Seasonal prevalence of Anopheles stephensi larvae and existence of two forms of the species in an urban garden in Calcutta City.

A study on the occurrence and seasonal prevalence of Anopheles stephensi (principal vector of malaria in Calcutta) larvae in an urban garden in central Calcutta revealed their presence in collections of fresh and/or rain water in various kinds of temporary and permanent breeding habitats. Breeding sites in shady areas were preferred in comparison to those receiving direct sunlight. The density of An. stephensi larvae was highest in earthen and lowest in wooden pots throughout the study period, as supported by ANOVA and Wilcoxon's signed rank test. During monsoons the larval density was uniformly high in all four, i.e. earthen, stone, cement and wooden kinds of pots. The variation in the density of An. stephensi larvae due to the effect of season and nature of pots was highly significant. Co-existence of An. stephensi larvae with Aedes aegypti, Ae. albopictus and Cx. quinquefasciatus was also observed at times in certain water collections. An. stephensi population in the urban garden in central Calcutta was found to comprise of two forms the type and variety mysorensis.