BACKGROUND: The insecticide dichloro-diphenyl-trichloroethane (DDT) is widely used in indoor residual spraying
(IRS) for malaria control owing to its longer residual efficacy in the field compared to other World Health
Organization (WHO) alternatives. Suitable stabilization to render these alternative insecticides longer lasting could
provide a less controversial and more acceptable and effective alternative insecticide formulations than DDT.
METHODS: This study sought to investigate the reasons behind the often 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%.
RESULTS: The pyrethroids insecticides outperformed the carbamates and DDT in the accelerated ageing tests. Thus
UV exposure, high temperature oxidation and high humidity per se were ruled out as the main causes of failure of
the alternative insecticides. Gas chromatography (GC) spectrograms showed that phosphogypsum stabilised the
insecticides the most against alkaline degradation (i.e., hydrolysis). Bioassay testing showed that the period of
efficacy of some of these formulations was comparable to that of DDT when sprayed on mud surfaces or cattle
manure coated surfaces.
CONCLUSIONS: Bioassay experiments indicated that incorporating insecticides into a conventional paint binder or
adsorbing them onto phosphogypsum can provide for extended effective life spans that compare favourably with
DDT’s performance under accelerated ageing conditions. 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.