CHAPTER XIX 

 INSECTS AND INSECTICIDES 



In order successfully to prevent insect injury to any crop, 

 it is necessary to take into consideration various factors, as 

 for instance : the life history of the insect ; its habits and struc- 

 ture in its various stages; the method and rate of reproduc- 

 tion ; how, when and on what part of the plant it feeds, both 

 in its immature and adult condition ; where and in what stage 

 it passes the winter ; and its susceptibility to the action of 

 poisons and contact insecticides, as well as the effect of these 

 substances on the plant concerned. In order to devise a 

 practical means of control, it is also necessary to take into 

 consideration the conditions under which the plant is grown, 

 the methods of propagation, cultivation and harvesting as 

 well as the commercial value of the crop. 



In the preceding chapters, an attempt has been made to 

 give the more important facts known touching on these points. 

 In the present chapter are presented some general considera- 

 tions in regard to the structure and development of insects 

 that are of interest from the standpoint of control, followed 

 by a discussion of the more important insecticides used against 

 vegetable insects. 



Structure of Insects 



Insects possess an external skeleton or shell composed of a 

 series of segments or rings which is divided into three well- 



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