6 



tills class of injurious insects are those which contain arsenic as the 

 poisonous principle. When properly prepared their efficacy depends 

 upon the thoroughness of the application. The insect attacks the 

 foliage, it eats the poison which is sprayed upon the leaves, and in 

 this way it is destroyed. It follows, therefore, that only thorough 

 applications are likely to be satisfactory. Among the poisonous 

 substances used, Paris green is the commonest. This is found in 

 various forms. Arsenic is the poisonous principle in its composi- 

 tion. Other forms of arsenical poison are London purple, paragrene 

 and arsenite of soda or lime. The application of these poisonous 

 materials has developed into a well-defined practice in the last few 

 years. There are two thoughts to be kept in mind. (1) Thorough- 

 ness of application, and (2), economy. The proper mixing of the 

 materials and their application to the trees are matters which are 

 regulated by judgment and good spraying pumps. 



11. There is another class of insects luhich obtain their food hy 



siicl'l)ig the juices or 

 saj) from the i^lant 

 which they infest. — 

 This class of insects 

 is very important. 

 They are very mi- 

 nute. They are not 

 supplied with jaws 

 with which to chew 

 or eat tlieir food ; 

 instead they have a 

 sucking tube with 

 which the juices of 

 the plant are ex- 

 tracted. As exam- 

 ples we have the 

 great number of lice infesting house plants and field crops. To this 

 sucking class also belong the many injurious scales, such as oyster- 

 shell bark-scale, scurfy scale and San Jose scale. These insects are 

 destroyed by applying either caustic or oily substances which injure 

 their bodies, or which may produce suffocation. The sprays are 



408 



The currant icorm is a leaf -eating, tliat is, a biting or 



chewing insect. 



