The emergence of pyrethroid resistance in mosquitoes is complicating malaria elimination efforts in Africa and alternative insecticides have to be considered for indoor residual spray. Unfortunately, the high volatility of WHO-approved organophosphate alternatives, e.g. malathion, translates into an early loss of residual efficacy. This laboratory study explored the concept of trilayer films as potential wall or ceiling linings. In the proposed design, the fugitive liquid insecticide is trapped in an inner core layer. The two sheath layers act as low-permeability membranes controlling the release of the insecticide. The concept was explored using poly(ethylene-co-vinyl acetate) (EVA) and low density polyethylene (LDPE) as core and sheath polymers respectively. The polarity of the EVA polymer matrix allowed incorporation of substantial quantities (up to 30 wt%) of malathion. The low polarity of the LDPE provided the necessary barrier properties and, in addition, allowed film blowing to be conducted at relatively low processing temperatures.
Trilayer films containing about 6 wt% malathion were prepared on a film blowing line. Scanning electron microscopy confirmed the trilayer film structure. Confocal Raman microscopy studies revealed a malathion concentration gradient across the thickness of the polyethylene layers. Mass loss measurements and FTIR spectroscopy studies showed that the malathion release followed first-order kinetics. Bioassays, on samples aged at 22 °C, indicated that the residual efficacy against mosquitoes can be maintained for up to about six months. This suggests that trilayer films impregnated with organophosphates, may have potential as alternative mosquito control interventions in pyrethroid resistant settings.