434 



AN ECONOMIC ENTOMOLOGY. 



CHAPTER IV. 



INSECTICIDES. 



Some of the principles governing the appHcation of insecticides 

 have been referred to and stated in previous parts of this work, 

 but it is deemed advisable to refer specifically to the different 

 kinds of materials, and to mention the range of the most useful 

 among those now upon the market. Broadly speaking, we may 

 say that insecticides are of two kinds,— those that kill by being 

 eaten, or stomach poisons, and those that kill by contact. Some 

 substances, such as hellebore and tobacco, belong to both cate- 

 gories, — that is to say, they are poisonous when eaten, and, 

 when properly applied, also kill by contact. 



Of the stomach poisons, none are in more general use or of 

 greater importance than the preparations of arsenic, and of these 

 three only need be considered here, — Paris green, or arsenite of 

 copper, London purple, or arsenite of lime, and "gypsine," or 

 arsenate of lead. All of these have very much the same range 

 of usefulness, and may be treated together. In each case the 

 active principle is arsenic, and the combination with the other 

 material is simply to put it into an insoluble form to prevent 

 injury to the plants ; for arsenic is quite as violent a poison to 

 plants as it is to animals. In Paris green tlie arsenic is in combi- 

 nation with copper, and in many cases we get, besides the in- 

 secticide effect, a direct influence upon fungi. An average good 

 sample contains about as follows : 



Per Cent. 



Moisture 74 



Arsenious oxide (AsgOa) 68.82 



Copper oxide (CuO) 30.59 



100.15 



A small quantity of the arsenic remains here in a soluble con- 

 dition, especially when the weather is warm ; hence we frequently 

 find that spraying on a hot day is much more likely to burn the 

 foliage than when done on a cool day, or late in the afternoon. 



