96 MISC. PUBLICATION 273, U. 8. DEPT. OF AGRICULTURE 
2. Bark showing no outward sign of insect attack. Tunnels 
under bark and sometimes entering wood; these increase 
in size with the growth of the white, often legless grubs 
which make them. 
a. Mines flattened, oval in cross section, usually packed 
with arc-like layers of boring dust made by 
slender grubs shaped like horseshoe nails; first 
thoracic segment greatly enlarged and flattened, 
with a horny plate on both top and _ bottom 
flatheaded borers, page 182. 
b. Mines broadly oval in cross section, made by elongate 
grubs which are thick in front with tapering 
bodies, thoracic segments enlarged, with horny 
plate on top only_______- roundheaded borers, page 154. 
c. Mines round in cross section ending in pupal cells 
partly in the wood, often lined with shredded wood 
fibers. Usually at base, root collar, or roots of 
weakened trees. Sometimes in terminals and 
GW Sele eae eet anes A pero teetts bark weevils, page 138. 
B. Tree apparently healthy or in some cases top-killed. Large masses 
of pitch exuding from wounds on trunk or with the bark, on a 
dying top, appearing pitchy and dry, and separating from the 
wood as though scorched. Slender caterpillars with three pairs 
of legs found working in the pitech_______________ pitch moths, page 139. 
BARK BEETLES 
The common term “bark beetle” (79) is applied to a group of 
small beetles belonging to the family Scolytidae. They are the most 
destructive group of insects to be found associated with western 
coniferous forests. Recent estimates place the annual loss of timber 
in the Western States as a result of their activities at 2 billion board 
feet. The bulk of this destruction is caused by three species of 
Dendroctonus, the western pine beetle, the mountain pine beetle, 
and the Black Hills beetle. : 
The bark-beetle adults are small, cylindrical insects, ranging in 
size from the tiny Crypturgus, about one-twentieth of an inch in 
length, to the larger species of the genus Dendroctonus, that attain a 
length of approximately three-eighths of an inch. Most species are 
unicolored, dark brown, reddish brown, or black, and are either 
shining or dull, though a few species have variegated markings. 
The head, which is more or less hidden by the thorax, has chewing 
mouth parts, with well developed mandibles. 
The adults of cambium-mining bark beetles have the very dis- 
tinctive habit of boring through the bark and making a tunnel 
between bark and wood in which to lay their eggs. The complete 
work or engraving of the bark beetles is therefore characterized by 
having two types of tunnels—egg galleries, made by parent adults, 
and larval mines, formed by the growing larvae. These tunnels 
form a particular pattern on the inner surface of the bark, which is 
distinctive for each species and usually very similar for each genus. 
In starting an attack the male or female beetle bores an entrance 
tunnel through the bark, usually at a slightly upward angle. An 
egg tunnel is then constructed along the surface of the wood, cutting 
through the inner bark and often slightly or deeply scoring the 
sapwood. As the work progresses, fine boring dust and excrement 
are extruded through the entrance hole and collect in the bark 
crevices. In some cases pitch and sap exude from the entrance hole 
