100 



Fig. 3. — Average per cent 

 of initial residue remaining at 

 intervals after 20 by 20 inch 

 I Iter papers bearing 10 mg of 

 r.jveral insecticides were placed 

 in a constant temperature 

 chamber at KO°F. 



100 



TIME IN HOURS 



about equal 



Fig. 4. — Average per cent 

 of initial residue remaining at 

 intervals where 10 mg de- 

 posits of aldrin and DDT were 

 exposed on filter papers to 

 varied temperatures. 



tributable to formulation — apply about equally to ai 

 residues, vapor pressure remains the one important vari- 

 able which might largely determine relative persistence 

 of the residues produced by various pesticidal chemicals. 



PRACTICAL APPLICATION OF GENERAL PRINCIPLES 

 What are the practical implications or considerations 

 involved? Presumably we are interested in residues pri- 

 marily for their public health aspects. Therefore, our 

 first concern should be how we may most efficiently util- 

 ize chemicals to control pests and at the same time assure 

 the public the greatest possible degree of safety. Where 

 we have a choice of pesticides that will do a given job, 

 we must decide in each instance which of the materials 

 available will be most effective and which can be used 

 with the greatest degree of safety. 



Obviously, the inherent toxicity of the chemicals in 

 c|uestion to man and other warm-blooded animals must 

 be taken into account, but it may be a minor factor and 

 by no means the dominant consideration. Very ottcn 



one may find that, from the standpoint of residues, the 

 most toxic substance will be the safest one to use by a 

 considerable margin. In most instances, the more toxic 

 chemicals are used or applied in proportionately smaller 

 amounts than are the less toxic materials, and frequently, 

 but not always, the more toxic compounds arc the most 

 volatile and are therefore short-lived. 



On the basis of data presented and of many fragments 

 of data obtainable elsewhere, one must conclude that 

 the rate of loss through evaporation for an insecticide is 

 basically a function characteristic of that compound (an 

 index combining vapor pressure, size, and shape of crys- 

 tal formation, etc.). Through all the work in field and 

 laboratory it was noted that the percentage of residue 

 loss from day to day and from week to week was re- 

 markably consistent for each compound and quite or 

 completely independent of the dosage rate or magnitude 

 of the initial deposit. Thus, it appeared that insofar as 

 loss through evaporation is concerned, residues resulting 

 from different rates ol application of a given substance 



