Laboratory and Field Studies of Insecticide Impregnated Fibres for Mosquito Control

Laboratory and field studies were undertaken in an attempt to improve mosquito control with insecticide-treated fabrics. The efficacies of the emulsifiable concentrates (E.C.) of three pyrethroids were assessed on three types of fibre in order to establish the optimum combination for bednet use. Various pyrethroid formulations were compared for persistence and wash-fastness. Incorporation of polystyrene into the E.C. of permethrin increased its wash-resistance. Scanning electron microscope studies revealed a film of permethrin on impregnated nylon fibre which was not visible on similarly impregnated cotton fibre. Using an acoustic actograph to record flight activity and video to record behaviour, impregnated nylon was found to cause greater irritancy than impregnated cotton. The contact times for mosquitoes to pick-up lethal doses from permethrin and lambdacyhalothrin treated netting were established and related to the time that a hungry mosquito spends searching for a bloodmeal. Studies on sublethal effects of treated netting such as knock-down, leg fracture and feeding inhibition were carried out. Experimental hut trials of bednets were undertaken in The Gambia to compare five different insecticides, an insecticide mixture and two formulations incorporating polystyrene. The unwashed permethrin formulations strongly deterred hut entry by An.gambiae s.l. and Mansonia spp. but this effect was lost after washing the nets three times. The unwashed insecticide-treated nets killed a significantly higher percentage of An.gambiae s.l. that entered the huts than the untreated net. Washing treated nets decreased the percentages killed by them, with the exception of the wash-resistant permethrin which showed no significant change in the percentage of mosquitoes killed. This formulation killed a significantly higher percentage than the normal permethrin formulation after washing. The unwashed mixture of permethrin and pirimiphos-methyl treated net performed well over the 6 weeks of the trial, but chemical analyses at the end of this time showed very little of its pirimiphos-methyl remained on the net. The effects of all the nets on numbers entering and exiting from huts and numbers found killed and bloodfed are reported. A trial was carried out in a village to compare nets treated with the wash- resistant formulation of permethrin, normal E.C.s of permethrin and lambdacyhalothrin and placebo. The numbers of mosquitoes found over 16 weeks during weekly bednet searches of the insecticide treated nets were greatly reduced compared with the placebo treated nets. Analysis of bioassay mortality showed that the permethrin formulation containing polystyrene (wash-resistant) was significantly less affected by washing under village conditions than the normal permethrin and lambdacyhalothrin formulations.