Anopheles larval productivity and diversity in Mwea irrigation scheme, Kirinyaga district, Kenya
Mwangangi, Joseph Mumo
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The use of irrigation to flood agricultural land during rice cultivation has over the years been associated with an increase in the number of disease vectors and corresponding increase in health burden due to malaria and other vector and waterborne diseases. In this study, field and laboratory studies were used to examine the primary factors responsible for regulating the aquatic stages of malaria vectors in a rice agro-ecosystem prior to implementation of a larval control programme. The objective of this study was to determine the environmental and agricultural factors that regulate malaria vector productivity and diversity in Mwea Irrigation Scheme, Kirinyaga district, Kenya. The study was conducted in 3 villages representing planned (Mbui Njeru), and unplanned rice cultivation with varying amount of land under rice (Kiamachiri and Murinduko). The physico-chemical variables were measured using different field based hand held equipments or visual assessment. Experimental plots were used to closely monitor the factors associated with Anopheles larval densities in the rice fields. A total of 29,252 immature stages of anopheline mosquitoes were collected in the three villages comprising of'78.23% (n = 22,885) early instars, 10.91% (n = 3,192) late instars and 10.85 % (n = 3,173) pupae. Anopheles gambiae s.l. was the most abundant and was found in all habitats that were positive for late anopheline instars in the 3 villages. Larval abundance was significantly higher in Murinduko compared to the other villages (F(2, 182) = 38.685, p< 0.01). Rainfall was positively associated with Anopheles gambiae s. l. larval abundance in Kiamachiri (r = 0.759) and Mbui Njeru (r = 0.602) but negatively associated in Murinduko (r = - 0.267). Multiple logistic regressions showed Anopheles larval density to be significantly associated with many interrelated biotic and abiotic variables including presence of other invertebrates, percentage Azolla cover, distance to nearest homestead, water turbidity, water temperature, conductivity, pH, and water depth. Anopheles productivity from different habitat types showed that paddies had most emergent mosquitoes (n = 143) followed by marshes (n = 65). Succession of Anopheles species was evident with An. gambiae colonizing the paddies throughout the rice growth cycle with peaks during the early stage of rice growth while An. rufipes and An. coustani occurred during the late vegetative stages. Larval densities were significantly higher at the centre of the paddy compared to the periphery during the transplanting period (F (1,166) = 4.809, P = 0.030) but the difference was not significant during the tillering period (F (1,362) = 0.037, P = 0.848). The survivorship of immatures in the paddies showed that there was 98.26% mortality of larvae. In conclusion, rice paddies and associated canals are the most productive habitats types throughout the year while peridomestic habitats are important during the long and short rains. The results further indicate that several biotic and abiotic factors interact to regulate Anopheles larval densities in aquatic habitats. These findings demonstrate the need to target larvicidal application in the entire paddy between transplanting and tillering stages in order to achieve effective larval control.