INSECTICIDES. 351 



tings may be replaced in fresh sand. If others can be 

 readily obtained, however, it will generally be better to 

 burn them up and make new ones. 



Aside from the form mentioned above, "damping 

 off" may be caused by one or more species of Botrytis 

 and Phyllosiicta, and upon beans by Oolletotricum Lin- 

 demuthianum, which thrive under about the same con- 

 ditions and which require the same treatment. 



CHAPTER XXVII. 



INSECTICIDES. 



In selecting material for the destruction of insects, 

 it is quite necessary that the nature of the insect to be 

 treated should be understood. Frequently insecticides 

 are applied without effect, when a little knowledge of 

 the insect would have shown that the materials used 

 were uot adapted to the purpose. 



ARSENITES. 



While Paris green, London purple and other arsen- 

 ates, hellebore, etc., are valuable when the insects EAT 

 the flowers, foliage, or other external portions of the 

 plants, they are in no way effective against the plant lice, 

 scale, and similar insects. 



If the above mentioned arsenites are mixed with one 

 hundred parts (by weight) of plaster, and dusted over 

 the plants when the foliage is wet, or used in water at 

 the rate of a teaspoonful to twelve quarts, such insects as 

 eat the portions to which they are applied will be 

 destroyed. Hellebore can be diluted with five times its 

 weight of plaster, or a teaspoonful can be used to a 

 gallon of water. 



