The urban area of Dakar, Senegal contains more than 5000 market-garden wells that
provide permanent sites for mosquito larvae, in particular Anopheles arabiensis
which is the major vector of malaria. A study of the bio-ecology of mosquito
larvae was conducted over one year with a monthly visit to 48 of these wells. The
total outcome was 9,589 larvae, of which 80.1% were Culicinae and 11.9%
Anophelinae. All the larvae from stages III and IV (n=853) were identified to ten
species. An. arabiensis represented 86% of the anophelines collected and
An. ziemanni 14%. The most common Culicinae species included Aedeomyia africana,
Culex quinquefasciatus and Mimomyia splendens. The maximum anopheline density was
observed at the end of the dry season in June, while the maximum Culicinae
density was observed at the end of the rainy season in September. Sixty-seven
percent of the wells did not harbor any An. arabiensis larvae and in the
remaining 33% the larval densities were low, averaging 0.54 larvae in stages
III-IV per tray sample. In the aim of identifying the factors which determine the
abundance of the larvae in these wells, a co-inertia (multivariate) analysis was
carried out taking account of the physicochemical variables of the water : depth,
turbidity, temperature, pH, conductivity, Na+, Cl-, HCO3-, CO3-- and NO3-
concentrations and the biological variables concerning the presence and the
density of the different mosquito species, predators of mosquitoes (fish,
Dytiscidae, Notonectidae, odonates, etc.), of molluscs (Bulinus and
Biomphalaria), of surface plants (water lettuces, Lemna and filamentous algae).
The co-inertia analysis highlighted that the numbers of An. arabiensis are
associated to those of Cx. quinquefasciatus and of Cx. decens for the
physiochemical data but they have not been associated to any other species of
mosquito for floro-faunistic environmental data. The conditions associated with
abundant An. arabiensis were : hot (28-30¡C), clear and not too deep water (<0.5
m); a strong concentration of HCO3-and CO3-- and a weak concentration of NO3- and
NaCl; a low density of larvivorous fish and invertebrate predators (notably
odonates); the presence of water lettuces; and an absence of Lemna. These results
suggest that many factors contribute together to influence the ecology of the
immature stages of An. arabiensis.