Insecticidal and Repellent Activity of Plectranthus amboinicus (Lour.) Spreng Leaf Extracts Against Callosobruchus maculatus (Fab.)

Natural pesticides derived from plant-based substances are effective alternative to conventional pesticides. A study was conducted on the biopesticide chemicals extracted from powdered. P. amboinicus leaves using different solvents and their effectiveness against cowpea beetles, Callosobruchus maculatus. In this study, leaf powders of P. amboinicus were extracted with various solvents and tested qualitatively and quantitatively for phytochemical constituents using GC-MS. The highest concentrations of terpenoids, fatty acids, phenolic compounds, and steroids were found in petroleum benzine and ethanolic extracts. This insecticidal effect is attributed in part to the presence of secondary metabolites identified in the extracts. These petroleum benzine extracts of P. amboinicus also contained biopesticides such as thymol, beta caryophyllene, farnesol, phytol, Codlelure, (Z)-11-Hexadecenal, Erucic acid, and squalene. A higher repellency (44.8±0.8%) was achieved at a higher concentration (50 mg/ml) of petroleum benzene extract of P. amboinicus after 360 minutes of treatment followed by ethanol extract (40.8±1.49%) and, benzene (37.6±0.97%), chloroform (36.8±0.8%) and water extract (34.4±0.97%). We measured the levels of toxicity after 24 hours, 48 hours, and 72 hours of exposure. After 72 hours, Petroleum benzene caused the highest level of toxicity (LD50 = 20.89 mg/ml) of Callosobruchus maculatus, followed by ethanol (LD50=28.18 mg/ml), water (LD50 = 32.35 mg/ml), chloroform (LD50 = 40.73mg/ml and Benzene (LD50 = 46.77mg/ml). Based on our results, we suggest P. amboinicus phytocompounds are helpful in protecting stored grains from Callosobruchus maculatus.