Malaria vector Bionomics and the role of Microbial larvicides in anopheline Mosquito larval Management in ariceland agro-ecosystem

Abstract

The current study was conducted with the aim of understanding the population dynamics of malaria vectors, their contribution to malaria transmission and potential role of Bacillus thuringiensis var israelensis and B. sphaericus (VBC- 60120) combined microbial formulations in malaria vector management in an irrigated rice agro-ecosystem. Field entomological studies were conducted between March 2005 and June 2007 in four villages in Mwea Rice Irrigation Scheme, Central Kenya. The population dynamics of Anopheles mosquitoes were examined by conducting weekly larval and adult mosquito sampling. Adult female anopheline mosquitoes were screened for blood meal sources and Plasmodium falciparum circumsporozoite proteins. The efficacy o f the combined microbial formulation (VBC-60120) was evaluated and eventually applied in anopheline breeding habitas in Karima village. Eleven anopheline breeding habitats with differential spatial and temporal larval density were identified. Temporary pools and burrow pits were the most productive habitats but rice paddies and associated canals maintained mosquito production throughout the year. Nine Anopheles species were identified from larval and adult population in the area. Anopheles arabiensis Patton dominated both pre-adult and adult population. Time lag cross-correlation analysis revealed no association between adult malaria vector density and rainfall but the densities increased during rice planting period. All the mosquito species had a high preference for bovine (range 56.3-71.4%) over human (range 1.1- 23.9%) or goat (0.1-2.2%) blood meals. Anthropophily was inversely proportional to land under rice cultivation. A resident of Mwea irrigated rice agro-ecosystem receive an average of 0.68±0.01 bites per person per night from Anopheles species with peak biting rates occurring during rice planting season. EIR values ranged between 0.02 infective bites per person per night for An. funestus Giles and 0.53 (ib/p/n) for An. arabiensis Patton. Microbial larviciding resulted XVlll in a complete (100 %) reduction of late instar larvae within 24hrs and habitats re-colonization occured three days post larviciding. ANOVA tests did not reveal significant reduction of the adult vector population after larvicide application. In conclusion, Mwea irrigated rice agroecosystem supports a wide range of anopheline species and shows a fundamental difference in breeding habitats productivity. Hence, time and habitat specific anti-larval measures are possible and should be advocated. Furthermore, microbial larvicide formulations have a potential role in vector control in irrigated areas and should be integrated with zooprophylaxis to form a powerful control tool in irrigated rice agro-ecosystems.