Response of Anopheles gambiae s.l. (Diptera: Culicidae) to larval habitat age in western Kenya highlands

BACKGROUND: Larval control is of paramount importance in the reduction of vector populations. Previous observations have suggested that, larvae of Anopheles gambiae s.l occur more often in small temporary habitats while other studies showed that long-lasting stable habitats are more productive than unstable habitats. In addition, the physical and biological conditions and stability of larval habitats can change rapidly in natural conditions. Therefore, we examined the effect of larval habitat age on productivity, larval survival and oviposition preference of Anopheles gambiae. METHODS: We sampled the three different habitat ages (10, 20 and 30 days) on a daily basis for a period of six months to determine mosquito larval abundance. In addition, we tested the effect of age of water (habitat age) on the oviposition choice preference of An. gambiae, larval development time and survivorship, and wing lengths of emerging adults. Additionally, chlorophyll a and abundance of mosquito larval predators in these habitats were monitored. RESULTS: Anopheles gambiae s.l. larvae were significantly more abundant (P=0.0002) in habitats that were cleared every 10 days compared to the other habitats. In particular, there were 1.7 times more larvae in this habitat age compared to the ones that were cleared every 30 days. There were significantly (P<0.001) more mosquito larval predators in the ‘30 day’ habitats compared to the other habitats. Oviposition experiments revealed that significantly more eggs (P<0.05) were laid in fresh water and water that was 5 days old compared to water that was 10 and 15 days old. However, pupation rate, development times and wing lengths of male and female An. gambiae in the different habitat ages was statistically insignificant (P>0.05). CONCLUSION: The current study confirmed that age of the habitat significantly influences the productivity of malaria vectors in western Kenya highlands. Given that malaria vectors were found in all habitats with varying ages of water, simple environmental methods of maintaining the drainage ditches in the valley bottoms can help reduce larval abundance of malaria vectors. Such inexpensive methods of controlling mosquito breeding could be promoted to supplement other vector control methods, especially in areas where scarce resources are available for intensive mosquito control.