The Persistence of Toxicity in DDT-impregnated Hessian and its Use on Tsetse Traps

Abstract

Experiments were carried out to investigate the persistence of toxicity to tsetse of hessian, impregnated with DDT, exposed to the normal weather conditions of the Inland Savanna Region of the Gold Coast; also, to establish a satisfactory method of estimating the loss or change in its toxicity, and to study the effects of impregnation with DDT on the efficiency of tsetse traps.Hessian was impregnated by being soaked for ten minutes in a filtered saturated solution of commercial DDT in kerosene which gave 4·4 g. DDT per sq. ft. and tests were made at intervals with batches of Glossina tachinoides at 15- and 60-second contacts. The comparison of the survival times of these flies compared with those of controls showed an appreciable residual toxicity after 20 weeks  full exposure to the weather including 4·26 ins. of rain, a slight toxicity after 30 weeks, and a negligible toxicity after one year including 34·35 ins. of rain. The same preparation could still produce toxic symptoms in tsetse after 15-second contacts when exposed for one year to wind and daylight, but protected from rain and direct sunlight. There was no great difference on the whole between the results from 15-second and 60-second contacts.It was found that survival times lengthened in October when the humidity was high but were shortened in December when the conditions became hotter and drier.A further experiment was carried out with hessian carrying 4·4 g. DDT per sq. ft. and super-impregnated hessian with 7·4 g. DDT per sq. ft. Samples of both were exposed to the weather and others kept as controls. All samples were tested by 30-second contacts with G. tachinoides.It was shown that contact with the super-treated cloth was invariably fatal; the standard cloth had an 82 per cent, toxicity but the flies that received a lethal dose took longer to die on the whole. It was found to be the survival time and not the toxicity that varied with seasonal conditions.A full analysis of the survival times from the exposed samples in relation to those from the control samples showed that exposure to weather during late rains followed by dry season conditions caused only a slight but progressive deterioration of the killing power of exposed DDT-impregnated hessian. This amounted to 10 per cent, in 3–4 months including 7·42 ins. of rain in the case of standard cloth, and 10 per cent, in six months with the same amount of rain in the case of the super cloth.Hessian impregnated with 25 per cent., 50 per cent, and neat DDT emulsion was found to be highly toxic and to retain its efficiency satisfactorily for at least 12 weeks' exposure in the dry season.Hessian freshly impregnated with DDT in kerosene solution caused flies to die more quickly as long as traces of kerosene persisted but moisture, on the other hand, interfered with the action of DDT unless kerosene was also present.An “animal” tsetse trap covered with standard impregnated hessian showed a 40 per cent, superiority in the numbers of G. palpalis and G. tachinoides taken over a period of 20 weeks. Corresponding to the results of the laboratory tests this superiority was lost in October but reappeared in December.Comparisons of catches made by traps covered with standard impregnated hessian and hessian sprayed with 25 per cent. ATSO emulsion showed that, although the emulsion might initially give much better results, its superiority relative to the standard impregnation tended to diminish, especially during the wet season.The increased efficiency of an impregnated trap is more than the measurable amount of the increased catches since flies which have investigated the trap, but have not been caught, are likely to have received a lethal dose of DDT.The application of DDT to traps represents an economical use of the insecticide which can conveniently be applied in sufficient concentration to ensure that brief contacts will be fatal to tsetse over a period of several months.