The Treatment of Insects. 227 



many things are still unknown, and so many points still in dis- 

 pute, that personal knowledge and judgment about individual 

 cases are not only desirable, but are very essential. Directions 

 covering the majority of cases can be given, but now and then 

 one will come up which seems to differ from all others, and it 

 is then that this knowledge will prove most valuable. A few 

 of the general principles upon which this work rests are men- 

 tioned below. 



The principal organisms against which the cultivator has to 

 contend are insects and fungi. They are widely different in 

 their organization, and entirely different substances are required 

 for their destruction. Any substance which is used to destroy 

 or repel insects may be termed an insecticide; and any sub- 

 stance which destroys fungi, or which prevents their injurious 

 growth on vegetation, is a fungicide. No substance, so far as 

 known, will answer both purposes equally well. 



I. UPON INSECTS. 



Practically all the applications which are made to destroy in- 

 sucts are designed to act in one of two ways. The substance may 

 be destined to enter the digestive system of the insect and thus 

 cause death, just as many poisons cause death when taken into 

 the stomachs of higher animals. This method is by far the 

 cheapest, and when possible it is advisable to make use of it. 



The second method does not consist in putting poison on 

 the food of the insect, but the material is put directly upon the 

 insect itself. It then causes death either by stopping up the 

 breathing pores, or it penetrates the outer coverings and so 

 enters the body directly. This method cannot be used with 

 success against all insects, as some have very tough and dense 

 coverings which are not readily penetrated by any material that 

 we can use for the purpose. Beetles, for example, can scarcely 

 be destroyed in this manner. But all soft-bodied insects, such 

 as aphides, worms, and caterpillars, yield readily to the treat- 

 ment if sufficient material comes in contact with their bodies. 



This method of killing insects by means of substances which 

 cause death merely by penetrating the creature's body, is rather 

 expensive, and it is resorted to only when the pest cannot be 

 treated by poisoning its food. It thus comes that most worms 



