February, ’24] 
WEBSTER: INSECT CROP DAMAGE 
57 
damage more severely than one growing with an amount sufficient for a 
normal and continuous progress. Lack of moisture affects the insects 
very little since they are able to obtain this by consuming plant food. 
On the other hand, a crop with superfluous moisture may also 
suffer from insect attack, altho by reason of different insects. 
Deficient Moisture and Sod Web worms 
An outbreak of sod webworms in certain counties in southwestern 
Iowa in 1918 serves to illustrate how insect damage may be quite severe 
under dry conditions. There was a deficiency of moisture in April and 
May 1918, coupled with a temperature several degrees higher than 
normal for the month of May. In that part of the state where the in¬ 
jury was most common, the surface features of the soil have been 
much eroded. In a geological sense, it is an old country. The damage 
by the sod webworms was most evident on the brow of the hills. The 
lower land suffered less from the insects probably due to a higher moisture 
content of the soil which may have been correlated with a higher con¬ 
tent of organic matter. But the hilltops also were less injured. The 
most plausible explanation of all this is that the direct rays of the sun 
dried out quickly the surface moisture from the hillsides and so re¬ 
tarded plant growth but also served to accelerate the rate of develop¬ 
ment of the insects by reason of higher temperature, since the sod 
webworms feed on the roots close to the surface. 
* 
Deficient Moisture and Pale Western Cutworm 
In 1921, in western North Dakota, severe damage by the pale western 
cutworm occurred to various crops, especially to wheat. Altho the 
cutworms were more numerous in 1922 than during 1921, less actual 
damage took place, mainly because of the abundance of moisture during 
the spring of 1922. In May 1921, at Beach, North Dakota, less than an 
inch of rainfall was recorded. This amount was two inches below the 
normal. During the same month in 1922, there was approximately 4 
inches of rainfall, more than an inch over the normal amount for May. 
During 1921, the growth of all crops was retarded because of insufficient 
moisture. This lack of moisture had no such effect on the cutworms, 
which merely worked lower down in the soil and caused still more damage 
to crops by cutting off the stems below the growing point. With an 
abundance of succulent green food, a larger number of insects in 1922, 
caused a comparatively small amount of damage. 
