Survival and emergence of immature Anopheles arabiensis mosquitoes in market-gardener wells in Dakar, Senegal.

Anopheles arabiensis is the unique species of the An. gambiae complex observed in the wells dug by market-gardeners in the Dakar area. In order to relate the numbers of immature stages and emerging adults mosquitoes, population measurements were performed in eight wells in which An. arabiensis was the only mosquito species. Mean density of immature stages was measured using two sampling methods, the dipping with a tray by giving 50 dips in each well, and the quadrat with a frame on 2 or 3 m2 in each well. The absolute number of emergent adults was obtained by collecting mosquitoes under net-trap covering entirely each wells. The dipping method was quicker and more operational than quadrat method. Density estimations of larvae at stage I to IV did not significantly differed using dipping or quadrat methods. On the contrary, pupal density was underestimated when measured by dipping. Mosquito nets placed over wells increased significantly emergence rate of adults, thus measurement of emerging mosquitoes was possible only the first day following the net putting up. The total number of immature stages in each well was significantly correlated with the number of emergent mosquitoes. The mean number of mosquitoes emerging daily from one well corresponded to 5% of the total number of immature stages. Stage distribution for larvae I to IV and pupae, estimated by quadrat, was respectively 29%, 28%, 22%, 16% et 5% (total = 100%). Taking account the mean duration of various immature stages and the number of emerging mosquitoes by day, the equation of the survivorship curve from larval hatch (excluded) to emergence included was: y = 427.2-136.8 Log x. Therefore the mean mortality at immature stages was 80% i.e. an emerging rate of 20%. The results of this study, associated with those of previous ones, permit to evaluate the average productivity of malaria vectors in market-gardener wells in the Dakar area.