Malaria vector bionomics and the role of microbial larvicides in anopheline mosquito larval management in a riceland agro-ecosystem

Abstract

Anopheles gambiae sensu lato IS a major vector of malaria in western Kenya where environmental conditions are highly conducive for its survival and reproduction. Breeding mostly occurs in newly formed small and transient water bodies but as the habitats age they may become unattractive for breeding of the species. The aim of this study was to deduce some of the ecological implications of this strategy of early colonization of breeding sites by Anopheles gambiae s.l. Ten semi-natural habitats (5 small and 5 large) were studied longitudinally (sampled same population over time) for a period of 12 weeks with daily sampling of all larval and pupal stages and quantification of all other identifiable aquatic fauna. This was followed by a final census and retrieval of all inhabitants of each habitat. A new set of 10 habitats was investigated together with the old habitats using the same procedures for an additional period of 9 weeks. Larval cohorts lasting on average twelve days were easily noticeable from the stage distribution data indicating the possibility of rhythms in the oviposition behaviour of gravid females in the area. The anopheline mosquito immature populations also exhibited stage-specific temporal variation in numbers but without a consistent or predictable pattern over the study period. The survivorship of Anopheles gambiae larval and pupal stages was lower in small habitats (0.0597) than in the large habitats (0.094 I). The colonization rate varied through time as various factors in the habitats became manifest during the experimental period. The most persistent. abundant and conspicuous components of the pioneer aquatic fauna were the tadpoles and mosquito larvae. Known potential predators of Anopheles gambiae larvae were largely absent or few during the early stages of habitats colonization. Hence breeding early in small and temporary habitats may be beneficial to Anopheles gambiae immature stages. a complete (100 %) reduction of late instar larvae within 24hrs and habitats re-colonization cured three days post larviciding. ANOV A tests did not reveal significant reduction of the ult vector population after larvicide application. In conclusion, Mwea irrigated rice agro¬osystem supports a wide range of anopheline species and shows a fundamental difference in eeding habitats productivity. Hence, time and habitat specific anti-larval measures are possible ld should be advocated. Furthermore, microbial larvicide formulations have a potential role in ector control in irrigated areas and should be integrated with zooprophylaxis to form a powerful entrol tool in irrigated rice agro-ecosystems.