INSECT ENEMIES OF WESTERN FORESTS 129 



Key to the Diagnosis of Insect Injury to the Inner Bark (Cont.) 



a. Mines flattened, oval in cross section usually, packed with 

 arclike 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, p. 169 



b. Mines broadly oval in cross section, made by elongate grubs 

 which are thick in front with tapering bodies, thoracic seg- 

 ments enlarged, with horny plate on top only 



Roundheaded borers, p. 173 



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 twigs Bark weevils, p. 176 



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 pitch Pitch moths, p. 178 



BARK BEETLES 



The common term "bark beetle" (32, 88, 150) 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 2.8 

 billion board feet. The bulk of this destruction is caused by five 

 species of Dendroctonus — the western pine beetle, the mountain 

 pine beetle, the Black Hills beetle, the Douglas-fir beetle, and the 

 Engelmann spruce beetle. 



The bark-beetle adults are small, cylindrical insects, ranging in 

 size from the tiny Crypturgus, about y 20 inch long to the larger 

 species of the genus Dendroctonus, which attain a length of ap- 

 proximately % 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 tun- 

 nels 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, cut- 

 ting through the inner bark and often slightly or deeply scoring 

 the sapwood. As the work progresses, fine boring dust and excre- 

 ment are extruded through the entrance hole and collect in the 

 bark crevices. In some cases pitch and sap exude from the entrance 



