A high level of resistance to chlorpyrifos has been detected in Culex quinquefasciatus larvae from Dar es Salaam. To study the impact of this resistance on the routine anti-larval spraying programme with this chemical, pit latrines, cess pits and soakage pits were examined regularly after spraying. Larvae were eliminated immediately after spraying, but in most cases they re-appeared about 2 weeks later. Bio-assays with susceptible and resistant larvae in water samples from the pits showed that in most cases there would have been sufficient insecticide residue for at least 10 weeks to kill a susceptible population, but resistant larvae could survive in the water from about 2 weeks after spraying. Chemical analysis showed that the water was toxic to resistant larvae so long as it contained about 0.02 mg/1 chlorpyrifos. These observations might have suggested that the routine spraying programme every 10 weeks would be having very little impact on the population now that resistance has evolved. However, in fact, adult densities in houses in a year with spraying on schedule were much less than in years when many sprayings were missed owing to shortage of supplies. Thus the regular spraying programme still apppears to have a beneficial effect.
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