; b, exit burrow; c, pitch tubes and exit holes. 

 (Original.) 



