Geranium Oil Nanoemulsion Delivers More Potent and Persistent Fumigant Control of Callosobruchus maculatus in Stored Grain

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 (<i>Pelargonium graveolens</i>) oil nanoemulsion (GONE) versus bulk geranium oil (GOB) against <i>Callosobruchus maculatus</i>. 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 <i>C. maculatus</i>. 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 LC₅₀ and LC₉₀ values, indicating greater potency. It also significantly reduced progeny development. Residual fumigant bioassays at the LC₉₀ level showed that GONE retained efficacy against <i>C. maculatus</i> 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 <i>C. maculatus</i>.