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Competitive interactions among the aquatic stages of the malaria mosquito Anopheles gambiae s.l. (Diptera: Culicidae) may affect the resulting adult densities and, hence, the risk of malaria. We investigated the impact of the presence of a fourth-instar larva ( An. gambiae Giles s.s. or An. arabiensis Patton), the quantity of food, and the available space on the survival and development of freshly hatched larvae of An. gambiae s.s. and An. arabiensis. To analyse the results, two proportional hazard models were constructed. The first estimated the effects of all covariates on mortality rate and the second estimated the effects of the covariates on development rate into the third larval instar (L3). A time-dependent covariate for density, which changed during the experiment as a result of death or development to L3, was included in both models. In the presence of a fourth-instar larva (L4), survival of the experimental larvae was significantly reduced, but no difference was detected between the presence of L4 An. gambiae and L4 An. arabiensis. The observation that the majority of dead larvae were not recovered in trays with an L4 present suggested that cannibalism and predation occurred readily. Limitation in space significantly increased mortality of larvae, whereas a limitation of food reduced larval development rate, but did not cause mortality per se. From this, we concluded that both cannibalism and predation were enhanced as a result of more frequent interactions within smaller environments, but did not occur for reasons of food shortage. This study shows that inter- and intraspecific interactions among larvae of the An. gambiae complex strongly affect survival and development, and that the quantity of food and the available space are important determinants of the outcome of these interactions. Implications of the results are discussed with respect to the population dynamics of both malaria vectors in the field. [ABSTRACT FROM AUTHOR]

The population dynamics of the larval and adult life stages of the malaria vector Anopheles gambiae Giles were studied in Miwani, western Kenya, in relation to meteorological conditions. Larval density within a habitat, the number of larval habitats and sibling species composition were investigated as determinants of larval population dynamics. Female vector densities inside local houses and sibling species composition were investigated as determinants of adult population dynamics.Larval densities were estimated using a modified area-sampling method. Within the habitats, all instars showed a highly aggregated distribution, with the exception of second instars. A longitudinal study on the larval populations of A. gambiae s.l. in two different types of habitat (dirt track and ditch) was carried out, using a novel sampling procedure. A. gambiae s.s. and Anopheles arabiensis, the two sibling species occurring sympatrically in the study area, showed some spatial segregation between the two types of habitat. Rainfall was significantly correlated with the number of A. gambiae s.l. larval habitats during the first 6 weeks of study taking 1 week time lag into account, while over the entire 5-month study period correlations were less clear. With 1 week time lag, rainfall was also significantly correlated with the number of female A. gambiae s.l. collected from CDC-light traps in the study houses. Both larval and adult populations showed a significant increase in the proportion of A. gambiae s.s. within the mixed population of A. gambiae s.s. and A. arabiensis over time. Although not significantly correlated, the ratio of rainfall over precipitation/potential evapotranspiration (P/PE), indicative of the humidity conditions in the area, was probably the driving force of this increase. [Copyright &y& Elsevier]