488 
Journal of Agricultural Research y 0 i. xxx, No. 5 
perature is approached, although win¬ 
ters are cold, the weevil may maintain 
itself and become of economic import¬ 
ance periodically. A mean annual 
temperature of 40° F. closely marks 
the northern limit of economic distri¬ 
bution. 
Probable Distribution in America, 
Based on Climate 
The writer carefully analyzed the 
climatological data from the Western 
States, considering all the possible 
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Fig. 10.—Cliinograph for Delta, Colo. 
combinations of temperature and rain¬ 
fall found to be of importance in the 
ecology of the alfalfa weevil. The 
stations may be tentatively grouped 
under'three categories: 
1. Regions whose normal climate 
approaches the optimum for the alfalfa 
weevil may be regarded as subject to 
severe infestation and included in an 
area of “normal” occurrence. 
2. Regions whose normal climate 
departs slightly from the optimum 
may be regarded as subject to periodic 
infestation in times when the climatic 
variations are toward the optimum. 
These regions are areas of “occasional ” 
occurrence. The weevil is able to 
maintain itself at all times in these 
areas and becomes abundant when 
conditions become favorable. 
3. Regions whose normal climate 
varies widely from the optimum, in 
which optimum conditions will rarely 
occur, may be included in an area of 
“possible” occurrence. The weevil 
will probably not maintain itself in this 
area but may be repeatedly introduced 
and become of minor importance after a 
series of favorable seasons. 
The probable distribution of the 
alfalfa weevil in America, based upon 
these climatic requirements, is shown 
in the map (fig. 11). The normal 
occurrence of the weevil is limited 
entirely to the sub-Pacific and Pacific 
rainfall regions, which are indicated 
on the map. There are two localities 
outside of these regions which are 
included in the second zone, one in 
northern Wyoming and the other in 
western New Mexico. Rainfall condi¬ 
tions are favorable in Wyoming, but 
the mean temperature is low and the 
winters are cold and dry; therefore 
this area is not of great importance. 
Western New Mexico, on the other 
hand, has favorable temperature con¬ 
ditions, but the rainfall is of such a 
character that wet years are very 
likely to occur often enough to keep 
the insect in control. The possible 
occurrence of the outer zone, which 
are confined to the Plains and Arizona 
rainfall types are chiefly conditions 
comparable to Moscow, Russia (fig. 4), 
but with more extreme winter con¬ 
ditions. 
Broadly speaking, the alfalfa weevil 
is limited on the north, inside the 
Pacific and sub-Pacific rainfall regions, 
by low summer temperatures; on the 
east, at the edge of the sub-Pacific 
type, by heavy summer rainfall; and 
on the northeast, in the plains of 
Montana and Wyoming, by the sum¬ 
mer rainfall and by low winter tem¬ 
peratures, plus a very light snowfall. 
No data have been found upon which 
to base a limit on the south, but it is 
possible that the high temperatures of 
southern California and Arizona may 
prove fatal to larvae. All of this 
region is included in the normal zone 
in this study, and must remain so until 
the factor controlling southern dis¬ 
tribution becomes evident. 
The question of the possible adapta¬ 
tion of the alfalfa weevil to new climatic 
conditions in America has not been 
approached in this paper and does not 
seem of importance to the writer. 
The weevil is a native of Europe and 
