100 



GENERAL ZOOLOGY 



by what are called contact insecticides, — substances which 

 kill when they touch the body. Oil emulsions, nicotine com- 

 pounds, insect powders, corrosive materials such as certain 

 combinations of lime and sulphur, are used in the war 

 against sucking insects. As an indirect protection of crops 

 many preventive measures have been developed. These 

 are based upon a knowledge of life history and of habits 

 other than feeding habits. Fall plowing kills many insects 



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Fig. 62. A cornfield in which a band of road oil was placed as a barrier 

 after the field had been attacked on one side by chinch bugs 



In the part of the field to the left of the line every plant was destroyed. (After 



Illinois State Natural History Survey) 



which would live through the winter if the ground in which 

 the larvae or pupae are living were not disturbed. Rotation 

 of crops is a big factor in insect control, for a field constantly 

 planted with the same crop becomes a breeding place for 

 the insect enemies of that crop. Some of the very worst 

 crop destroyers, such as the chinch bug (Figs. 24 and 62) 

 and the Hessian fly (Fig. 52), are held in check by keeping 

 fields free from old straw and vegetation in which these in- 

 sects seek shelter for the winter. 



If it were not for civil war among the insects man would 

 have little chance to compete with them. Lady beetles 



