THE GENUS DENDROCTONUS. 23 



ferent sections of the country and between different species of 

 Dendroctonus, as mentioned under the more detailed discussion of 

 the several species. 



CLIMATIC INFLUENCES. 



DROUGHT. 



It is the common impression that the death of pine and spruce 

 timber in certain sections of the Rocky Mountain region is primarily 

 due to a weakened condition resulting from drought and that the 

 work of the insects is secondary. Under the influence of exceptionally 

 severe drought during several successive seasons this may be true to 

 a very limited extent, but our observations lead us to conclude that 

 drought does not offer specially favorable conditions for the multi- 

 plication and destructive work of barkbeetles. In fact, the reverse is 

 more likely to be the rule, since exceptionally dry conditions appear 

 to be more unfavorable for the development of the beetles than 

 humid conditions. The only exception we have noted in which injury 

 is greater in dry sections than humid ones is that of the Douglas fir. 

 In the more southern range of this tree, where the normal dry char- 

 acter of the climate and soil prevails, it suffers more from the Douglas 

 fir beetle (No. 13) than it does in the Northwest, where, under moist 

 conditions and rich soil, the tree reaches its best development. This 

 beetle is very abundant in the Northwest, yet as a rule it confines its 

 attack to the felled and injured timber and rarely attacks the healthy 

 trees. On the other hand, the western yellow pine suffers more 

 severely in the humid sections than it does in the more arid ones, as 

 demonstrated by the work of the Black Hills beetle (No. 10), which 

 is widely distributed over the eastern section of the Rocky Mountain 

 region, yet has been far more aggressive and destructive in the Black 

 Hills National Forest than in the much drier sections in southern 

 Colorado and northern New Mexico and Arizona, and has continued 

 its depredations in the Black Hills unabated through excessively wet 

 as well as excessively dry seasons. 



The western pine beetle (No. 1) is far more abundant and destruc- 

 tive in the northern and more moist climate of the mountains of 

 Idaho, Oregon, and California than is its near relative, the south- 

 western pine beetle (No. 2) in the drier forested areas of New Mexico 

 and Arizona. The mountain pine beetle (No. 9) is exceedingly 

 destructive to the lodgepole pine at high altitudes and under espe- 

 cially moist conditions. The same may be said of the eastern spruce 

 beetle and the Engelmann spruce beetle. It is evident, therefore, 

 that drought is not an important factor in contributing to the multi- 

 plication or destructiveness of this class of enemies. 



