August, ’24] 
BLACKMAN: HICKORY BARK BEETLE 
463 
possible to be sure that beetles from several such foci of infection may 
not have combined. 
The main facts of the history of this infestation to be emphasized and 
to be remembered in connection with the attempted explanation follow¬ 
ing are as follows:—(1) The consistantly rapid increase of the number of 
the hickory bark-beetle (as shown by the number of trees killed) 
from 1912—1914 inclusive; (2) the sharp reduction in the numbers of 
the beetles in 1915; (3) the lack of any marked increase or decrease in 
1916; (4) the steady but gradual increase and diffusion of the insects 
from 1917 to the present time. 
In the table given herewith the precipitation departures at Syracuse 
for each month of the years 1908 to 1922 inclusive is shown. By con¬ 
sulting this table it will be readily seen that there was an annual de¬ 
ficiency in rainfall for every year from 1908—1913 inclusive, a slight 
excess in 1914 and an excess of 10.24 inches in 1915. The total deficiency 
from 1908 to 1914 inclusive was 23.58 inches. A closer scrutiny of the 
data will show that much of this deficiency came during the three grow¬ 
ing months of the tree—June, July and August. 
An examination of the growth rings of thirty trees killed during the 
five years from 1912 to 1916 was made. A great diversity of conditions 
were found but these may be briefly recorded as follows. Of two trees 
killed in 1912 (the year of the incidence of the infestation in the wood lot 
studied) one showed very slow growth for a period of more than 20 
years before its being killed, while the other, closely adjacent to it, 
showed normal growth up till 1908 with but a slight reduction from 
1908—1912. Of five trees killed in 1913 two showed moderate reduction 
in the rings from 1908 to the time of their death, two showed similar 
reduction in growth from 1909, and one showed slight reduction from 
1906—1908, an increase in 1909 and slight reduction from then till its 
death. Trees killed during 1914 and 1915 showed the same variable 
conditions. In some the reduction in the rings was never excessive 
while in others it was very great. In some the reduction in growth ex¬ 
tended back many years, while in others it did not begin until 1910 or 
1911. It is certain from this data that the deficiency in moisture ex¬ 
tending over a period of several years has a decided effect upon the 
amount of new wood laid down by the tree. This effect is shown in 
some trees by an immediate and sharp reduction in the width of the 
growth rings, in others by a gradual reduction increasingly marked each 
year, while in other trees no effect is apparent until the deficiency has 
continued for several years. When the facts are all considered there can 
