Repellence and acute toxicity of a nano-emulsion of sweet orange essential oil toward two major stored grain insect pests

Control strategies in stored-product facilities mainly rely on synthetic pesticides. The development of new environmentally friendly alternatives, such as essential oils (EOs), is a key issue. The aim of this research was to develop a stable nano-emulsion containing a high amount of sweet orange (Citrus sinensis) EO and to evaluate its insecticidal activity against Tribolium confusum du Val (Coloptera: Tenebrionidae) and Cryptolestes ferrugineus (Stephens) (Coleoptera: Cucujidae). The experimental nano-emulsion showed a low surface charge (zeta slightly higher than -30 mV) and a droplet size within the sub-micrometre range (131.37 nm +/- 0.29), maintained also after 1 year of storage. The EO nano-emulsion presented good repellence toward both adult insects, which lasted until 24 h of exposure at the highest dosages. Furthermore, the developed nano-formulation showed acute toxicity against both insects when tested as fumigant and cold aerosol. In fumigation trials, adults of C. ferrugineus were more susceptible than T. confusum adults. Moreover, the insecticidal activity of cold aerosol was 5 and 7-fold higher than fumigation against both C. ferrugineus and T. confusum, respectively. The EO-nano-emulsion was effective in controlling and repelling the target pests. Cold aerosol treatments with EO nano-formulations is a promising alternative method for the sanitation of production areas, warehouses, and machineries.