22 LEAFHOPPERS AFFECTING CEREALS, ETC. 
its availability as food material. An undue increase of the leaf- 
hoppers, which should result in the diminution of the food supply, 
must necessarily affect the possibilities of multiplication and cause a 
certain reduction in the number of the insects. This is, however, by 
no means the only statement of conditions as, aside from these two 
forms which may be associated in the same area, a large number of 
other organisms, both plant and animal, will affect the problem. 
The occurrence of different birds and predaceous insects which prey 
upon the leafhoppers will naturally reduce their numbers and to that 
extent favor the plants which serve as their food, whereas the pres- 
ence of herbivorous animals, grasshoppers, cutworms, etc., serves to 
reduce the available food supply. Aside from these dominant forms 
there are also various fungus parasites which attack both insects and 
plants and which play their part in the complex of which the leaf- 
hoppers are such a conspicuous element. Furthermore the minute 
insect parasites which attack the leafhoppers add their part, tending 
to keep the latter reduced in numbers. The relation of these and 
other direct parasites which concern them may be considered under 
the general head of natural enemies. 
NATURAL ENEMIES. 
That leafhoppers maintain a fairly average abundance from year to 
year, for the most part causing no perceptible devastation, is due to 
the fact that there are so many different natural agencies tending to 
reduce their numbers or to keep them in check. Of these natural 
enemies birds, spiders, and predaceous and parasitic insects are 
probably the most important and require careful consideration. 
It seems improbable that leafhoppers are affected to any great extent 
by mammals, except as eggs may be swallowed by foraging species — 
cattle, sheep, etc. The only forms which would seem likely to feed 
upon them are the moles and these confine their work so largely 
beneath the surface of the ground that it is doubtful if they would 
secure many of the leafhoppers. There are no records to show any 
service in this direction. 
Birds. 
Birds would undoubtedly be thought of as an important factor in 
the natural control of leafhoppers. It would seem that they might 
feed very commonly upon these insects ; and yet very little has been 
published in the way of correct determination or definite records of 
the kinds of birds which feed upon them, or the extent to which these 
leafhoppers enter into their normal food. The most complete records 
in this line are those accumulated by the Bureau of Biological Survey 
of the Department of Agriculture, which has for many years past been 
making a record of the contents of birds' stomachs. Besides the 
