SIGNIFICANCE OF PESTICIDE RESIDUES: 

 PRACTICAL FACTORS IN PERSISTENCE 



OBVIOUSLY ALL FACTORS THAT INFLUENCE 



the magnitude of pesticide deposits or the persistence of 

 residues have some practical value or significance, and 

 one finds it difficult to classify the factors involved as 

 either practical or theoretical. Here the question of 

 practicality is really one of rationality — in other words, 

 correlation of the soundness of the evaluations we have 

 placed on every one of the known factors with the effi- 

 ciency with which we have utilized the available knowl- 

 edge. 



LIntil cjuite recently the insecticides in common use 

 could be divided into groups, such as the highly \olatile 

 fumigants, the rather unstable botanicals, and the stable 

 and persistent metallic salts. Thus it developed that for 

 many years most residue problems were associated with 

 highly stable and practically nonvolatile mineral salts. 

 Once such materials were applied to plants or other sur- 

 faces, they might be expected to persist unchanged for 

 indefinite periods of time or until removed by mechanical 

 processes. 



FACTORS AFFECTING RESIDUE LOSSES 



From time to time investigators working on residue 

 problems have isolated and evaluated some of the vari- 

 ous factors that affect the magnitude of residues and their 

 persistence. Most of these factors are fully discussed 

 and summarized in recent text or reference books (Brown 

 19''1; Frear 1942; Shepard 19'5l) and therefore will not 

 be reviewed here. It is possible, however, that one or 

 more important factors involved have been largely over- 

 looked or grossly underestimated. The terms "vapor 

 pressure" and "evaporation" are seldom given much 

 space, and in many cases are not even mentioned in 

 residue discussions. Perhaps this was fitting and proper 

 so long as we were primarily concerned with the residues 

 of lead arsenate and similar, essentially nonvolatile com- 

 pounds. We may have erred, however, in following the 



This was an invitation paper, presented in ^^il^vaukee. Wis- 

 cnnsin, in April, l!t.^)2. as part of a synipoKiuni sponsored by the 

 American Chemical Society. It was accepted for publication 

 in a contemplated numl>er of the Advances in Chemistry series, 

 but after this and several other mannscripts had £:one as far as 

 the t?ailey proof stau'e, the whole project was abaniioneti and tlie 

 series was never imblished. Meanwliile, Omitller & Hliim and 

 others had received the niannscript and had cited it as "In 

 Press." In response to the many innnii-ies as to the status and 

 disi>osiIinn of the inanuscript, to clear the record it is l)eini: 

 publisheil here. Despite the fact that mneh proRi-ess l\as been 

 nimii- in the interveninj^r j'ears. in fairness to those who read and 

 commented n])on the (n-iKinal niannscript. it is piiliUshed almost 

 unaltered and exactly the same as it would have appeared h.nl 

 publication Iteen consummated in l!i.^:l. 



This paper is published liv authority of the State of Illinois. 

 IRS Ch. 127. Par. 58.21. It is a contribution from the Section 

 of Economic KntomoloKry of the Illinois Xatm-al History Survey. 



Dr. Geoi-Re C. Decker, formerly Principal Si'iel\tist and Heail 

 of the Section of Koononiic hhitomoloKy. Illinois Natural History 

 Survey, is now retired. 



GEORGE C. DECKER 



same old lines of approach when we began to attack 

 problems associated with the use of the chlorinated hy- 

 drocarbons and other new, synthetic, organic insecticides. 



Very soon after DDT came under intensive study in 

 1944, Fleck (1944:853), reporting on the "Rate of Evapo- 

 ration of DDT," concluded that "the loss of DDT from 

 insecticidal spray deposits by volatilization will occur 

 too slowly to be of any importance." Two years later 

 Wichmann et al. (1946:218-233) apparently came to 

 about the same conclusion but did not clearly say so. 

 Unfortunately, in both instances the rate of evaporation 

 was determined by exposing known amounts of DDT 

 crystals (63. 36 mg and 200 mg) on glass plates which 

 were weighed at intervals. This method, of course, was 

 not conducive to the production of maximum losses by 

 evaporation. 



About the same time, Gunther et al. (1946:624-627), 

 reporting on rather extensive residue studies involving 

 progressive analyses of apple foliage and fruit samples, 

 showed that after 86 days "every treatment showed a 

 loss of from 71 to 95 per cent of the original quantiry 

 of DDT deposited." There was no suggestion or impli- 

 cation, however, that even a part of the loss might have 

 been due to evaporation. The final conclusion was 

 merely, "A distinction between mechanical weathering 

 and chemical decomposition has not been attempted in 

 this report." 



Fleck (1948:706-708) introduced a noteworthy paper 

 with the following appropriate and highly significant 

 statement, "The residual action of an insecticide is de- 

 termined 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." From 

 that point on, however, the theme of the paper is chemi- 

 cal change or decomposition; vapor pressure and evapo- 

 ration are ignored. 



It would appear that most workers have taken their 

 cue from these and similar reports, for although several 

 writers (Hadaway & Barlow 1951:854; Hcnsill & Gard- 

 ner 19'i0:102-10~; Walker 1950:123-127) have made 

 siMiie reference to vapor pressure, volatility, or evapora- 

 tion, no one has come forward to emphasize the impor- 

 tance of evaporation as a factor strongly influencing the 

 rate of residue loss where the chlorinated hydrocarbon 

 insecticides and similar materials are involved. Even 

 where a significant and consistent progressive loss of 

 residue has been noted, the tendency has been to at- 

 tribute the losses almost entirely to erosion, weather, or 

 chemical decomposition. 



More recently, in 1950, the writer and his associates 

 (Decker, Weinman, e^: Bann iq'>0:019-92"'). in reporting 



