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for them, and should be in a position to modify his treatment so as to 

 make them confoim with the character of the insect or the disease which 

 is being treated, and with the season. Every year and every day such 

 knowledge will be of value. So many things are still unknown, and 

 so many points still in dispute, that personal knowledge and judgment 

 about individual cases are not only desirable, but very essential. Di- 

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

 one will come up w hich 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 mentioned below. 



The Action of Insecticides and Fungicides. 



" The principal organisms against which the cultivator has to con- 

 tend are insects and fungi. They are widely different in their organ- 

 ization, and entirely different substances are i equired for their destruc- 

 tion. Any substance which is used to destroy or repel insects may be 

 termed an insecticide ; and any substance 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. 



1. Upon Insects. 



"Practically all the applications which are made to destroy insects' 

 are designed to act in one of two ways. The substance may be des- 

 tined 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 possi- 

 ble, 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 pen- 

 etrates 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 treatment 

 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 and caterpillars are destroyed 

 by mt ans of poisons which are eaten, though they yield to the other 

 treatment equally well. 



" The food of many insects, however, cannot be poisoned, since they 

 feed upon the juice of plants and do not eat the external coverings. It 

 is fortunate that most of these insects have soft b«dies, so that they yield 

 readily to treatment if the poison comes in contact with them. Their 

 mouth parts are formed for penetrating the external coverings of 

 plants to a depth sufficient to leach ihe sap; just as the mosquitoes' 

 bills are in the habit of penetrating human kind. All aphides belong 

 to this class, as well as the true bugs, these having mouth parts which 

 are adapted to suck, but not to chew. The utter uselessness of cover- 



