a1 London School of Hygiene and Tropical Medicine, Keppel Street, London, WC1E 7HT, UK
a2 Wellcome Research Laboratories, Ravens Lane, Berkhamsted, Herts, HP4 2DY, UK
Pieces of netting and sheeting of various types were impregnated by dipping them in permethrin emulsion. Bioassays were performed in which three species of mosquitoes were exposed to the impregnated fabrics for periods of between 15 s and 8 min. When pieces of netting and sheeting were dipped in permethrin emulsion at ambient temperature, the amount of insecticide absorbed was generally proportional to the weight of liquid taken up, i.e. there was no evidence of selective absorption. Diffusion of permethrin did not occur between pieces of netting and sheeting sewn together. The LD50 on cotton nets was found to be about three times as great as on nylon nets. Aedes aegypti (L.) was found to be more susceptible than Anopheles gambiae Giles, which was more susceptible than Culex quinquefasciatus Say. Although for most other groups of insecticides variation in time and dose have equivalent effects, for permethrin on netting this was not found to be true, i.e. on having the exposure time the LD50 was less than doubled.
(Received September 20 1988)
c1 To whom correspondence should be addressed.
p1 Present address: Department of Zoology, University of Dhaka, Dhaka 1000, Bangladesh.