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Abstract. Laboratory studies were carried out to investigate the role of larval habitatderived microorganisms in the production of semiochemicals for oviposition site selection by Anopheles gambiae Giles sensu stricto mosquitoes. Dual-choice bioassays with gravid females were conducted in standard mosquito cages. Field-collected or laboratory-reared mosquitoes, individually or in groups, were offered a choice between unmodified (water or soil from a natural breeding site) or modified substrates (filtered water, autoclaved soil or sterile media to which bacterial suspensions had been added). Egg counts were used to assess oviposition preferences.Mosquitoes preferred to oviposit on unmodified substrates from natural larval habitats containing live microorganisms rather than on sterilized ones. Variable responses were observed when sterile substrates were inoculated with bacteria isolated from water and soil from natural habitats.We conclude that microbial populations in breeding sites produce volatiles that serve as semiochemicals for gravid An. gambiae. These signals, in conjunction with other (non-olfactory) chemical and physical cues, may be used by the female to assess the suitability of potential larval habitats in order to maximize the fitness of her offspring.Key words: oviposition site selection, soil microbiota, semiochemicals, Anopheles gambiae Résumé. Des études de laboratoire ont été conduites afin de déterminer le rô le de substances chimiques attractives de microorganismes présents dans les sites larvaires dans la préférence de ponte des femelles gravides d'Anopheles gambiae s.s. pour ces sites. Un test biologique à double choix a été mis en place dans des cages à moustiques standards contenant des femelles gravides. Un choix entre substrat naturel (eau et sol non modifiés provenant des sites larvaires naturels) et substrat modifié (eau filtrée, sol stérilisé, ajout de suspensions bactériennes) est offert individuellement ou en groupe aux femelles collectées sur le terrain ou élevées en laboratoire. Le nombre d'oeufs pondus dans chaque substrat a été utilisé comme critère pour évaluer la préférence des femelles gravides pour un substrat particulier. Les femelles ont préféré pondre dans le substrat naturel contenant des microorganismes vivants plutô t que dans le substrat stérile. Des réponses variables ont été observées lorsque des bactéries provenant des sites larvaires naturels ont été inoculées dans le substrat stérile. Nous concluons que la population microbienne, présente dans les site larvaires, produit des substances chimiques odorantes attirant les femelles gravides d'Anopheles gambiae s.s. Ces signaux, associés à d'autres substances non odorantes et à des facteurs physiques, peuvent être utilisés par les femelles pour évaluer le potentiel d'un site larvaire, en vue du développement optimal de leur progéniture.

Females of Anopheles gambiae Giles normally oviposit in a large number of fresh, small, sunlit, and spatially spread temporary pools. Such pools are associated with lower levels of predation compared to large, longer-lasting habitats. We compared oviposition levels on preferred (water collected from natural anopheline larval habitats) and non-preferred (distilled water) aqueous substrates by gravid females that contained different densities of conspecific eggs or early and late instar larvae. The presence of conspecific larvae, but not eggs, had a positive or negative effect on the ovipositional responses of gravid An. gambiae females, depending on the quality (preferred or non-preferred by the mosquito) of the oviposition water and the density of larvae. Presence of larvae, at all densities, in distilled water deterred oviposition. However, in natural anopheline pool water, a low density of larvae increased oviposition, whereas a higher density inhibited oviposition. Our results suggest that two signals produced by this mosquito may be involved in regulating oviposition: a volatile pheromone emitted by conspecific larvae, which augments the effect of a volatile signal emitted by preferred habitats, and a non-olfactory cue associated with high densities of larvae that deters oviposition.

BackgroundAnopheles gambiae Giles is the most important vector of human malaria in sub-Saharan Africa. Knowledge of the factors that influence its daily oviposition pattern is crucial if field interventions targeting gravid females are to be successful. This laboratory study investigated the effect of oviposition substrate and time of blood feeding on daily oviposition patterns of An. gambiae mosquitoes.MethodsGreenhouse-reared gravid and hypergravid (delayed oviposition onset) An. gambiae sensu stricto and wild-caught An. gambiae sensu lato were exposed to three types of substrates in choice and no-choice cage bioassays: water from a predominantly anopheline colonised ground pool (anopheline habitat water), swamp water mainly colonised by culicine larvae (culicine habitat water) and distilled water. The daily oviposition pattern and the number of eggs oviposited on each substrate during the entire egg-laying period were determined. The results were subjected to analysis of variance using the General Linear Model (GLM) procedure.ResultsThe main oviposition time for greenhouse-reared An. gambiae s.s. was between 19:00 and 20:00 hrs, approximately one hour after sunset. Wild-caught gravid An. gambiae s.l. displayed two distinct peak oviposition times between 19:00 and 20:00 hrs and between 22:00 and 23:00 hrs, respectively. During these times, both greenhouse-reared and wild-caught mosquitoes significantly (P < 0.05) preferred anopheline habitat water to the culicine one. Peak oviposition activity was not delayed when the mosquitoes were exposed to the less preferred oviposition substrate (culicine habitat water). However, culicine water influenced negatively (P < 0.05) not only the number of eggs oviposited by the mosquitoes during peak oviposition time but also the overall number of gravid mosquitoes that laid their eggs on it. The differences in mosquito feeding times did not affect the daily oviposition patterns displayed.ConclusionThis study shows that the peak oviposition time of An. gambiae s.l. may be regulated by the light-dark cycle rather than oviposition habitat characteristics or feeding times. However, the number of eggs laid by the female mosquito during the peak oviposition time is affected by the suitability of the habitat.

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