The decay of DDT and parathion deposits under the influence of temperature, rain and ultraviolet radiation was described. Temperature and solar radiation were the main factors limiting the residual effect. High mean temperatures were more important than high maxima.DDT residues were more persistent than parathion residues. A fraction of the parathion residue persisted for many days indicating the presence of parathion inside leaf tissues.In the laboratory at high temperatures losses mainly occurred within the initial period of exposure; at moderate temperatures initial losses were much less but continued at a higher level over a longer period. Parathion residues were more temperature-sensitive than DDT residues.DDT deposits showed a linear decrease with rain; the parathion residues decreased geometrically at first, then slowly. Losses were not proportional to rainfall.Ultraviolet radiation was very effective in reducing the DDT and the parathion residue within a few hours. Moistening generally had a protective effect against radiation.
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