DEVELOPMENT OF LARVAL STAGES OF Aedes aegypti IN POLLUTED WATER HABITATS IN COLOMBO DISTRICT AND THEIR SUSCEPTIBILITY TO TEMEPHOS INSECTICIDE

Abstract

Dengue fever, primarily transmitted by Aedes aegypti and Aedes albopictus, affects approximately 400 million people worldwide annually. In Sri Lanka, recurring outbreaks have occurred since the 1960s, with severe epidemics in Colombo, especially in 2017. Recent findings indicate that Aedes aegypti can breed in polluted water and develop resistance to insecticides such as temephos. This study investigated the developmental characteristics and temephos susceptibility of Aedes aegypti larvae reared in polluted water (PW) and freshwater (FW) under laboratory conditions to assess the role of polluted habitats in mosquito larval development. Field-collected eggs were reared to the fourth (F4) laboratory generation, with water quality (temperature, dissolved oxygen, pH, conductivity, total dissolved solids, salinity) assessed for both PW and FW. Morphometric analysis of all larval instars included head, thorax, antenna, siphon, anal papillae, abdominal features, and total body length using binocular microscopy. Temephos susceptibility was evaluated using five concentrations, ranging from 0.0125 mg/L to 0.625 mg/L, on third- and early fourth-instar larvae. Testing involved 20 larvae per concentration across four replicates, with mortality assessed after a 24-hour exposure period. Statistical analysis employed log-probit methods to determine lethal concentration values. Results revealed that both populations exhibited temephos resistance, with PW larvae showing significantly higher resistance levels. LC₅₀ values were 0.019 mg/L (95% CI: 0.014-0.027) for FW versus 0.046 mg/L (95% CI: 0.00- 0.109) for PW populations. LC₉₉ values reached 0.102 mg/L (95% CI: 0.055- 0.660) for FW and 0.602 mg/L (95% CI: 0.306-23.933) for PW populations. Morphometric analysis revealed significant differences in PW larval siphon and anal papillae development, showing faster growth and distinct developmental patterns compared to the consistent progression of FW larvae. These results indicate that polluted habitats foster temephos resistance and morphological adaptations in Aedes aegypti, highlighting the need for revised vector control and integrated management strategies to prevent dengue transmission.