368 GENERAL SCIENCE 



strips of wheat are planted early to attract the fly. After 

 the eggs are laid these strips are plowed under and the 

 wheat planted later escapes. 



There are numerous other insects, bugs, and moth 

 larvae which do considerable damage to plant life. The 

 usual remedy is the destruction of the infested parts 

 and proper spraying. 



Poisons for Crop Pests. It is estimated that insects 

 annually destroy one tenth of the crops of the United 

 States. In order to have strong healthy plants and sound 

 fruits, these ever-present insect pests must be exter- 

 minated. To accomplish this result various poisons have 

 come into quite general use. These poisons may be 

 divided into two general classes according to the type 

 of insect that is to be destroyed. Some pests, like the 

 potato bug, feed directly on the leaf of the plant and may 

 be killed by placing poison on the leaf. Poisons which are 

 eaten by the insect are called stomachic poisons. Then 

 there are other pests which get their nourishment by suck- 

 ing the juices of the plant or the plant leaf. To kill such 

 animals it is necessary to use a poison which acts on the 

 body by enveloping it and drying up its tissues by cutting 

 off its air supply or by poisoning it through absorption. 



Stomachic Poisons. Paris green is a poison which 

 is in quite common use for those insects which swallow 

 their food. It is prepared by combining hot solutions 

 of arsenious acid and copper acetate. Paris green sepa- 

 rates from this mixture as a fine, bright green powder, 

 which is almost insoluble in water. However, it will 

 readily dissolve in ammonia water, giving a dark blue 

 solution. If the Paris green is adulterated with gypsum, 

 a white powder will be suspended in the ammonia water ; 

 this will gradually settle to the bottom. 



