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

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.

Cuticula of leaves and the residue problem

“…As shown in Figs. 18 and 19 (FROHBERGER 1949, WARD and BURT 1956, BURT and WARD 1956) persistence on dead material such as glass and paper is much higher than on leaves and fruits. Furthermore, it can be demonstrated that the living state is decisive: living leaves of the same species show much less persistence (measured as increase of the LD50 of flies) than do killed leaves.…”

“…Loss of DDT deposits from glass plates and growing leaves (above) as quantity of DDT residue per em.' of leaf surface, and (below) as persistence measured by insecticidal activity (adapted from WARD and BURT 1956) time (PIELOU and WILLIAMS 1962 a and c). The effect of surface-active agents (surfactants) such as wetters, spreaders, and stickers on the deposits of pesticides on leaves is not always readily predictable.…”

“…Generally the factors affecting persistence can be grouped as follows: Removal or breakdown of the toxicant brought about by atmospheric elements (BURT and WARD 1956).…”

“…Oxidation as a result of exposure to air, decomposition induced by heat, and degradation either occurring spontaneously or accelerated by catalytic material may effect persistence. Changing of the physical nature of the spray film as a result of a decrease of adhesion of the insecticide particles, crystallization of a solution, liquefaction of a solid, and volatilization of the pesticides all cause a change in the deposits and residues on foliage (GUNTHER and BLINN 1955, BURT and WARD 1956, EBELING 1963.…”

A thermal preference method of bioassay of the toxicity of insecticidal films to house-flies.

“…For the second experiment, x 2 = 20-0 (20 degrees of freedom); this is not significantly heterogeneous. Burt & Ward (1955) showed that deposits from DDT suspension increase in toxicity to Tribolium castaneum (Hbst.) when stored; it appears that during the first 24 hours there is also an increase in the replicability of the contact effect of the deposit.…”