Ovicidal and oviposition deterrence properties of Tabernaemontana divaricata (Apocynaceae) against Aedes albopictus and Culex quinquefasciatus in Sri Lanka

Plant-based insecticides have an increasing demand in mosquito control programs due to the eco-friendly nature of botanical products. We examined the ovicidal and oviposition deterrent properties of ethanol extracts of Tabernaemontana divaricata leaves against Aedes albopictus and Culex quinquefasciatus mosquitoes. Ovicidal bioassays were conducted to determine the lethal concentrations required to kill 50% and 90% of mosquito eggs (LC₅₀ and LC₉₀). Oviposition deterrent activity of leaf extracts was evaluated using 1% treatment solutions (2 g leaf powder: 200 ml distilled water). The number of eggs laid in treatments and controls was recorded for 7 days. LC₅₀ and LC₉₀ for Ae. albopictus were 626.6 and 892.5 ppm, respectively. For Cx. quinquefasciatus, LC₅₀ and LC₉₀ were 553.2 and 621.1 ppm, respectively. A strong positive correlation was reported between egg mortalities and concentrations (Aedes albopictus r = 0.993, p = 0.001; Culex quinquefasciatus, r = 0.992, p = 0.001). Eggs of Cx. quinquefasciatus were more susceptible to leaf extracts compared to those of Ae. albopictus. The deterrent properties of the plant extracts were relatively high during the first few days of the treatment (99.8% for Ae. albopictus and 100% for Cx. quinquefasciatus). The gradual reduction of the deterrent properties was observed after the second day of treatment for Ae. albopictus and the third day for Cx. quinquefasciatus. The oviposition activity index (OAI) ranged from − 0.968 to -0.120 for Ae. albopictus and − 1 to -0.53 for Cx. quinquefasciatus. The results reveal the ovicidal and oviposition deterrent properties of the leaf extracts of Ta. divaricata against Ae. albopictus and Cx. quinquefasciatus.