RELATIVE RESISTANCE OF THE RICE WEEVIL, SITO- 
PHILUS ORYZA L., AND THE GRANARY WEEVIL, S. 
GRANARIUS L., TO HIGH AND LOW TEMPERATURES 1 
By E. A. Back, Entomologist in Charge , and R. T. Cotton, Entomologist, Stored 
Product Insect Investigations, Bureau of Entomology, United States Department 
of Agriculture 
During several years of study on the grain weevils the writers have observed 
that the rice weevil, Sitophilus oryza L., and the granary weevil, S. granarius L., 
although belonging to the same genus and closely resembling each other in size, 
structure, and habits, differ markedly in their resistance to high and low tem¬ 
peratures. Whereas the rice weevil is predominant in the Middle and Southern 
States, it has not established itself in the extreme Northern States and is unable 
to withstand the northern winters except in protected situations. On the other 
hand, the granary weevil is seldom if ever found in the South below North 
Carolina, but is the predominant form in the Northern States. In order to 
obtain more precise information on the relative effects of various temperatures 
on the two weevils, a number of experiments were carried on in the laboratory 
with small refrigerator units and incubators, in which the temperatures could be 
controlled. The results obtained were striking and seemed worthy of record. 
A constant temperature of zero Fahrenheit proved quickly fatal to both 
species, although the granary weevil was able to withstand even this low tem¬ 
perature slightly longer than the rice weevil. Adults of the rice weevil were 
killed after *an exposure of four hours, and adults of the granary weevil after an 
exposure of five hours, to this temperature. 
Up to a certain point, as the temperature was increased, the difference in 
resistance between the two species became more and more apparent. At a tem¬ 
perature of 5° F. an exposure of four and a half hours was sufficient to kill adults 
of the rice weevil, whereas an exposure of seven and a half hours at the same 
temperature was required to kill adults of the granary weevil. 
With a temperature ranging from 15° to 20° F., adults of the rice weevil were 
killed in 3 days and adults of the granary weevil in 14 days. With a temperature 
ranging from 20° to 25° F. a majority of adults of the rice weevil were killed 
in 3 days, although a few showed feeble movements after an exposure of 6 days. 
These longer-lived individuals all died shortly thereafter, without regaining normal 
activity. Adults of the granary weevil were killed at this temperature only 
after an exposure of 33 days. 
With a temperature ranging from 25° to 30° F., adults of the rice weevil were 
killed in 8 days and adults of the granary weevil in 46 days, or nearly six times 
as long. At 30° to 35° F. a majority of adults of the rice weevil were killed in 
8 days; a few, however, showed faint movements after an exposure of 16 days, 
but died without regaining normal activity. An exposure of 73 days at this 
temperature was necessary to kill adults of the granary weevil. 
With a temperature ranging from 35° to 40° F., adults of the rice weevil were 
killed in 18 days. A few specimens of the granary weevil showed faint move¬ 
ments after being exposed to this temperature for 111 days. With a tempera¬ 
ture ranging from 40° to 45° F. adults of the rice weevil were killed in 80 days, 
1 Received for publication Apr. 19,1924—issued Nov., 1924. 
Journal of Agricultural Research, 
Washington. D. C. 
( 1043 ) 
Vol. XXVIII, No 10 
June 7, 1924 
Key No. K-132 
