Silver Nanoparticles Biosynthesized from Metarhizium anisopliae Crude Extract: Insecticidal Activity, Residual Efficacy, and Metabolic Responses in Solenopsis invicta

Green biosynthesis of silver nanoparticles using natural biological extracts represents a promising route to develop eco-friendly nanopesticides; nevertheless, many biogenic nanoformulations suffer from poor photostability and rapid loss of bioactivity under field sunlight exposure. In this study, silver nanoparticles (Des-AgNPs) were one-pot biosynthesized using crude fermentation extract from the entomopathogenic fungus Metarhizium anisopliae (Metschn.) Sorokīn, which served as both a reducing agent and a surface-capping ligand. Solenopsis invicta Buren was employed as a model pest to evaluate the physicochemical features, insecticidal potency, outdoor residual stability, and underlying metabolic toxic mechanism of the resulting nanomaterial. The synthesized Des-AgNPs exhibited high crystallinity, favorable dispersibility, and excellent colloidal stability. Laboratory bioassays demonstrated potent insecticidal activity against S. invicta workers, with a 5-day LC50 of 174.74 mg/kg. Notably, outdoor exposure assays revealed that Des-AgNPs significantly outperformed the unformulated fungal crude extract in residual efficacy: Des-AgNPs retained high insecticidal activity after 96 h of environmental exposure, whereas the free crude extract underwent a pronounced decline in toxicity within 24 h. Untargeted LC-MS-based metabolomics was further applied to decipher the multi-target toxic responses triggered by Des-AgNPs. In total, 540 differential metabolites (321 upregulated, 183 downregulated) were identified. KEGG pathway enrichment revealed significant disruptions in glutathione metabolism, cytochrome P450-mediated xenobiotic detoxification, tyrosine/dopamine neurotransmitter metabolism, and ABC-transporter-driven efflux pathways. These metabolic perturbations reflected oxidative stress, impaired energy metabolism, compromised cell membrane integrity, disrupted neural transmission, and biomacromolecule damage in the model pest. Collectively, our results demonstrate that entomopathogenic-fungal-extract-capped silver nanoparticles can overcome the photodegradation drawback of free fungal bioactive metabolites. This mycosynthetic strategy provides a feasible framework for constructing stable, high-performance nanobiopesticides, and the workflow could be extended to other fungal extracts and target insect pests beyond the present model system.