INSECTICIDES AND THEIR APPLICATION 161 



also ground into a powder which is applied directly to the 

 insects. A tobacco decoction is frequently made by boil- 

 ing the powder or stems 

 in water or by pouring 

 boiling water over them. 

 There is thus formed a 

 concentrated solution 

 which is diluted with cold 

 water until there are about 

 two gallons of decoction 

 for each pound of tobacco 

 used. The decoction may 

 be sprayed upon the in- 

 sects to be killed. Per- 

 haps the way in which tobacco is most commonly used as 

 an insecticide is that of slowly burning the stems in green- 

 houses, producing a smoke which permeates the plants 

 and kills the pests upon them. 



VAPORS AND GASES 



There are certain insects which may most easily be 

 killed by the use of vapors or gases. For this purpose 

 bisulphide of carbon and hydrocyanic acid gas are chiefly 

 employed. The former is a liquid that readily volatilizes 

 on exposure to the air and kills all insects with which it 

 comes in contact. It is used to best advantage against in- 

 sects affecting dry seeds and grains in storehouses, and 

 against insects affecting the roots of plants. 



Hydrocyanic acid gas is a deadly vapor made by pouring 

 sulphuric acid on cyanide of potassium. It is used to de- 

 stroy white flies in greenhouses and for various enemies of 

 citrus fruits in Florida and California. In the case of 



