168 THE PRINCIPLES OP AGRICULTURE 



diseases. Clean tillage, rotation of crops, plant- 

 ing varieties which are least liable to attack, and 

 careful attention to prevent all the conditions 

 which seem to favor the breeding of insects and 

 the spread of diseases, are quite as important as 

 destroying the enemies ; for "an ounce of pre- 

 vention is worth a pound of cure." 



295. Insects are destroyed by three general 

 means : (a) by killing them directly, as by hand- 

 picking, digging out borers ; (b) by killing them 

 by means of some caustic application to their 

 bodies ; (c) by poisoning them by poisoning 

 their food. In some instances, insects may be 

 kept away by covering the plants with some 

 material, as lime, to which the insects object ; 

 but this method of fighting insects is usually 

 unsatisfactory. A substance which is used to 

 destroy an insect is called an insecticide. 



296. (b) The caustic applications or insecti- 

 cides must be used for those insects which suck 

 their food (291). Kerosene, kerosene emulsion, 

 soap washes, lime-and-sulfur, miscible oils, to- 

 bacco, and the like, are the materials used; and 

 plant-lice, scale insects, plant-bugs, thrips, and 

 leaf-hoppers are the insects thus treated. 



297. (c) The poisonous applications are used 

 for the chewing insects that prey upon the 

 outside of the plant (not for borers, which are 

 usually dug out) . Paris green and other arsenicals 



