Larvicidal, Ovicidal, and Repellent Activities of Leucas stachydiformis (Hochst. ex Benth.) Briq Essential Oil against Anopheles arabiensis

Larvicidal, ovicidal, and repellent activities of the essential oil extracted by hydrodistillation from the leaves of the endemic Ethiopian plant Leucas stachydiformis (Hochst. ex Benth.) Briq were investigated against Anopheles arabiensis, the dominant malaria vector species in Ethiopia with the objective of searching for a plant-based malaria vector control strategy from medicinal plants. The larvicidal effect was tested against the fourth instar An. arabiensis wild larvae whilst freshly laid ova of An. arabiensis were used to determine the ovicidal activity of the essential oil at concentrations ranging from 6.25 to 400 ppm. Concentrations of 41.6–366.7 µg/cm² were used to evaluate the repellent activity of the essential oil on 3–5 days old adult female An. arabiensis. The oil composition of L. stachydiformis was also analyzed using GC-MS. The study revealed that the oil possesses the highest larvicidal activity at 400 ppm and 200 ppm after 24 h and 48 h of treatment. LC₅₀ values for the fourth larval instar after 24 h and 48 h of treatment were 43.4 ppm and 34.2 ppm, respectively. After 72 h of exposure, the oil displayed 100% ovicidal activity at 400 ppm with an IH₅₀ value of 32.2 ppm. In the repellency test, at concentrations of 366.7, 133.3, and 41.6 µg/cm², the oil gave a total percentage protection of 67.9 ± 4.2%, 37.2 ± 2.8%, and 32 ± 2.2%, respectively, for 4 h. The highest concentration (366.7 µg/cm²) gave 100% protection up to 90 min. GC-MS analysis of the oil revealed the presence of 24 compounds representing 90.34% of the total oil with caryophyllene oxide, germacrene D, and trans-caryophyllene constituting more than 50% of its components. Results of the present study suggest that the essential oil of L. stachydiformis has the potential to be used for the control of An. arabiensis mosquitoes.