95 MISC. PUBLICATION 273, U. S. 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 132. 

 &. 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 134. 



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, 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 pitch 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 Crypturgiis, about one-tAventieth 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 — Qgg 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 

 Q>gg 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 



