INSECT ENEMIES OE INSECTS AND THEIR RELATION 

 TO AGRICULTURE 



By CuETis P. Clauskn" 

 Senior Entomologist, United States Department of A<!ri('ul1urc 



Insects of various types play a very important role in affecting 

 the production of practicall}'^ all agricultural crops and products 

 and have an important bearing upon the health of man and animals. 

 Everyone is familiar with the common insects which each year attack 

 garden and field crops. The aphids upon roses and other plants, the 

 caterpillars and beetles which destroy the foliage of shade and fruit 

 trees, not to mention the maggots and caterpillars in fruits and 

 vegetables, daily come to our attention. Practically all of these 

 which most frequently present themselves, whether as plant feeders, 

 as burrowers in wood, or as destroyers of clotliing, or which feed 

 directly upon man himself, are injurious and troublesome, and in 

 the public mind all insects consequently come under this classifica- 

 tion. This condemnation, however, is entirely undeserved by a vast 

 array of insect species which are entirely harmless in themselves, and 

 at the same time are actively engaged in destroying the injurious 

 species. That insects should prey upon one another is no more 

 strange than that the larger animals should do so. The manner in 

 which this is accomplished in the insect world, however, is much 

 more diversified. 



Under normal conditions in nature a state of equilibrium exists 

 between all the elements which go to make up the plant and animal 

 world. No one species attains a pronouncedly dominant position, 

 and, on the other hand, the species which prey upon it do not 

 increase to an abnormal extent. Various influences may disturb 

 this balance from time to time, though these are only temporary 

 and the normal condition is soon restored. The advent of man, 

 however, has completely and permanently upset the equilibrium in 

 large areas throughout the world. The elimination of the natural 

 flora and the substitution in many localities of a single agricultural 

 crop have been followed by very unexpected results. These vast 

 areas of new vegetation made conditions ideal for the development 

 of insects which feed upon these particular plants, and instead of 



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