Insects on the Farm 



165 



(a) Food, or Internal Poisons, are substances which 

 poison by being taken into the digestive tract of the 

 insect. This class includes various arsenical compounds, 

 such as Paris green, London purple, lead arsenate. 

 Poisons of this class are used for insects that chew their 

 food, as the leaf-eating forms, unless the use of the poison 



Fig. 102. Spraying in the late dormant season. 



renders the plants dangerous for food, such as cabbage. 



(b) Contact Poison. Substances that destroy by 

 attacking the body of the insect, such as washes of 

 caustic alkalies, oils, etc. They are used for sucking in- 

 sects, i.e., those having beaks, such as the San Jose scale. 



(c) Fumigation Poisons. Substances which enter the 

 breathing pores of the insect and cause death by poison- 

 ing or suffocation. Smoke, and the deadly hydrocyanic 

 acid gas, Pyrethrum, or "insect powder," and carbon 

 bisulphide, belong to this class. 



