66 
THE SCOLYTID BEETLES. 
favorable conditions for the concentration of individuals from widely 
scattered colonies and broods, and thus increases their power of attack- 
ing and killing the surrounding living trees. By this means their 
forces are greatly augmented, and much of the surrounding timber is 
killed. If conditions favorable for the continued concentration of the 
beetles prevail from year to year, an invasion like that of 1891 and 
Fig. 28.— Spruce timber killed by the southern pine beetle, mountains of Transyl- 
vania County, N. C (Original.) 
1892 may be started, which may far exceed any forest fire hi the 
history of the country, both in the extent of area covered and in the 
number of trees of commercial value killed. 
Unfavorable conditions for the multiplication of the insect and its 
destructive attacl 
on 
living timber will be found in large areas where 
the older matured trees have been removed and where continued 
timber-cutting operations are carried on under some regular system of 
