LAUSR

Simple Summary Plectranthus amboinicus (Lour.) Spreng., commonly known as Indian borage, has been reported to have insecticidal activity against various insects. In this study, the insecticidal properties (contact and fumigant toxicities) derived from P. amboinicus essential oil were investigated against the stable fly, Stomoxys calcitrans, and the horse fly, Tabanus megalops. The results showed that P. amboinicus essential oil has both contact and fumigant toxicities against the target species and thus has potential as an alternative control agent. Abstract The stable fly, Stomoxys calcitrans (Diptera: Muscidae), and the horse fly, Tabanus megalops (Diptera: Tabanidae), are important ectoparasites of livestock in Thailand. These species affect animal health and cause economic losses. This study investigated the insecticidal activity of Plectranthus amboinicus essential oil against S. calcitrans and T. megalops through contact and fumigant toxicity tests and evaluated the effects of the essential oil on these flies through histopathological and scanning electron microscopic (SEM) studies. The results of the contact toxicity test indicated that the median lethal dose against S. calcitrans and T. megalops was 12.05 and 131.41 µg/fly, and the 90% lethal dose was 45.53 and 200.62 µg/fly, respectively. The results of the fumigant toxicity test showed that the median lethal concentration against S. calcitrans and T. megalops was 1.34 and 7.12 mg/L air, and the 90% lethal concentration was 4.39 and 30.37 mg/L air, respectively. Histopathology revealed neuronal degeneration in the brain of S. calcitrans and interstitial neuronal edema of the brain and ovarian necrosis in T. megalops. No external morphological changes were observed via SEM. Given its insecticidal properties against S. calcitrans and T. megalops, P. amboinicus essential oil could be developed into a natural insecticide to control these fly species.