SPRAY MATERIALS 137 



Spray materials are divided into two classes, namely, insect- 

 icides, which are chemicals used for the killing of insects, and 

 fungicides, which are chemicals used to prevent or destroy 

 plant diseases. The insecticides are divided into stomach 

 poisons and contact poisons, based upon the way in which 

 they kill the insect. 



Insecticides and fungicides are used in two ways, either in 

 a dry state as a fine powder or dissolved in water and dis- 

 tributed as a fine spray. If the material is dusted on the 

 plants, it is distributed by the use of a dust gun which forces 

 the powder out by the aid of a fan. If a spray is used, various 

 kinds of spraying machines are employed. 



Sprays for the Plants and How to Prepare Them. — Poisons 

 for Biting Insects. — There are a number of insecticides which 

 are used for biting insects. These poisons are taken into 

 the stomach of the insects and kill them through their action 

 on the digestive tract. There are two classes of stomach 

 poisons, namely, organic poisons, which are usually vegetable 

 compounds, and inorganic compounds, which are of inorganic 

 materials. The vegetable poisons are poisonous to the 

 insects but not to the human system and therefore are safer 

 to use on some plants, as, for instance, cabbage, lettuce, 

 or currants. The poisonous property of these insecticides 

 is usually volatile, that is, when the material is exposed to 

 the air for a few days it loses its poisonous nature. Because 

 of this fact it must be strictly fresh when it is used, and it 

 must be kept air-tight, when it is stored, otherwise it is 

 worthless. The two most important insecticides which 

 belong to this class are the white hellebore and the pyrethrum. 

 The inorganic insecticides are poisonous to insects as 

 well as to animals, and greater care must be used in their 

 application to the plants. There are many poisons in this 

 class, but the base of all of them is usually some form of 

 arsenic. A few of the most common ones are arsenate of 

 lead, Paris green, and London purple. 



These poisons are applied in both the dust form and the 

 spray form. The method which is selected depends upon 

 the plant and the insect which is being treated. 



