Experimental hut trials of bednets impregnated with synthetic pyrethroid or organophosphate insecticide for mosquito control in The Gambia

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Abstract
Nylon bednets impregnated with different insecticides were evaluated in 1988 against wild adult mosquito populations, mostly Mansonia africana (Theobald) and Anopheles gambiae Giles sensu lato, entering experimental verandah‐trap huts in The Gambia. Each bednet had six 10 ± 10 cm holes made in the walls to simulate torn conditions and permit female mosquitoes to enter and feed on sleepers. Individual net treatments, determined by gas chromatography of net samples from before and after 12 weeks use of the bednets, were: permethrin 670 ± 159 and 405 ± 190mg/m2 (40% loss), cypermethrin 37 ± 8 and 16 ± 9 mg/m2 (57% loss), deltamethrin 10 ± 7 and 10 ± 8mg/ m2 (no loss), lambda‐cyhalothrin 2.6 ±0.9 and 1.6 ±0.5 mg/m2 (38% loss), pirimiphos‐methyl 4017 ± 117 and 1160 ± 319mg/m2 (71% loss). Washing three times in the traditional manner with local cow‐fat soap reduced the initial dosages by about 85% of cypermethrin and lambda‐cyhalothrin, 99.8% of pirimiphos‐methyl and left no detectable residues of deltamethrin or permethrin. The unwashed permethrin‐treated bednet reduced the number of mosquitoes entering a hut by 60% of An.gambiae s. l. and 68% of Mansonia spp. This deterrency was less pronounced with the other insecticides and was lost by washing the bednets. Each insecticide, especially lambda‐cyhalothrin and pirimiphos‐methyl, caused significant mortality rates of mosquitoes that entered huts with impregnated bednets, and prevented the majority of An. gambiae s.l. and Mansonia females from bloodfeeding. Washing completely removed the efficacy of deltamethrin and permethrin treated bednets, whereas nets treated with cypermethrin, lambda‐cyhalothrin or pirimiphos‐methyl remained significantly insecticidal after washing. Aerial toxicity from the pirimiphos‐methyl treated bednet killed 80% of An.gambiae s.l. confined overnight in the hut at the end of the trial, whereas the pyrethroid‐treated bednets gave negligible mortality rates of mosquitoes.

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