The Persistence and Fate of DDT on Foliage. I.—The Influence of Plant Wax on the Toxicity and Persistence of Deposits of DDT Crystals

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Abstract
The persistence and toxicity of DDT crystals on thin layers of plant wax were studied. Deposits of crystalline DDT at rates of 2 to 8 microgm. per sq. cm. were applied to glass plates, plain or coated with a 0·5 μ layer of wax obtained from leaves of sisal or cabbage. The plates were stored for various periods at 18°C. and at 43°C. (representing temperature and tropical leaf-temperatures). DDT remaining after storage was estimated biologically by means of its contact action on adult Tribolium castaneum (Hbst.), and chemically by a modification of the Schechter-Haller method. At 18°C, there was no significant change in any of the deposits when estimated chemically. But biological estimation showed that, although the toxicity of the deposits on waxed plates was unchanged, that on glass plates was doubled after storage for eight days. The reason for this change was not discovered. At 43°C., the amount of DDT, estimated both chemically and biologically, diminished rapidly and was negligible after 26 days. The loss of toxicity at 43°C. was shown to be accompanied by loss of weight of the deposits. Loss at 43°C. from plain glass plates could be much reduced by storing them in an atmosphere saturated with DDT vapour. It appeared, therefore, that loss of toxicity at the higher temperature was due to volatilisation of DDT and not to decomposition.