172 PBINCIPLES OF PLANT CULTUBE 



feed on the juices of the plant which they infest, and 

 do not directly devour its tissues, as do the eating in- 

 sects ; but they reduce its vitality by their continued 

 drain upon the reserve food. The so-called scale in- 

 sects 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 resistant scales. 



Sucking insects are not susceptible to poisonous in- 

 secticides, hence we must resort to materials that clog 

 their breathing pores, that dissolve their eggs or scales 

 or that form an air-tight coating over them.^ The ma- 

 terials most often used for destroying this type of insects 

 are lime-sulfur, kerosene emulsion and tobacco decoctions. 



317. The life histories of injurious insects, which 

 cannot here be taken up, may profitably be studied by 

 the plant-grower. A standard work on economic ento- 

 mology will furnish the needed information. 



'■ The cottony cushion scale, Icerya purchasi, which was very 

 destructive to the orange in Cahfornia, has been nearly sup- 

 pressed by the introduction of an Australian parasite, the 

 Vedalia cardinalis. 



