Effects of organic and inorganic fertilisers on mosquito populations in rice fields of southern India

The effects of nitrogenous (inorganic) fertilisers, organic manures and blue-green algae (BGA) biofertiliser on mosquito populations (Diptera: Culicidae) were studied in rice fields of Madurai, Tamil Nadu, south India, with particular attention to Culex vishnui Theobald, Cx. pseudovishnui Colless and Cx. tritaeniorhynchus Giles, the vectors of Japanese encephalitis (JE). The application of urea, a nitrogenous fertiliser, in rice fields significantly increased the grain yield and the population densities of mosquito larvae and pupae (anophelines as well as culicines) in a dose-related manner. Fields treated with inorganic fertilisers (N, P, K) had significantly higher population densities of mosquito immatures than fields treated with organic manures (farmyard manure and green manure). Without nitrogenous fertiliser, BGA increased paddy yield without enhancing mosquito production. Therefore, the use of BGA with less nitrogenous fertiliser is recommended, which is beneficial economically and agronomically to the farming community and also significantly reduces mosquito production in rice fields. Increased use of nitrogenous fertiliser over the past two decades may have contributed to the increased severity of Japanese encephalitis epidemics, vectors of which breed in rice fields.