64 INSECTICIDES — GENERAL PRINCIPLES 



In other words, poison sprays for leaf-eating insects are applied to the 

 plant, to be eaten along with part of the plant. Contact sprays are 

 applied to the iiisect, and only incidentally to the plant, since we could 

 not hit the one without hitting the other. With these, the great aim 

 is to apply the material so thoroughly that it will certainly come into 

 contact with all the insects concerned. 



A third method of using chemicals is to take advantage of the suf- 

 focating qualities of certain gases. Such materials are known as Jumi- 

 gants. They are of great value in fighting pests living in stored prod- 

 ucts, such as the grain weevils. It is necessary, in using fumigants, 

 that the materials treated shall be in a closed receptacle. 



An anomalous material, applied frequently in the open air but kill- 

 ing the insect by the volatile oil that it gives off, is the so-called Per- 

 sian insect powder. 



Examples of insects that have biting mouth parts and are killed by 

 poison sprays are the grasshoppers, roaches, beetles, and practically all 

 caterpillars. 



Examples of insects that have sucking mouth parts, and cannot be 

 killed by poisons, but must be fought with contact sprays or by some 

 other means, are the thrips, fleas, and the true " bugs " — such as the 

 squash bug, plant lice, scale insects, and leaf hoppers. 



