Abstract:
The insecticide dichloro-diphenyl-trichloroethane (DDT) is widely used in indoor residual spraying (IRS) for malaria control owing to its longer residual efficacy compared to other World Health Organization (WHO) alternatives. It was envisaged that by investigating mechanisms of degradation of these alternative insecticides, a better understanding would be obtained on strategies of stabilising them and rendering their efficacy comparable with or better than DDT, hence providing less controversial and more acceptable and effective alternative insecticide formulations to DDT. This study sought to investigate the reasons behind the reported longer lasting behaviour of DDT by exposing all the WHO approved insecticides to high temperature, high humidity and ultra-violet light. Interactions between the insecticides and some mineral powders in the presence of an aqueous medium were also tested. Simple insecticidal paints were made using slurries of these mineral powders whilst some insecticides were dispersed into a conventional acrylic paint binder. These formulations were then spray painted on neat and manure coated mud plaques, representative of the material typically used in rural mud houses, at twice the upper limit of the WHO recommended dosage range. DDT was applied directly onto mud plaques at four times the WHO recommended concentration and on manure plaques at twice WHO recommended concentration. All plaques were subjected to accelerated ageing conditions of 40 °C and a relative humidity of 90%. In the accelerated ageing tests, Fourier transform infra-red (FTIR) interferograms showed that pyrethroids were the most stable insecticides compared to carbamates and DDT. High temperature oxidation, ultra violet light and humidity were ruled out as the cause of failure of the alternative insecticides. Gas chromatography (GC) interferograms showed that phosphogypsum stabilised the insecticides the most against alkaline degradation. Bioassay testing showed that the period of efficacy of these formulations was comparable to that of DDT sprayed at 4 times the upper limit of the WHO recommended dosage range. Bioassay testing also showed that these insecticidal “paint” formulations stabilised the insecticides on cattle manure coated surfaces as compared to DDT sprayed directly on similar surfaces. Bioassay experiments indicated that incorporating insecticides into a conventional paint binder or adsorbing them onto phosphogypsum provided effective life spans, under accelerated ageing conditions, comparable to or exceeding that of DDT directly applied to typical soil substrates. Best results were obtained with propoxur in standard acrylic emulsion paint. Similarly, insecticides adsorbed on phosphogypsum and sprayed on cattle manure coated surfaces provided superior lifespans compared with DDT sprayed directly on a similar surface. Copyright
