Comparative insecticidal efficacy and biochemical impact of nano-encapsulated citronella and geranium essential oils against Spodoptera littoralis (Lepidoptera: Noctuidae)

Abstract This study assessed the insecticidal efficacy of bulk and nano-formulated essential oils of citronella ( Cymbopogon nardus ) and geranium ( Pelargonium graveolens ) against the cotton leafworm Spodoptera littoralis (Boisd.) under controlled laboratory conditions. Nano-formulations were prepared using polyethylene glycol (PEG) via oil-in-water emulsification, achieving high encapsulation efficiencies (EE%) of 96.16% for citronella and 94.16% for geranium oils, with corresponding loading capacities (LC%) of 77.43% and 76.35%, respectively. Transmission Electron Microscopy (TEM) confirmed spherical nanoparticles with diameters ranging from 20 to 60 nm. Bioassay results revealed that nano-citronella exhibited the highest insecticidal activity, with the lowest LC₅₀ value (0.4111 × 10 4 ppm) and a toxicity index of 100, indicating approximately tenfold greater potency than its bulk counterpart. Nano-geranium showed a moderate enhancement in toxicity (LC₅₀ = 3.95 × 10 4 ppm) compared to the bulk oil. Biochemical analyses demonstrated pronounced disruptions in enzymatic activity: chitinase activity ranged from 4.43 ± 0.94 µg NAGA/min in bulk citronella treatments to 23.70 ± 2.90 µg NAGA/min in nano-citronella. Invertase activity increased markedly, reaching 175.34 ± 3.47 µg glucose/min in nano-citronella treatments, while total protein content varied from 750.77 ± 3.85 µg/mg tissue in nano-geranium to 1315.80 ± 1.27 µg/mg tissue in bulk citronella treatments. Developmental parameters were significantly affected with nano-citronella LC₅₀ treatments, causing prolonged larval duration (19.6 ± 0.71 days) and reduced pupal weight (178.7 ± 3.18 mg) compared with control groups. Molecular docking and molecular dynamics simulations identified geraniol as a potent natural inhibitor of bacterial chitinase (PDB ID: 1CTN), forming stable π–alkyl and van der Waals interactions with Trp252, Phe373, and Ser341, and exhibiting a strong binding free energy (–20.08 kcal/mol). The geraniol–chitinase complex displayed enhanced conformational stability (RMSD = 1.15 ± 0.15 Å) and compactness (Rg = 26.9 ± 0.09 Å) relative to the unbound enzyme. Overall, these findings demonstrate that nano-encapsulation substantially enhances the insecticidal efficacy, biochemical disruption, and molecular stability of essential oils, highlighting their potential as eco-friendly, high-efficiency biopesticides within integrated pest management (IPM) programs.