EXPERIMENT STATION BULLETINS. 537 



HOW CONTACT INSECTICIDES KILL. 



IMPORT OF THE PROBLEM, 



The Study presented in this bulletin has been made under a project 

 entitled, "How do Contact Insecticides Kill Insects?" The term ''Con- 

 tact Insecticide," as understood in the project, refers to all those sub- 

 stances which may kill through first coming into contact, or into some 

 intimate relation with the outer surface of the iusect-bodv, in contradis- 

 tinction to those which must be eaten in order to become effective. 

 Contact insecticides are applied in one of three forms — as dry powdered 

 solids, as liquids (which may contain sludge), or in the form of gases 

 or vapors. The latter are usually spoken of as fumigauts. 



By definition then it follows that suck insecticides must become 

 effective through one or perhaps both of two means : 



1. Through influences conditioned only by proximity, or mere con- 

 tact with the insect's external integument or with the tubular in- 

 vaginations of the body wall — -the tracheae and hind-intestine. 



2. Through effects brought about after a possible absorption into 

 the body tissues. 



A few statements of possibilities in relation to the two means named 

 here may now be permitted. In the first case the insect may be 

 mechanically incumbered by the stickiness or hardness of the insecti 

 cide. The effect of pitchy and resinous w^ashes in this respect is com- 

 mon knowledge. Still other possibilities are n6t so certainly known 

 to hold true. With the more penetrating insecticides such as fine 

 dusts, kerosene, soap-solutions, creolin and the miscible oils in gen- 

 eral, it seems reasonable to suppose that the spiracles might be covered 

 over, or that the insecticide might pass through them into the tracheae 

 and thus interfere with the passage of air into the respiratory system. 

 Besides, there are various fine openings through certain parts of the 

 chitinous covering of insects, and the obstruction of such openings 

 might be of serious consequence to the insect. 



In case liquid or fumigant insecticides are absorbed, such absorp- 

 tion might take place through the fine openings in the chitin just men- 

 tioned, or through thin portions of the body integument such as may 

 be found forming the delicate body-wall of many small insects. If any 

 penetrating insecticide should pass into the tracheae or through the 

 anal opening, it would seem that the chances of absorption through the 

 thin walls of the tracheae or of the rectum would be still greater. Now 

 in case any agent is absorbed through some of these avenues, it might 

 then cause the death of the insect through one or more of several differ- 

 ent ways. It might act as a general protoplasmic poison such as would 

 cause coagulation or corrosion of the substance in any cells touched. 

 It might be more specifiic, affecting the whole or some part of the 

 nervous system, as a narcotic or paralytic; acting as a blood-poison; im- 



