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Elementary Principles of Agriculture 



be killed by spraying the leaves with poisons. Others, 

 like the sucking insects, may be killed by spraying 

 directly onto the insect some substance 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 hydrocyanic acid gas, used 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 



