Fig. 5. — Aldrin residues in 

 milligrams at intervals after 

 20, 30. 40, and 50 rag de- 

 posits on 20 by 20 inch filter 

 papers were placed in 80°F. 

 temperature chambers. 



24 

 TIME IN HOURS 



will, under the same conditions of exposure, reach the 

 vanishing or zero point at exactly the same time regard- 

 less of the magnitude of the original deposits. This 

 sounded logical, but left a feeling of apprehension. In 

 all instances, however, when this theory was tested by 

 exposing various insecticides at varying rates of appli- 

 cation, the a.ssumption proved to be valid. All deposits 

 of each compound similarly exposed reached the vanish- 

 ing point at the same time (Fig. 5). 



Now, let us see what the general principles just de- 

 veloped mean in terms of practice. In the first place, 

 with several materials available, what would be the pos- 

 sibility or probability that if used to control a certain 

 pest one or more of the materials would leave a detect- 

 able residue at harvest time? Where adequate experi- 

 mental data are available, the answer to that question is 

 fairly simple. Since it has been shown that under any 

 given set of conditions the residues produced by a given 

 insecticide will arrive at the vanishing point or zero 

 level at a specific time, regardless of the rate of applica- 

 tion or the magnitude of the initial deposit, a very good 

 indication of which materials will be most likely to show 

 residues at harvest time can be obtained from Fig. 1, 2, 

 and 3. They indicate the time required for the residue 

 of each material studied to reach the base line or zero 

 point. Obviously, lindane would have the best chance 

 of showing no residue, followed in order by aldrin. 

 chlordane, dieldrin, toxaphene, and DDT, witli little 

 likelihood that a DDT residue will ever reach the van- 

 ishing point unless aided very materially by other im- 

 portant factors, such as plant growth or erosion. Some 

 idea of the odds that residues of the various materials 

 would be gone by harvest, or by any other given time, 

 might be obtained by comparing tlie data on days re- 

 quired to reach zero (Table .i). 



From these data, which may be subject to consider- 

 able error, one may get a fair picture of the relative 

 importance and apparent practical value of several fac- 

 tors tiiat affect the r.itc o( rcsidu2 I iss. k'rom the labo- 



Table 3. — Days of exposure required for residues of various 

 inseaicides to decline to zero point under field and laboratory 

 conditions. 



ratory data one obtains a forceful impression of the dif- 

 ference in evaporation rates of the semivolatile materials, 

 such as lindane and aldrin, in contrast to the relatively 

 nonvolatile materials, such as toxaphene and DDT. The 

 importance of evaporation in the case of all materials 

 including dieldrin is evident, but a study of the values 

 for toxaphene and DDT discloses the very great signifi- 

 cance of erosion and weathering where mature foliage 

 was involved, and the added effect of plant growth in 

 diluting the residue in the case of the growing clover 

 plants. 



APPLICATION OF MATHEMATICAL PRINCIPLES 



Assuming that the interval between the date of last 

 treatment and harxest will be such that none of the 

 materials, not even lindane, will have time to vanish 

 completely, the probable magnitude of the residue of 

 any individual compound will be in direct proportion 

 to the dosage rate or the magnitude of the original de- 

 posit. As has been shown in the comparison of several 

 dosage rates, since the per cent of original deposit re- 

 maining at the end of each interval of time is the s.ime, 

 the residues at any specified interval will be in direct 

 proportion to the magnitude of the original deposit. 

 For example, in Fig. 5, where the original deposits were 

 in the approximate ratios of 2, 3. 4. and 5, the residues 

 remaiiiini; in each case on the 1st, 2nd. 4th. and 6th 



