8 



advantage of a relatively high vapor pressure, the residue 

 of a given insecticide would tend to disappear more rap- 

 idly than the residue of another insecticide lacking high 

 vapor pressure. 



Time will not permit a lengthy discussion of other 

 problems, but perhaps the enumeration of a few other 

 practical considerations worthy of thought may be in 

 order 



(1) Where we have been making progressive resi- 

 due analyses under field conditions, we have found that 

 in some instances after a residue had shown a typical 

 straight-line decline to near zero, we suddenly had a 

 very great rise in the residue level (sometimes 20 to 100 

 ppm). This was inevitably traceable to drift from other 

 spray operations in the vicinity, in some cases up to 750 

 feet distant. This raises the question of how much 

 confidence we can place on harvest residues alone unless 

 we are sure beyond doubt that no contamination oc- 

 curred between our recorded treatment and harvest. 



(2) It was observed that where there is any air 

 movement at all, the magnitude of an initial deposit 

 increases row by row from the windward margin of a 

 plot and that there is a very appreciable deposit several 

 rows beyond the plot. One may therefore question the 

 validity of small-plot data. 



(3) Since the solvents employed will influence the 

 time of crystallization and the type of crystals produced, 

 to what extent have formulation differences clouded our 

 results and to what extent can we utilize formulation 

 differences to hasten or retard the disappearance of a 

 residue? 



LITERATURE CITED 



Brown, A. W. A. 1951. Insect control by chemicals. John Wiley 

 & Sons, New York. 



Caldwell. John R.. and Harvey V. Mover. 1935. Determi- 

 nation of chloride. Industr. and Engin. Chem., Analyt. Ed. 



"(1>: 38-39. 



Decker. G. C, C. J. Weinman, and J. M. Bann. 1950. A 

 preliminary report on the rate of inseaicide residue loss from 

 treated plants. Jour. Econ. Entomol. Ai(6) : 919-92". 



Fleck. Elmer E. 1944. Rate of evaporation of DDT. Jour. 



Econ. Entomol. 37(6): 853. 

 . 194". Report on methods for analysis of DDT and 



insecticidal preparations containing DDT. Assoc. Off. Agr. 



Chem. Jour. 30(2) : 319-324. 



. 1948. Residual aaion of organic inseaicides. In- 

 dustr. and Engin. Chem. 40(4) : 706-"08. 



Frear, Donald E. H. 1942. Chemistry of inseaicides and fungi- 

 cides. D. Van Nostrand Co., Inc., New York. 



Gunther. F. a., D. L. Lindgren. M. I. Elliot, and J. P. 

 LaDue. 1946. Persistence of certain DDT deposits under field 

 conditions. Jour. Econ. Entomol. 39(5): 624-62". 



H ADA WAY. a. B., and F. BarlOW. 1951. Sorption of solid 

 inseaicides by dried mud. Nature 167 (4256): 854. 



Hensill. G. S., and L. R. Gardner. 1950. Some poisonous 

 residue faaors in use of rwo new organic inseaicides. Ad- 

 vances in Chemistry Ser. 1 : 102-10". 



Shepard, Harold H. 1951. The chemistr>' and aaion of in- 

 secticides. McGraw-Hill Book Co., New York. 



Stepanow. a. 1906. Ueber die Halogenbestimmung in or- 

 ganischen Verbindungen raittels metallischen Natriums und 

 Aethylalkohol. Ber. 39:4056-405". Med. Chem. Lab., Univ. 

 of Moscow. (Chem. Abs. 1:397). 



Umhoefer, Robert R. 1943. Determination of halogens in 

 organic compounds. Industr. and Engin. Chem.. Analyt. Ed. 



15(6): 383-384. 



Walker. Kenneth C. 1950. Parathion spray residue on soft 

 fruits, apples, and pears. Advances in Chemistry- Ser. 1 : 



123-127. 



WicHMANN. H. J., W. I. Patterson. P. A. Clifford. A. K. 

 Klein, and H. V. Claborn. 1946. Decomposition and 

 volatiliry of DDT and some of its derivatives. Assoc. Off. 

 Agr. Chem. Jour. 29(2) : 218-233. 



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