170 Principles of Plant Culture. 



chard are most valuable assistants in this work. The 

 apple- maggot* is more effectually controlled in this man- 

 ner than by any other known method. 



316. Sucking Insects include many species. They feed 

 on the juices of the plant which they infest, and do not 

 directly devour its tissues, as do the eating insects; but 

 they reduce its vitality by their continual drain upon 

 the reserve food. The so-called scale insects belong to 

 this class. These are especially difficult to destroy, since 

 they are dormant the greater part of the year, and in 

 this condition are protected by their comparatively re- 

 sistant scales. 



Sucking insects are not susceptible to poisonous insect- 

 icides, hence we must resort to materials that clog their 

 breathing pores, as kerosene (294), that dissolve their 

 eggs and scales, as potash solutions or that form an air- 

 tight coating over them, as the resin washes (295). f 



317. The Life Histories of Injurious Insects, which can 

 not here be taken up, may profitably be studied by the 

 plant grower. A standard work on economic entomology 

 will furnish the needed information. 



B PLANTS AS AFFECTED BY VEGETABLE PARASITES. 



318. Many of the most serious enemies of cultivated 

 plants belong to this class. As a rule, vegetable parasites 

 contain no chlorophyll, and hence are incapable of form- 

 ing their own food. While most of them belong to the 

 lower orders of plants, a few species are highly developed 

 and produce true flowers and seeds. 



* Trypeta pomonella. 



f The cottony cushion scale, Icerya purchasi, which was very destruc- 

 tive to the orange in California, has been nearly suppressed by the intro- 

 duction of an Australian parasite, the Vedalia cardinalis. 



