INSECT ENEMIES OF WESTERN FORESTS QQ 



Trees attacked by Dendroctonus beetles can first be distinguished 

 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 enti'ance tunnels, but in the case of heavily attacked 

 or decadent trees pitch tubes are often either missing or are 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, accurately to correlate the degree of discoloration 

 with the status of brood development, as this varies with different 

 tree species, regions, and seasons. The most conclusive evidences of 

 attack are the ^gg and larval galleries on the inner surface of the 

 bark. These form a pattern which is 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 {Dendroctorms hrevicoTriis Lee.) (20) 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 it 

 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 apparently vig- 

 orous trees of all age classes having bark sufficiently thick to protect 

 it in its development. Trees under 6 inches in diameter are seldom 

 attacked by this beetle nor does it 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. 48). 



The adult beetles are about the smallest of the western species of 

 Dendroctonus and measure from one-eighth to about one-fifth of an 

 inch in length. 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 

 ^gg 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. 49). 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, w^here their development is completed. 

 Flight and attacks start late in spring or early in the summer and 

 continue until stopped by cold weather. There are from one to two 

 generations annually in the northern part of the range and from 

 two and one-half to four generations in the southern portion, where 

 its activity continues almost without interruption throughout the 

 year. 



Woodpeckers, clerid beetles, and ostomatid beetles are important 

 natural enemies of this insect, though its abundance is more often 

 determined by climatic influences and the resistance of the host tree. 

 Prolonged winter temperatures of —20° F. and lower have been 

 found to cause heavy brood mortality. Rapid, vigorous tree growth 

 increases host resistance and discourages epidemics. 



