10 



SPRAYING CROPS 



a destructive effect upon insect and fungus life over 

 the outer surfaces of trees, shrubs, vines, and herba- 

 ceous plants. These substances are usually applied in 

 a finely powdered condition, and the tiny particles 

 stand guard over the plants, killing with remorseless 

 certainty any insect or fungus pest that attempts to 

 pass through their lines to reach the plant. The par- 

 ticles themselves do not enter the plant, but remain on 

 the outside until driven off by the combined action of 

 wind, rain, dew, and sunshine. This is not true, how- 

 ever, of the contact-killing insecticides, which are 

 applied directly to the offending insects and do not 

 remain on the plant in an effective condition. 



Substances used to destroy insects are called 

 insecticides; those used to destroy fungi are called 

 fungicides. 



INSECTICIDES USED IN SPRAYING 



The insecticides used in spraying may be broadly 

 divided into two classes : ( i ) internal poisons, or 

 those which take effect by being eaten along with the 

 ordinary food of the insect ; and (2) external irritants, 

 or those which act from the outside, — penetrating the 

 breathing pores, or causing death by irritation of the 

 skin. The most important insecticides are the poisons. 

 Of these the most popular are the various combina- 

 tions of arsenic. 



Paris green is a chemical combination of arsenic 

 and copper, containing about 58 per cent of arsenious 

 acid; about 31 per cent of copper oxide and 10 per 

 cent of acetic acid. Technically it is called copper 

 aceto-arsenite. It is almost insoluble in water ; but 

 there is often a small percentage of it soluble, and 

 to prevent the injury this may do to foliage it pays 

 to add a little fresh lime water (made by slaking fresh 

 lime in water) to the spraying mixture. It may be 



