CHAPTER VII 



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INSECTICIDES IN GENERAL: STOMACH POISONS 



Though the farm practices and special methods outlined in the pre- 

 ceding chapter are' of great importance for the control of insect pests 

 in many cases, they are ineffective and cannot be made use of in many 

 others. Under these circumstances other methods of attack must be 

 resorted to, and in. general, insecticides of various kinds, fitting the. 

 particular nature of the injury and of the insect causing it in each case, 

 have proved successful. Insecticides are substances which may be 

 placed upon a plant, or elsewhere, to be eaten by the insect and which 

 when eaten, kill the insect ; or materials which on coming in contact with 

 the body of the insect, kill it as a result of that contact. Poisonous 

 gases and vapors v/ould also be included as insecticides, as thus defined. 



CLASSES OF INSECTICIDES 



The materials used as insecticides fall into two general groups: (1) 

 Those which are placed upon the food eaten by the insect, swallowed 

 with it, and which upon entering the stomach are dissolved, producing 

 inflammation and finally death. Such poisons can, of course, be used 

 only for insects with chewing mouth parts which bite off and swallow 

 solid food, sucn as pieces of leaves, stems, etc.; (2) Those which, when 

 they come in contact with the body of an insect either enter the spir- 

 acles and penetrate their chitinous lining and kill the tissues beyond; 

 or which corrode the body; daprive it of oxygen; or by softening the 

 coverings over the body (scale insects) cause these to adhere to the 

 plant it is on, killing the insect in any case. The materials of the first 

 group are usually called stomach poisons; those of the second, contact 

 insecticides. The latter could also be used for biting insects but the 

 difficulties in the way of their being successfully applied are such that 

 stomach poisons are used whenever possible. 



In reaching the insects concerned, either with stomach poisons or 

 contact insecticides, the methods of conveying the material to where 

 the insect is, and of an even and thorough distribution of it are important. 

 Those substances which are solids in the form of fine powders can be 

 blown onto the tree or whatever the insect may be on, but some are 

 liquids. Accordingly, "powder guns" for spreading the poisonous 

 dusts have been used with considerable success, and pumps with a fine 

 nozzle at the end of a line of hose are used for the liquids. 



