53^ Journal of Agricultural Research voi. xiii. no. h 



SUMMARY 



From the general results reported in this paper it appears that the 

 physical properties as well as the chemical properties have an important 

 bearing upon the efficiency of the contact spray. Even though the spray 

 may contain a very active poison, it will not be effective unless it con- 

 forms to certain physical requirements— that is, the ability to vaporize 

 and penetrate in the form of a vapor or to spread over the insect and 

 penetrate in the liquid form. The results reported by McClintock, Hough- 

 ton, and Hamilton (7) show very clearly that this is true. The results 

 in the use of quassia with or without soap as reported by Mclndoo and 

 Sievers (9) are another example, and it is a common observation that the 

 addition of soap to nicotine sprays increases their efficiency. The follow- 

 ing are some of the principles which must be kept in mind in studying 

 the effects of contact insecticides. 



(i) Contact insecticides may be divided into two groups: (a) Those 

 which spread over the body of the insect and penetrate the tracheae, 

 (b) Those which are not able to spread over the insect and do not pene- 

 trate the tracheae. 



(2) Contact insecticides which are either soluble in ether or chloroform 

 or are fat solvents are able to spread over the insect and enter the trachea. 



(3) The rate of spread of these insecticides is governed by their vis- 

 cosity and cohesion. 



(4) Compounds with a viscosity as high or higher than castor oil 

 spread so slowly that in general they may be classed as poor insecticides. 



(5) Compounds more volatile than xylene evaporate too quickly for 

 efifective work. 



(6) Sprays in the form of emulsions may enter the tracheae as such, 

 or the oil remaining after the emulsions is broken down may spread over 

 the insect and enter the spiracles. 



(7) Relatively nonvolatile oils penetrate the body of the insect directly 

 through the walls of the tracheae as liquids, the rate depending upon the 

 viscosity. 



(8) Volatile oils may penetrate the walls of the tracheae in either vapor 

 or liquid form. 



(9) Sprays which are unable to enter the tracheae in liquid form may 

 penetrate and pass through the tracheal walls as vapor. 



(10) Fumigants gain entrance and pass through the tracheal walls in 

 vapor form. 



(11) Slightly volatile compounds tend to condense upon the tracheal 

 walls, owing to the fact that small quantities are sufficient to saturate 

 the atmosphere. Owing to this high saturation, these condensations tend 

 to penetrate the chitin rather than to reevaporate. Volatility is an index 

 of the ability of the compound to gain entrance into the insect and is 

 therefore closely correlated wiith toxicity. 



