164 



Elementary Principles of Agriculture 



agement should be given to these common enemies. 

 Field pests can sometimes be killed by running heavy 

 rollers over the fields, or by plowing or harrowing. The 



leaf-eating forms 

 can frequently be 

 killed by spraying 

 the leaves with 

 poisons. Others, 

 like the sucking 

 insects, may be 

 killed by spraying 

 directly onto the 

 insect some sub- 

 stance that kills 

 by contact, such 

 as oils, alkali 

 washes, etc. The 

 poison must not 

 be strong enough 

 to injure the plants. In some cases, the insects may be 

 killed by treating the plants with poisonous fumes or 

 gases, such as tobacco smoke, and the deadly hydro- 

 cyanic acid gas, uesd especially for San Jose scale. Where 

 plants are sprayed to prevent fungous diseases, the poison 

 for insects may be applied in the same solution at the 

 same time. There are many kinds of special machines 

 for applying fungicides and insecticides. They are fully 

 described in special books and bulletins. 



232. Classification of Insecticides. Substances that 

 are used to poison insects are called insecticides. There 

 are many substances used to kill insects. They may be 

 grouped into three classes, according to the manner in 

 which they poison the insect. 



Fig. 101. This apple might have been kept sound 

 by spraying. From Cornell University Junior 

 Naturalist. 



