days were still in those same relative ratios. In addition, 

 the ratios were maintained to the point at which, insofar 

 as the graph is concerned or mathematical calculations 

 can determine, some time on the 12th day (300 hours), 

 according to both theory and practice, all residues dis- 

 appeared simultaneously. 



If one wished to compare two materials, he would 

 have to take into account the slope of the line which 

 can be established by use of the formula Y = a -)- bX 

 (where X is the logarithm of time; b, the regression co- 

 efficient; and a, a constant derived from the equation, 

 a ^ Y — bX), and the normally recommended or 

 probable rates of application. The higher the evapora- 

 tion rate and the lower the normally recommended rate 

 of application, the greater the probability that the residue 

 will disappear or reach an insignificantly low level in 

 X days or any other given period. Conversely, the lower 

 the evaporation rate and the higher the dosage or appli- 

 cation rate, the greater will be the probability that a 

 very significant residue will be present after any given 

 period. 



It is to be hoped that further study of the mathe- 

 matical principles involved may lead to the development 

 of a formula or formulae which will make it possible 

 to compare two or more insecticides and predict rather 

 precisely the probable relative magnitude of their re- 

 spective residues X days after treatment, or the relative 

 number of days required for each to reach a residue of 

 Y magnitude. If this becomes a reality, then it will be 

 possible to multiply the values obtained by some suitable 

 index of chronic toxicity supplied by the toxicologist 

 to obtain in advance a fair estimate of the probability 

 that a specified use of a pesticide would result in a sig- 

 nificant food contamination hazard. Until such time as 

 suitable formulae are developed, helpful comparisons 



may be maile by utilizing data on dosage rates and 50 per 

 cent or 10 per cent life values. 



It is possible also that graphic presentations, such 

 as Fig. 6, will prove helpful in visualization of some of 

 the complex dosage-rate vs. residue-loss relationships. 

 For example, in comparing lindane or aldrin with the 

 reportedly less toxic materials, DDT or toxaphene, one 

 finds that on the 15th day the residues in ppm are, rough- 

 ly, lindane, 1.0; aldrin, 10; DDT, 152; and toxaphene, 

 220. On the 20th day they are lindane, 0; aldrin, 2; 

 DDT, 130; and toxaphene, 185. It would appear that 

 in estimating ultimate safety, one would have to balance 

 these or other comparable values for any given time 

 against the relative toxicities of the materials in question. 

 While the ppm lines (Fig. 6) are admittedly calculated 

 from hypothetical considerations, a check on seven points 

 where good data are available indicates all lines are well 

 within a 10 per cent error. 



DISCUSSION 



The data and discussion presented here in no way 

 establish vaporization as the sole or even the dominant 

 factor in determining the rate of residue losses. We are 

 all too familiar with the importance of weathering, dilu- 

 tion due to plant growth, etc., to discount the impor- 

 tance of these factors. In the field phases of investiga- 

 tions reported here it was impossible to isolate such 

 factors and study them separately. The losses observed 

 in the field investigations were in reality the total ac- 

 cumulative effects of all factors combined. In the final 

 analysis, however, it seems significant that in comparing 

 the residue losses of the different materials tested they 

 tended to persist in the inverse order of their vapor 

 pressures. This could only mean that with the added 



560 



Fie. 6. — Hypothetical res 

 idues in parts per million 

 and per cent of initial resi- 

 due at intervals after treat- 

 ment, assuming four mate- 

 rials were sprayed on apple 

 foliapc at normally recom- 

 mended rates of application. 



6 13 



DAYS AFTER TREATMENT 



