52 



INSECTS AND HUMAN WELFARE 



Predatory insects also play an important part in reducing 

 the abundance of j)hytopliagous insects. In habits they are 

 directly comparable to the beasts of prey among the higher 

 animals, which destroy the weaker and more timid mammals. 

 They do not show the same close correlation to their prey 

 exhibited by insect parasites and cannot usually be depended 



Fig. 25. A common beetle (Helops viicans) killed by a parasitic fungus. 



upon to restrict their depredations to any particular insect 

 pest. Quite frequently, however, they are a great aid in 

 combination with parasites and several exotic ones intro- 

 duced into the United States have aided in the control of the 

 gipsy moth, scale insects, etc. 



Another factor which must not be overlooked, relates to 

 the provenience of our agricultural crop plants. These are 

 the result of selection during the course of many centuries, 

 and have been gathered together from the most diverse parts 

 of the world. Thus most of them are grown to a very great 



