370 WEST VIRGINIA EXPERIMENT STATION. 



insects were common, or at least not rare, both before the be 

 ginning and after the ending of the destructive outbreak ; but 

 in this case the insect was apparently exceedingly rare before 

 the beginning ; and apparently extinct in the invaded territory 

 after the ending of the trouble caused by it. 



When we come to consider the causes of the sudden death of 

 the insect over such a vast area, including such widely varying 

 conditions of altitude and climatic conditions, we have at once 

 a complicated and difficult problem to solve. 



ON THE CAUSES OF THE UNIVERSAL DEATH OF THE INSECT. 



The sudden and universal death of the destructive pine bark 

 beetle throughout the State, in the winter and spring of 1893, 

 was a most fortunate occurrence, and at the same time a most 

 remarkable one, not easy to explain. It was as if a supreme 

 power, dissatisfied with the undue increase of the species and 

 the widespread devastations wrought by it, had sent a plague 

 to exterminate it. In fast it is apparently the rule that, when 

 an animal of almost any species increases to excessive abund- 

 ance, it is, sooner or later, reduced to its normal position by 

 its natural enemies, parasitic and predaceous iusects, or unfav- 

 orable climatic conditions, or all combined. 



It is evident that the extermination of the destructive pine 

 bark beetle was brought about by one or more of these causes, 

 which we will discuss separately. 



CLIMATIC CONDITIONS. 



My first conclusion upon finding the insect dead in the bark 

 of trees examined in the early spring of 1893, was that they had 

 been killed by the excessive cold of the previous winter; and 

 from the records of the temperature by the Weather Bureau of 

 the Department of Agriculture, in the annual summaries of 

 1892-3, 1 it would appear that there was some good evidence up- 

 on which to base this conclusion. 



1. Table of maxium and minimun temperature Dec. 1892 and 1893. 



