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 insect, 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 fumi- 
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 
n 
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. 
