INSECT ENEMIES OF WESTERN FORESTS 131 



that are not capable of coping with a vigorous pitch flow. Light 

 attacks by primary species on living trees often fail because the 

 flow of pitch is so copious that the attacking beetles are overcome 

 or driven from their galleries. The oleoresins are known to be 

 repellent and toxic to the beetles and so aid in resisting light 

 attacks. 



In the following discussion the bark beetles attacking western 

 forest trees will be treated under their principal host trees. 



PINE BARK BEETLES 



No group of commercially valuable trees in western forests has 

 more insect enemies than the pines, and of these, bark beetles are 

 the most numerous and destructive. The most aggressive bark 

 beetles attacking western pines are the so-called pine beetles, 

 which belong to the genus Dendroctonus. Several species in this 

 group are capable of attacking and killing normal healthy trees. 

 The damage they do in western pine forests runs into millions of 

 dollars annually (87). 



The next most important group comprises the pine engraver 

 beetles belonging to Ips, Pityogenes, and related genera. These 

 beetles usually work under thinner bark and make very striking 

 and distinctive forked or star-shaped gallery patterns. While they 

 normally breed in weakened, dying, or felled trees, or in broken 

 branches and slash, and are to that extent beneficial in hastening 

 the disintegration of forest debris, they occasionally develop in 

 sufficient numbers to become primary enemies of young trees and 

 of the tops of older ones. 



There is also a third group of bark beetles comprising a large 

 number of species that are secondary in their attack and are sel- 

 dom responsible for the death of any trees. Many of these are 

 found feeding under the dying bark of pines that are being killed 

 by other bark beetles, fire, or other causes, and sometimes are con- 

 fused with primary species. Space will not be taken for a descrip- 

 tion of all the bark beetles that may be encountered, for it is 

 usually sufficient for all practical purposes if the forester learns 

 to recognize species of chief importance. 



Dendroctonus Beetles 



The pine beetles that are members of the genus Dendroctonus 

 (meaning tree killers) (72) make up by far the most destructive 

 group of bark beetles attacking pine trees in North America. All 

 species breed under the thick bark of the trunk of living or dying 

 trees or in fresh stumps or logs of various pines. Some species 

 prefer felled, weak, or dying pines, whereas others apparently 

 prefer normal, healthy pines for their attack. 



The adults are stout, cylindrical, dark, reddish-brown to black 

 bark beetles ranging from % to about % inch long. The eggs, 

 larvae, and pupae are similar to those of other bark beetles. These 

 beetles are monogamous in habit and each pair constructs a single 

 egg gallery which, starting from the outside, penetrates to the 

 cambium and is extended between the bark and wood. Egg gal- 



