Abstract
Plant essential oils offer eco-friendly alternatives to insecticides, though their instability limits effectiveness. This study evaluated the physicochemical stability and fumigant efficacy of geranium (Pelargonium graveolens) oil nanoemulsion (GONE) versus bulk geranium oil (GOB) against Callosobruchus maculatus. Geranium oil nanoemulsions (GONEs) were prepared via spontaneous emulsification using 8% oil and varying surfactant levels. The 10% surfactant formulation produced the most uniform and stable nanoemulsion, with an average droplet size of 91.85 ± 0.02 nm and a low polydispersity index of 0.16 ± 0.02. No significant changes in droplet size were observed after 30 days of storage at room temperature and 9 °C, confirming the formulation's stability. A fumigant bioassay was conducted using five concentrations (50, 100, 150, 200, and 250 µL/L air) of GOB and GONE over 24, 48, 72, and 96 h. Both forms exhibited concentration- and time-dependent toxicity against C. maculatus. Complete mortality was achieved sooner and at lower doses with GONE (72 h at 150 µL/L air versus 250 µL/L air for GOB; 96 h at 150 µL/L air for GONE versus 200 µL/L air for GOB). Geranium oil nanoemulsion consistently produced lower LC(50) and LC(90) values, indicating greater potency. It also significantly reduced progeny development. Residual fumigant bioassays at the LC(90) level showed that GONE retained efficacy against C. maculatus adults longer than GOB, causing 50% mortality 12 days post-treatment compared to 21% for GOB. Overall, nanoformulation enhanced the potency and persistence of geranium oil, highlighting its promise for protecting stored grains from C. maculatus.