452 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 5 



THE POISON EXPONENT: A SYMBOL OF THE TOXICITY OF 

 CHEMICALS IN THEIR RELATION TO INSECTS ' 



By T. E. HoLLOWAY, Bureau of Entomology, U- S. Department of Agriculture, 

 Audubon Park, N'eic Orleans, La. 



During the season of 1910 the writer, under the direction of Mr. 

 W. D. Hunter, conducted a series of experiments with certain poisons 

 on larvae of the cotton boll worm, Heliothis ohsol'eta. While preparing 

 a report of these experiments the writer felt the need of some method 

 of expressing the toxic value of chemicals to insects, and he finally 

 concluded that this value might be obtained by comparing the num- 

 ber of days of life of the poisoned insect and of the unpoisoned insect. 

 To. illustrate : if the poisoned insect lived one day while the unpoisoned 

 or check insect lived three days, both being subject to the same en- 

 vironmental conditions except that of the poison, the relationship 

 between the life of the poisoned insect and the life of the unpoisoned 

 insect might be expressed by the following proportion : life of poisoned 

 insect is to life of unpoisoned insect as 1 is to 3. Or, 



Life of poisoned insect _ ^ _ „„ 



Life of unpoisoned insect 



which number may be taken as the indicator of the toxic value of the 

 poison. Lack of data prevented the publication of this theory. 



On account of other work the writer was unable to continue the 

 poison investigations until this year, when preparations were made 

 to determine the toxic values of a number of poisons to Heliothis. An 

 outline- was made which was similar to that used in 1910, but under 

 field conditions it was changed in the matter of food and cages for the 

 larvse. The experiments were then started, but before they had pro- 

 gressed very far an important obstacle was noticed. The poisoned 

 larvse died within a day or two, but the unpoisoned larvae lived for a 

 much longer period, in the natural course of their life cycle. It was 

 obvious that the environmental conditions to which the unpoisoned 

 larvse were subject after the death of the poisoned larvse were of not 

 the slightest value in the determination of correct toxic values. In 



1 Published by permisssion of the Chief of the Bureau of Entomology. 



2 This outUne was thoroughly discussed with Mr. W. D. Hunter and Mr. W. 

 Dwight Pierce, of the Bureau of Entomolog5^ Mr. Pierce suggested experimenting 

 with each instar of Heliothis, and gave the writer a list of poisons to be tested. The 

 writer takes this opportunity of thanking Messrs. Hunter and Pierce for their kind- 

 ness in cooperating. 



