method. The sensitivity of this method is generally 

 considered to be in the neighborhood of 1.0 ppm, though, 

 in the vast majority of cases, replicates were in close 

 agreement down to 0.5 and 0.3 ppm. The use of this 

 method, however, leaves open to speculation the proba- 

 bility that the trends shown may continue below the 

 1.0 ppm level. 



In the process of developing techniques for the va- 

 porization study — which began with the exposure of 

 known quantities of material in petri dishes or on glass 

 slides and terminated in the use of 10 to 50 mg deposits 

 of crystalline material uniformly dispersed over 20 by 20 

 inch sheets ot semicrepe, white filter paper — it was shown 

 that with a large mass of material heaped or piled on a 

 small area the per cent loss in weight at various intervals 

 was very small, but as the mass of material per unit of 

 surface area was decreased, the rate of loss increased. 

 Finally it was found that the residues deposited on filter 

 paper responded to external conditions much the same 

 as did normal residues on plants (Table 2). 



Table 2. — Residue loss in 14 days' exposure under green- 

 house conditions (temperature 80 ± 5°F., R.H. 75 ± 5 per cent) 

 for seven insecticides. 



Under controlled conditions in the laboratory, where 

 many of the variables could be eliminated, the writer 

 and his associates (Decker, Weinman, & Bann 1950: 

 919-927) found that the data obtained were exceedingly 

 consistent and in most cases, when plotted, all points 

 were on, or in close proximity to, the lines mathemati- 

 cally fitted to the data. Under such conditions, residue 

 losses for eight and, in some instances, more insecticides 

 were compared at five temperatures. In all experiments 

 the order of loss was the same, with lindane disappear- 

 ing first and DDT last. As was previously reported 

 (Caldwell and Meyer 1935:38-39) for the field experi- 

 ments, the residue-loss curves for pure compounds 

 showed that the loss was a straight-line logarithmic func- 

 tion of time (Fig. 3). In general, the d.ita tor impure 

 substances, such as technical aldrin, chlordane, and toxa- 

 phene, when plotted, departed from the straight line 

 and produced typical wavy lines characteristic of mixtures 

 of compounds differing in their volatility. This was 

 found to be true both on foliage in the field and under 

 controlled conditions in the laboratory. This tendency 

 is evident in Fig. 1 and 2. 



Changes in temperature, of course, produce corre- 

 sponding changes in the vapor pressure of each substance. 



and thereby alter the rate of evaporation or residue loss. 

 The work in progress at Illinois showed definitely that, 

 although changes in temperature affected rates of evapo- 

 ration or residue loss, in general they produced some- 

 what corresponding effects on all of the materials studied, 

 as shown for aldrin and DDT in Fig. 4. While an in- 

 crease in wind or air movement tends to accelerate the 

 rate of residue loss, it affects all materials more or less 

 alike and does not greatly alter their relative positions. 



A review of all the conflicting data obtainable led to 

 the obvious conclusion that several factors influence the 

 rate of evaporation and that vapor pressure alone is not 

 an accurate measure of that loss. This is particularly 

 true for the initial period of each test when the spray 

 is being applied and drying on the surface; and it is 

 evident throughout some tests. Evaporation, therefore, 

 must be considered as the summary effect of the various 

 factors which influence residue loss through vaporiza- 

 tion: vapor pressure (probably the most important), 

 ratio of mass to surface exposed, air movement, type of 

 formulation used, etc. It follows logically, then, that 

 a substance like DDT, with a vapor pressure of approxi- 

 mately 33 X 10' at 25" C, exposed as a mass in an open 

 container would lose weight very slowly and might be 

 considered practically nonvolatile. As a thin layer of 

 fine, fluffy crystals exposed to warm, moving air, the 

 rate of loss would be increased manyfold. and it might 

 then be regarded as fairly volatile. 



Presumably evaporation has been established as an 

 important, if not a dominant, factor influencing residue 

 persistence for many of our presently used and poten- 

 tially available insecticides. That brings us back to 

 Fleck's highly significant statement (1948:706-708), 

 "The residual action of an insecticide is determined by 

 its vapor pressure, its sticking power, its solubility, its 

 absorption into the surface to which it is applied, and 

 its resistance to chemical change. " 



In most instances one should know or determine in 

 advance the relative importance of such (actors as solu- 

 bility, resistance to chemical change (stability), and 

 probability of absorption into plant tissue or coatings. 

 Such factors can then be properly evaluated or elimi- 

 nated from further consideration. That lea\es vapor 

 pressure and sticking power to be considered, and to 

 these I would add the probability of dilution owing to 

 plant growth and formulation variables. While there 

 would, of course, be exceptions to the rule, it is quite 

 probable that the last three factors would apply about 

 equally or proportionately to all pesticide deposits that 

 may come under consideration. That is, regardless of 

 the pesticide used, the factor of plant growth would 

 diminish or dilute all equally. Likewise, in the case o( 

 sticking properties and formulation variables, it seems 

 probable that the methods of formulation and applica- 

 tion employed would determine tenacity and would 

 apply equally to all materials without regard to their 

 toxicity or volatility. If three oi the four big factors — 

 tenacity, dilution by plant growth, and differences at- 



