468 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 17 
amount of water than it would under like conditions in a dry period of 
much sunshine. 
Perhaps the most conclusive evidence of the influence of these 
several factors upon the bark beetles is furnished by the decided check¬ 
ing of their injuries due to the excess in rainfall during July and August 
in 1915. Corroborative evidence of the opposite effect of a deficiency in 
precipitation is, however, furnished by the development of the epidemic 
from 1912-1914 inclusive and by the renewed increase in damages in 
the years following 1915 when a deficiency again became evident. As 
will be seen by referring to the table, the first six months of 1915 showed 
a slight deficiency in precipitation so that the trees were in no better 
condition as to stored-up resistance than in previous years. In the 
second half of the year, however, there was an excess of 11.22 inches and 
of this excess more than half came in the months of July and August 
when the effect upon the beetles and their larvae would be greatest. 
As has already been stated, only 34 trees were killed, although the pre¬ 
vious year 111 had suffered mortal injury. As we have already seen the 
reason for this could not have been due to the condition of the trees at 
the time the beetles emerged from their old hosts and there can be no 
reasonable doubt that the partial control if due to weather conditions 
was due to such as existed after their emergence. 
The rainfall in July was 3.69 inches above the normal for the month. 
There was rainfall on 19 days and 21 days were cloudy or partly cloudy. 
In the first half of the month 10 days showed rainfall varying from .01 
to 2.63 inches. This is the period when the adults are emerging and 
feeding upon the twigs and petioles—when they are most exposed to 
injury. In August the excess precipitation was 2.12 inches. Rain fell 
on 19 days, there being 11 rains during the first 15 days. There were 
but 3 clear days and of the remaining 28 days (cloudy or partly cloudy), 
13 are listed as cloudy. This is the period when excess of precipitation, 
especially when accompanied by a greater amount of cloudiness than 
normal, would if our deductions are correct, cause a greater sap-pressure 
in the tree and in this way check the work of both adults and larvae bor¬ 
ing in the living tissues. 
At the time, the writer did not fully realize the significance of what 
was occurring and the observations made were not as detailed and 
systematic as would be desired. However, in September a number of 
trees—including not only those killed but also a large number not 
mortally injured—were examined. The most striking fact with regard 
to the killed trees was that the brood galleries were much shorter than 
