ECOLOGY OF LARVAL MOSQUITOS, WITH SPECIAL REFERENCE TO ANOPHELES ARABIENSIS (DIPTERA, CULCIDAE) IN MARKET-GARDEN WELLS IN URBAN DAKAR, SENEGAL

The urban area of Dakar, Senegal, contains >5,000 market-garden wells that provide permanent sites for mosquito larvae, in particular Anophe les arabiensis Patton, the major vector of malaria. A study of the bio ecology of mosquito larvae was conducted over 1 yr with a monthly visi t to 48 of these wells. Overall, 9,589 larvae were collected of which 80.1% were Culicinae and 11.9% Anophelinae. Larvae from stages III and IV (n = 853) were identified to 10 species. An. arabiensis represente d 86% of the anophelines collected and An. ziemanni Grunberg 14%. The most common Culicinae species included Aedeomyia africana Neveu-Lemair e, Culex quinquefasciatus Say, and Mimomyia splendens Theobald. Maximu m anopheline abundance was observed at the end of the dry season in Ju ne, whereas maximum Culicinae abundance was observed at the end of the rainy season in September. Most wells (67%) did not harbor any An. ar abiensis larvae and in the remaining 33% the larval abundance was low, averaging 0.54 larvae in stages III-IV per tray sample. To identify f actors that determine the abundance of larvae in these wells, a co-ine rtia (multivariate) analysis was carried out to account for physicoche mical variables (depth, turbidity, temperature, pi-i, conductivity, Na +, Cl-, HCO3-, CO3--, and NO3- concentrations) and biological variable s (abundance of mosquito species, predators [e.g., fish, Dytiscidae, N otonectidae, odonates ], molluscs [Bulinus and Biomphalaria], and surf ace plants [water lettuce, Lemna, and flamentous algae]). The co-inert ia analysis indicated that the abundance of An. arabiensis was associa ted with Cr, quinquefasciatus and Cx. decens for the physiochemical da ta but was not associated with other mosquito species for floro-faunis tic data. The conditions associated with abundant An. arabiensis were warm temperature (28 - 30 degrees C), clear and not too deep water (<0 .5 m), elevated concentrations of HCO3- and CO3--, low concentrations of NO3- and NaCl low populations of larvivorous fish and invertebrate predators (notably odonates), the presence of water lettuce, and an ab sence of Lemna. These results indicate that many contributing factors influence the ecology of the immature stages of An. arabiensis.