JOMAL OF AGEICDLTDRAL ISEARCH 



Voiv. XIII Washington, D. C, June io, '1918 No. 11 



PHYSICAL PROPERTIES GOVERNING THE EFFICACY 

 OF CONTACT INSECTICIDES^ 



By William Moore, Head of Section of Research in Economic Zoology, and S. A. 

 Graham, Assistayit in Entomology, Minnesota Agricultural Experiment Station 



INTRODUCTION 



It was considered by Shafer (zd)^ that the vapor of contact insecticides 

 such as kerosene, gasoline, creolin, and pyrethrum were responsible for 

 the death of insects to which these materials were applied. It was there- 

 fore assumed as a working basis that the volatility of organic compounds, 

 which has previously been shown to be an index of the toxicity of their 

 vapors to insects {11, 12), would also be an index of the toxicity of these 

 compounds when used as contact sprays. In working with insect eggs 

 {14), however, it was found that materials not sufficiently volatile to kill 

 insects or their eggs by their vapor within a reasonable length of time, 

 were among the most effective materials when applied to the eggs as 

 liquids. Further studies in which different fractions of kerosene were 

 used {13) revealed the fact that the least volatile fractions were the most 

 effective as contact insecticides, while they failed to kill insects which 

 were exposed only to their vapor. 



With these results in mind it was considered advisable to determine the 

 physical properties governing the entrance into the insect of a contact 

 insecticide, and wherein this differs from the penetration of the vapor. 



WETTING AND SPREADING OF THE INSECTICIDE 



It is common observation that when some contact insecticides strike 

 an insect they form into round droplets which roll off the body, while 

 others spread out, forming a film over the insect. This phenomenon of 

 the spreading out of the insecticide over the body has been often termed 

 "wetting" or "spreading" and is often confused with the wetting and 

 spreading of the insecticide over the surface of the leaves sprayed, the 

 terms "wetting" and "spreading" being used synonymously. Vermorel 

 and Dantony {18, ig, 20), Lefroy (5), and more recently Cooper and 



1 Published, with the approval of the Director, as Paper 112 of the Journal Series of the Minnesota Agri- 

 cultural Experiment Station. 



2 Reference is made by number (italic) to "Literature cited." p. 537-538. 



Journal of Agricultural Research, Vol. XIII, No. n 



Washington, D. C. (523) June 10, igiS. 



nv Key No. Minn. 28 



