The replacement of Culex nebulosus Theo. by Culex pipiens fatigans Wied. (Diptera, Culicidae) in towns in Nigeria

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Abstract
An account is given of the disappearance in Kaduna, Northern Nigeria, of Culex nebulosus Theo. from the town streams and drains and other polluted collections of water, habitats in which until comparatively recently it appears to have been the dominant species, and its replacement by C. pipiens fatigans Wied., which was formerly rarely recorded in these sites. Similarly, the latter species has replaced the former as the commonest Culicine mosquito resting in houses. Increased urbanisation and the widespread use of residual organo-chlorine insecticides within recent years have both been advanced as possible explanations for this change.A laboratory experiment with larvae of C. p. fatigans from Kaduna and of C. nebulosus from villages on its perimeter showed the latter species to have a slightly, but significantly, quicker rate of larval development than C. p. fatigans; this would possibly confer an advantage on it in circumstances where the two species are in competition for the same larval habitats. Field and laboratory observations showed that both species can tolerate high organic pollution in their larval habitats. There appeared to be little evidence in Kaduna, or in other towns in Nigeria, to support the theory that urbanisation has been responsible for the replacement of C. nebulosus by C. p. fatigans. Laboratory and field trials demonstrated that larvae of C. nebulosus were extremely sensitive to DDT, and although resistance was not found in C. p. fatigans, this species showed tolerance of the insecticide. It is believed that, in Kaduna and most probably in other towns, the application of DDT as a larvicide to streams and drains has been responsible for the eradication of C. nebulosus there, and in the absence of competition from C. nebulosus and because of the greater tolerance of DDT shown by C. p. fatigans the latter has been able to establish itself as the dominant species.