98 MISC. PUBLICATION 273, U. 8. 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 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. 
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 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 OR DENDROCTONUS BEETLES 
The pine beetles which are members of the genus Dendroctonus 
(meaning tree killers) (46) 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 egg gallery, which, starting from the 
outside, 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 egg 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. 
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