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. 
