382 Diseases of Greenhouse Crops 



of liquid sprays or in powders. Insecticides may 

 be classified as internal or stomach poisons, and ex- 

 ternal or contact poisons. 



(a) Stomach Poisons. Paris green is one of 

 the oldest stomach poisons. When chemically pure, 

 it is composed of copper oxide, acetic acid, and 

 arsenious acid. It destroys cutworms, caterpillars, 

 beetles, grubs, slugs, etc. It should be applied pref- 

 erably as a liquid, using one pound of the poison 

 and two pounds of lime to two hundred gallons of 

 water. The Paris green tends to sink to the bottom 

 of this mixture, unless constantly stirred while be- 

 ing applied. This chemical is often adulterated with 

 white arsenic, causing it to scorch the treated plants 

 badly. Therefore for greenhouse crops the use of 

 arsenate of lead is to be preferred, since it is less 

 liable to scorch the foliage, and also because it ad- 

 heres better. Its chemical composition consists of 

 acetate of lead and arsenate of soda. It is applied 

 to the best advantage as a liquid, composed of about 

 three pounds of powdered arsenate or five pounds of 

 paste arsenate to one hundred gallons of water. 



Arsenite of zinc may also be used. It is a very 

 finely divided fluffy white powder which can be thor- 

 oughly distributed and which adheres well to the 

 foliage. It is intermediate between Paris green and 

 lead arsenate in strength, and costs less than either. 



7/ is essential when arsenicals are used to see that 

 they are correctly labeled^ and kept under loch and 

 key^ as they are poisonous to both man and animal. 



Hellebore or white hellebore is somewhat less dan- 



