CHAPTER V 

 INSECTICIDES 



MATERIALS used for the destruction of insects are commonly 

 called insecticides, and are roughly divisible into four classes : 



1. Poisons, which kill by being eaten and are usually composed 

 of various forms of arsenic and are therefore often called arseni- 

 cals. 



2. Contact insecticides, which kill by either clogging up the 

 spiracles, the openings of the respiratory system, or by entering 

 the trachea, and thus causing suffocation, or by their corrosive 

 action on the skin. 



3. Repellants, which deter the insect from attacking the plant 

 or animal to which they are applied. 



4. Gases, which are used for fumigating buildings, stored prod- 

 ucts and greenhouses where other means are not practicable. 



1. Poisons 



Poisons are applied to the food of the insect and must be eaten 

 by it to be effective. It is evident, therefore, that they are only 

 effective against biting (mandibulate) insects, or for those which 

 lap up their food from the surface, and that they are of no avail 

 against the true sucking insects, such as the true bugs which suck 

 the juices from beneath the surface of the plant. Poisons are not 

 always, however, the most effective means of combating biting 

 insects, which are sometimes more effectively controlled by con- 

 tact insecticides or other means. 



Nearly all of the stomach poisons are derivatives of arsenic 

 and are therefore termed arsenicals. As they are dangerous to 

 human life they should be kept well labeled, locked up when not 

 in use, and vessels in which they have been used should be care- 

 fully cleaned. 



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