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During May and June 1969, the distribution and abundance of adult and larval mosquito populations were studied on an irrigation scheme designed for rice production in Nyanza Province, Kenya and compared with an unmodified ecosystem 12 km to the southwest. Twelve species, 4 anopheline and 8 culicine, were recorded as adults. Biting studies using human bait showed that activity was almost exclusively at night. Although the numbers taken in night catches at the 2 sites (23,641 and 22,966 respectively) were very similar, the species composition was different. In the village adjacent to the ricefields Anopheles gambiae Giles made up 65% of the catch, Mansonia uniformis Theo., 28% and Culex pipiens fatigans Weid., 5%, while in a village in the unmodified area, M. uniformis constituted 86% of the catch, A. gambiae less than 1% and only 3 C. p. fatigans were taken. Larvae of A. gambiae were most abundant in nursery padi while among older, denser plants C. antennatus Beck was the dominant species. In ecologically similar microhabitats in the unmodified area, A. gambiae was the dominant species although 4 times less abundant than in young padi; adult and larval abundance are related. Due to the cropping regime, 2 population peaks a year are to be expected. Elsewhere in East Africa arboviruses have been isolated from 4 of the species recorded here. Birds were abundant in the rice, particularly heron, stork and ibis, and 2 species, the Cattle Egret ( Bubulcus ibis ) and Sacred Ibis ( Threskiornis aethiopicus ), play a role in the maintenance Sindbis and West Nile viruses elsewhere in southern Africa. Rodents, particularly Arvicanthis niloticus and Mastomys natalensis , were found and could play a role in the epidemiology of arboviruses, rickettsial infections, plague and leptospirosis. Other diseases which may become associated with the irrigation scheme are considered.
Gordon Surtees (Sun,) studied this question.