Preimaginal development of Anopheles stephensi in brackish water habitats and its co-occurrence with dengue vectors in coastal habitats of Northern Sri Lanka

Anopheles stephensi, traditionally recognized as an important urban malaria vector, has recently demonstrated considerable expansion in its preimaginal developmental ecology, highlighting its remarkable ecological adaptability and plasticity. This study, conducted between January and September 2024 in Columbuthurai, a densely populated coastal locality in Jaffna, investigated the preimaginal development and oviposition preferences of An. stephensi under freshwater (FW) and brackish water (BW) conditions through field surveys and ovitrap-based experiments. Fortnightly larval surveys were conducted to assess potential breeding habitats. In addition, twenty conventional ovitraps containing FW (0 g/L salt) and BW representing salinity levels of 2, 4, 6, 8, and 10 g/L salt were deployed in twenty randomly selected households, each receiving one FW and one randomly selected BW ovitrap. Traps were monitored weekly, and collected larvae were reared under laboratory conditions for species identification. To further validate oviposition preferences, four additional ovitraps containing FW, 4 g/L BW, and 10 g/L BW were placed in selected households. Preimaginal stages of An. stephensi were detected in stagnant open pools, discarded tyres, containers, and disused boats, with salinity levels ranging from 0 to 25 g/L. Ovitrap investigations confirmed egg-laying in both FW and BW habitats, with salinity levels ranging from 0 to 17 g/L. These habitats frequently co-contain dengue vectors, including Aedes aegypti, Ae. albopictus, and Ae. vittatus. Notably, An. stephensi showed a statistically significant preference (p < 0.05) for BW ovitraps over FW ovitraps. This first report of its development in small brackish water containers alongside dengue vectors emphasizes the urgent need for integrated vector surveillance and coastal mosquito control strategies.