2O4 F. J. BRINLEY AND R. H. BAKER. 



RELATION OF CONCENTRATION OF HCN TO THE LENGTH 



OF EXPOSURE. 



Experiments were carried out to determine whether a low con- 

 centration of cyanide for a long period of time was more fatal than 

 a high concentration for a short period of exposure. When 

 using Ca(CN)2 under field conditions, where one is not able to 

 obtain a long period of exposure, it is necessary to use a very high 

 concentration. 



The results obtained from more than a thousand tests with two 

 species of aphids and one species of thrips, show that con- 

 centration and length of exposure are inversely related up to a 

 certain time, i.e., toxicity = concentration X the time. Beyond 

 a period of 15 minutes it seems that concentration is independent 

 of the time, as it takes practically the same concentration to 

 kill in one hour as in 30 minutes (Figs. 2, 3 and 4). 



UJ 



I- 



o 

 u 



_J 

 a: 



u 



10 20 30 M-0 60 



EXPOSURE IN MINUTES 



FIG. 3. Curve showing the relation of concentration of HCN to length of 

 exposure and the effect of temperature upon the toxicity of HCN to Aphis rumicis. 

 Solid line, 25 C.; broken line, 30 C. 



