156 FARM FRIENDS AND FARM FOES 



sene, whale-oil soap, and pyrethnim or insect powder are 

 among the most important tracheal poisons. 



Some plant-feeding larvae have such slimy skins that they 

 are readily killed by dusting with powdered air-slaked lime, 

 or ashes. The " slugs " on pear, cherry, and rose leaves 

 are examples of these. 



Internal Poisons 



The poisons most commonly used against biting insects 

 are certain chemical compounds composed in part of ar- 

 senic. Arsenate of lead, arsenite of lime, and Paris green 

 are notable examples of such insecticides. 



For a long time Paris green was the most important of 

 the arsenical insecticides. It is a chemical combination of 

 arsenic and copper, containing, when pure, about fifty-five 

 per cent of arsenic. It is in a finely powdered condition, 

 and, when placed in water, the fine particles do not dis- 

 solve, but remain in suspension. Because of this fact, one 

 of the most popular ways of applying arsenical poisons to 

 plants in danger from the attacks of 

 biting insects, is in a water mixture 

 by means of a force pump and spray 

 nozzle. 



In its simplest form this process 

 consists of mixing about a teaspoon- 

 ful of the poison in a pail of water. 

 Bucket stirring thoroughly to get the powder 

 Pump evenly distributed through the water. 

 A small force pump, to which is at- 

 tached a rubber tube with a spray 

 nozzle at the end, is then set in the pail. By working the 

 handle of the pump, the water with the poison particles 

 in suspension is forced through the tube and nozzle, com- 



