Physicochemical characterization and optimization of Tulsi-mediated silver nanocomposites with enhanced insecticidal activity against Spilosoma obliqua.

Spilosoma obliqua Walker (Lepidoptera: Erebidae) is one of the major pests of jute plants, severely reducing fibre yield and consequently lowering agricultural productivity. To address this problem through an eco-friendly approach, the green synthesis of a series of nano-biopesticide formulations, named E, P, Q, R, S, and T, based on nano-bio-composites obtained from varying proportions of Ocimum sanctum and in situ-formed silver nanoparticles, is proposed. Furthermore, their larvicidal efficacies are compared. These green-synthesised nano-bio-composites were characterised using FTIR and Raman spectroscopy, XPS, XRD, FE-SEM, EDX, HR-TEM, SAED, DLS, zeta potential analysis, cyclic voltammetry, GC-MS, and UV-vis spectroscopy. The study elucidated a variation in the extent of chemical interactions between silver nanoparticles and the bioactive constituents of the leaf extract, which notably influenced the physicochemical properties and could be correlated with their antilarval actions. The relative contributions to larval mortality in the jute hairy caterpillar based on LC50 values calculated after 24, 48 and 72 hours of exposure revealed the following toxicity order: R > Q > P > S > T > E. The findings further indicated that an optimal ratio of Ag(0) and Ag(i) species in sample R, together with associated bioactive molecules anchored to the nanoparticle surface, induced a synergistic effect that significantly enhanced the nano-biopesticide's toxicity against the targeted larvae. Overall, this study highlighted a cost-effective, greener, benign, and sustainable strategy for effective jute pest management.