SECTION III--FATE OF CHEMICALS 
INSECTICIDE RESIDUES IN PLANTS, ANIMALS, AND SOILS 
W. E. Westlake and J. P. San Antonio! 
It is impossible to give anything approaching. a comprehensive discussion of our 
knowledge of insecticide residues in a paper as brief as this one must be. It has been 
necessary, therefore, to select a few examples illustrating the behavior of certain repre- 
sentative types of insecticides. The discussion will be limited to insecticides, although 
much of the chemistry involved in determining the amounts present and the fate of these 
compounds applies equally to other types of pesticides. 
Various chemical and physical factors influence the rate of degradation and the 
amounts of a parent compound and related products that may be present at any given 
time. On plant surfaces and in soils, temperature, light, humidity, air movement, 
volatility of the compounds, and microorganism activity areimportant factors influencing 
the breakdown and mechanical dispersion of residues. The action of enzymes is probably 
the most important single factor within plants and animals. 
Since the disappearance of insecticide residues is of the utmost importance, par- 
ticularly on plants, the present discussion will dealprincipally with some of the chemical 
changes that have been observed or proposed. Progress in research on metabolites of 
non-nutritive substances in general is graphically illustrated by a comparison of the 
first and second editions of R. T. Williams’ ‘‘Detoxication Mechanisms”’ (19). The first 
edition contained 288 pages and the second, published 12 years later, in 1959, contains 
796 pages. During this same period significant advances have similarly been made in our 
knowledge of the occurrence and fate of residues following the application of insecticides. 
When an insecticide is applied to plants, animals, or soils, there are many factors 
that may influence chemical changes and the rates of such changes depend upon the nature 
of the compound and the particular conditions to which it is subjected. The chemical 
reactions that occur may produce compounds that are either less or more toxic than the 
one applied. 
CHLORINATED HYDROCARBON INSECTICIDES 
DDT has been the subject of comprehensive studies to determine its rate of degrada- 
tion and the products formed. Fleck and Haller (8) studied the dehydrochlorination of this 
insecticide and found that iron and aluminum salts catalyzed dehydrochlorination under 
1Chemist, Entomology Research Division, and Plant Physiologist, Crops Research Division, respectively, Agricultural Research 
Service, U.S, Department of Agriculture, 
105 
