132 MISC. PUBLICATION 273. U.S. DEPT. OF AGRICULTURE 



leries 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. Den- 

 droctonus 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 Dendroctonus beetles from that of 

 other groups of bark beetles. 



Trees attacked by Dendroctonus beetles can first be distin- 

 guished by reddish boring dust caught in bark flakes or crevices 

 and around the base of the tree, or by pitch tubes that form on 

 the bark at the mouth of the entrance tunnels, but in heavily at- 

 tacked or decadent trees pitch tubes are often either missing or 

 so small that they can be seen only from a short distance. Later, 

 discoloration of the foliage furnishes a more noticeable evidence 

 of attack. It is difficult, however, to correlate accurately the dis- 

 coloration with the status of brood development, as this varies 

 with different tree species, regions, and seasons. The most con- 

 clusive evidences of attack are the egg and larval galleries on the 

 inner surface of the bark. These form a pattern so characteristic 

 for the work of each species that, when considered with locality 

 and host tree, the identification of the species responsible for the 

 attack is relatively simple. 



The western pine beetle (Dendroctonus brevieomis Lee.) (31) 

 is the most important insect enemy of ponderosa and Coulter pine 

 within the range of these trees from Baja California north into 

 Oregon, Washington, Idaho, Montana, and western Canada. Other 

 pines may be attacked under exceptional conditions. Normally this 

 beetle breeds in a few overmature trees, in windfalls, unhealthy 

 trees, or in trees weakened by drought, stand stagnation, or fires. 

 Under epidemic conditions it becomes aggressive and kills appar- 

 ently vigorous trees of all age classes having bark sufficiently thick 

 to protect the insect in its development. Trees under 6 inches in 

 diameter are seldom attacked, nor does this beetle breed in limbs. 

 The heaviest losses of mature merchantable ponderosa pine have 

 resulted from outbreaks of this insect in California, Oregon, and 

 Washington. It is less important in the more northern limits of 

 its range. Losses as high as 50 percent of the timber in 5 years 

 have been recorded, and many large blocks of pine timber have 

 been commercially ruined by its depredations (fig. 57). 



The adult beetles are about the smallest of the western species 

 of Dendroctonus and measure from % to about % inch long. The 

 larvae found in the outer bark are white, curved, and about the 

 size of a grain of rice. Their work is distinguished from that of 

 other bark beetles within the same range by the winding egg 

 galleries which cross and recross each other, forming a network 

 of irregular markings on the inner surface of the bark and on the 

 surface of the sapwood (fig. 58). The larvae feed in the inner 

 bark, working away from the egg gallery for about half an inch 

 and then turn into the outer bark, where they complete their de- 

 velopment. Flight and attacks start late in the spring or early in 

 the summer and continue until stopped by cold weather. There are 



