Abstract
Currently, many plant extracts have been employed to green synthesize metal nanoparticles toxic to insect pests and vectors. However, scarce efforts focused on the main molecules responsible of reduction and stabilization processes. In this study, we assessed the larvicidal activity of hexane, dichloromethane (DCM), acetone and aqueous extracts of Catharanthus roseus on the insect pest Earias vittella. Among them, DCM extract exhibited maximum larvicidal activity against E. vittella. Therefore, it was fractionated using a silica gel column. Based on the TLC profile, 7 fractions were isolated. Fraction 5 showed the highest larvicidal activity (81.33%) when tested at 1000 ppm. Fraction 5 showed a single spot on TLC and was subjected to UV-Vis spectroscopy, (NMR)-N-H1, FTIR and mass spectroscopy. Based on the spectral data, we isolated stearic acid, which was further used as a reducing agent to synthesize Ag nanoparticles. The formation of Ag nanoparticles was recognized by the presence of broad peak at 416 nm corresponding to the surface plasmon resonance of Ag nanocomposites in UV-Vis spectroscopy. Ag nanoparticles were spherical in shape with size ranging from 35 to 55 nm. The nanoparticles showed significant antifeedant (87.13%) and larvicidal (93.77%) activities against E. vittella when tested at 200 ppm and exhibited LC50 values of 45.46 ppm in antifeedant experiments and 25.12 ppm in larvicidal assays on E. vittella. The nanoparticles showed acute toxicity against mosquitoes Culex quinquefasciatus and Aedes aegypti with LC50 values lower than 40 ppm on both vectors, showing their promising potential to develop newer and effective biopesticides to control arthropod pests.