98 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 



from their galleries. The oleoresins are known to be repellent and 

 toxic to the beetles and so aid in resisting light attacks. 



In the follo\Ying discussion the bark beetles attacking western 

 forest trees will be treated under their principal host trees. 



Pine Baek 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 attacl^ing and killing normal healthy trees. The damage 

 they do in western pine forests runs into millions of dollars annually. 



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. Wliile 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 seldom 

 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 confused with 

 primary species. Space will not be taken for a description of all the 

 bark beetles that may be encountered, for it is usually sufficient for 

 all practical purposes if the forester learns to recognize those species 

 of chief importance. 



PINE BEETLES OE DENDEOCTONUS BEETLES 



The pine beetles which are members of the genus Dendroctonus 

 (meaning tree killers) {JpS) 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 one-eighth inch to about three-eighths 

 inch in length. 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 Qgg gallery which, starting from the 

 outsicle, penetrates to the cambium and is extended between the bark 

 and wood. Egg galleries differ in that some wind in a tortuous 

 manner, crossing and recrossing the galleries made by other pairs 

 of beetles, while others are straight and parallel to the grain of the 

 wood. In all cases Dendroctonus ^gg galleries are always packed 

 with boring dust, except for an inch or two at the end where the 

 beetles are working. This will distinguish the work of the Dendroc- 

 tonus beetles from that of other groups of bark beetles. 



