Characterization of Ocimum basilicum synthesized silver nanoparticles and its relative toxicity to some insecticides against tobacco cutworm, Spodoptera litura Feb. (Lepidoptera; Noctuidae).

Tobacco cutworm, Spodoptera litura Feb. (Lepidoptera; Noctuidae) is a notorious cosmopolitan pest of numerous crops. Frequent evolution of insecticide resistance and polyphagous nature favors the survival of this voracious pest. Nanotechnology offers an alternative technique to overcome the limitations of existing pest management strategies. In the present study, the silver nanoparticles were synthesized from Ocimum basilicum leaf extract and UV-spectra at 464.17 nm confirmed their synthesis while crystalline nature of nanoparticles was evaluated by X-rays diffraction (XRD) pattern and scanning electron microscopy (SEM) revealed their octagonal to spherical shape. Insecticidal potential of O. basilicum synthesized silver nanoparticles (ObAgNPs) was evaluated in comparison with some synthetic insecticides owning different mode of actions, including Coragen®, Proclaim®, Tracer® and Talstar® against S. litura. Probit analysis of 2nd, 3rd, 4th and 5th instars of S. litura showed that lethal concentrations (LCs) of tested nanoparticles and pesticides were positively correlated with older instars. All values of LC50, LC90 and LC95 were statistically different from each other with maximum on the 4th instar and minimum on 2nd instar of S. litura. Tukey's HSD test revealed that ObAgNPs caused maximum mortality (21.67-96.67%) in 2nd instar larvae at the rate of 100-1500 mg/L followed by Coragen® (18.33-91.67%), Proclaim® (13.33-78.33%) and Talstar® (13.33-68.33%), while Tracer indicated the lowest larval mortality (11.67-66.67%) at the rate of 60-120 mg/L and a similar trend was assessed for all the successive instars of S. litura. Higher activity of carboxyl esterase and glutathione-s-transferases confirmed the development of insecticide resistance against the synthetic chemicals. Conclusively, ObAgNPs were found to be eco-friendly but the most effective biogenic compounds for the suppression of S. litura population as compared to the selected synthetic chemicals which may be hazardous for the environment as well as human health. This study will assist the pesticide industries to re-standardize the dose rates against Spodoptera litura.