Oviposition Site Preference and Egg Hatchability of Anopheles gambiae: Effects of Land Cover Types

We studied the oviposition site preference and egg hatchability of Anopheles gambiae Giles with water collected from farmlands, forests, and natural wetlands. Water types significantly affected oviposition preference. Mosquitoes deposited significantly more eggs in rainwater in both the dry and wet seasons than waters from forests and wetlands, suggesting that An. gambiae prefers water with few impurities for oviposition. In the dry season, An. gambiae females also deposited significantly more eggs in waters from farmlands than those from forests and natural wetlands, but these differences were not statistically significant during the wet season. In both indoor and natural conditions, egg mortality in natural wetland habitats was significantly higher than in farmland habitats. The average water temperature in natural wetland habitats was significantly lower than farmland habitats in the natural conditions, but it remained the same under indoor experimental conditions, suggesting that factors other than water temperature play an important role in egg hatchability. Together with the findings from previous studies on the effects of land cover on larval survivorship, our results support the hypothesis that variations in habitat conditions induced by different land cover types contribute to the heterogeneous spatial distribution of An. gambiae larvae in the western Kenya highland

Effects of larval competitors and predators on oviposition site selection of Anopheles gambiae sensu stricto.

We examined whether predators and competitors influence selection of oviposition sites by Anopheles gambiae Giles. Mosquitoes in cages laid significantly fewer eggs in rainwater conditioned with a predator (backswimmers, Notonecta sp.) than in unconditioned rainwater. Rainwater conditioned with a putative competitor (tadpoles, Xenopus sp.) also had fewer eggs than unconditioned rainwater. Similarly, mosquitoes laid significantly fewer eggs in rainwater conditioned with five and 50 An. gambiae larvae than in unconditioned rainwater. When larvae were present, significantly more eggs were laid in containers with five larvae than in containers with higher densities, but the differences in number of eggs laid were not significant among the densities of 40, 70, and 100 larvae. This study demonstrated that caged An. gambiae females avoid oviposition in habitats with supposed competitors and predators

Effects of Larval Competitors and Predators on Oviposition Site Selection of Anopheles gambiae

We examined whether predators and competitors influence selection of oviposition sites by Anopheles gambiae Giles. Mosquitoes in cages laid significantly fewer eggs in rainwater conditioned with a predator (backswimmers, Notonecta sp.) than in unconditioned rainwater. Rainwater conditioned with a putative competitor (tadpoles, Xenopus sp.) also had fewer eggs than unconditioned rainwater. Similarly, mosquitoes laid significantly fewer eggs in rainwater conditioned with five and 50 An. gambiae larvae than in unconditioned rainwater. When larvae were present, significantly more eggs were laid in containers with five larvae than in containers with higher densities, but the differences in number of eggs laid were not significant among the densities of 40, 70, and 100 larvae. This study demonstrated that caged An. gambiae females avoid oviposition in habitats with supposed competitors and predators

Association between land cover and habitat productivity of malaria vectors in western Kenyan highlands.

We examined the effects of land cover type on survivorship and productivity of Anopheles gambiae in Kakamega in the western Kenyan highlands (elevation = 1,420-1,580 meters above sea level). Under natural conditions, An. gambiae sensu lato adults emerged only from farmland habitats, with an estimated productivity of 1.82 mosquitoes/meter(2)/week, but not from forest and swamp habitats. To determine the effects of intraspecific competition and land cover types, semi-natural larval habitats were created within three land cover types (farmland, forest, and natural swamp), and three different densities of An. gambiae sensu stricto larvae were introduced to the larval habitats. The mosquito pupation rate in farmland habitats was significantly greater than in swamp and forest habitats, and larval-to-pupal development times were significantly shorter. At higher densities, the larvae responded to increased intraspecific competition by extending their development time and emerging as smaller adults, but initial larval density showed no significant effects on pupation rate. Land cover type may affect larval survivorship and adult productivity through its effects on water temperature and nutrients in the aquatic habitats, as shown by the significantly higher water temperature in farmland habitats, enhanced pupation rates and shortened development times from the addition of food to habitats, and a significant negative correlation of the occurrence of An. gambiae larvae with canopy cover and emergent plants in natural habitats. These results suggest that deforestation and cultivation of natural swamps in the western Kenyan highland create conditions favorable for the survival of An. gambiae larvae, and consequently increase the risks of malaria transmission to the human population