56 THE PARASITES OF PLANTS 



4 that the food for the living cells of plants is mostly 

 formed in the leaves, and that whatever destroys the 

 leaves cuts off a part of the food supply of the plant. 

 Every American farmer knows that he must destroy the 

 potato beetle or it will nearly destroy his potato crop. 

 Since the potato beetle eats the leaves, by putting poison 

 on the leaves we can poison the beetle. For this purpose, 

 a deadly poison called Paris green is much used. One 

 ounce of Paris green may be stirred into twelve gallons 

 of water, and the mixture sprinkled on the plants. Or 

 an ounce of Paris green may be well mixed with nine 

 pounds of land plaster and the mixture dusted on the 

 plants. Most other insects that eat the leaves of plants 

 may be destroyed in the same way as the potato beetle. 

 Plant Lice. Those who have the care of house plants 

 have seen a small green insect on the under side of the 

 leaves. This insect, commonly called 

 the green fly or the plant louse ( Figure 

 25), does not eat the leaves as the po- 

 tato beetle does. And yet it injures 

 the plants on which it lives. It does 

 this by sucking out the sap, thus rob- 

 bing the cells of water and food. Since 

 it does not eat the leaves, we can not 



FIG. 25. Plant louse. . 



poison it by poisoning the leaves, as in 

 the case of the potato beetle. To destroy the green fly 

 and other sucking insects, use some substance that pre- 

 vents their breathing. Strong soap suds, tobacco water, 

 or kerosene emulsion, sprayed on this class of insects, 

 usually destroys them. (See page 254.) In greenhouses, 

 tobacco smoke is much used for the green fly. 



