Combinations of Lemongrass and Star Anise Essential Oils and Their Main Constituent: Synergistic Housefly Repellency and Safety against Non-Target Organisms.

Simple Summary: The housefly (Musca domestica) is a global vector of several pathogens and a nuisance to humans and animals. Repellents reduce the risk of housefly vector diseases. Green repellents from plant essential oils (EOs) and their active compounds were, at the time, a promising option against houseflies. This study evaluated the housefly-repellent activities of every tested EO, their main constituents, and several combinations of them. The combinations were lemongrass (Cymbopogon citratus) EO + trans-anethole, lemongrass EO + star anise (Illicium verum) EO, geranial + trans-anethole, and star anise EO + geranial. The efficacies of every formulation were compared against each other and DEET (a synthetic repellent). All combinations were more effective in repellency than all single-component formulations and DEET. More significantly, all of them were safe for four non-target species: guppy (Poecilia reticulata), molly (Poecilia latipinna), dwarf honeybee (Apis florea), and stingless bee (Tetragonula pagdeni). These combinations could be developed into valuable green repellents for housefly vector disease management. The present study evaluated the housefly repellency of single-component formulations and combinations of lemongrass and star anise essential oils (EOs) and their main constituents. The efficacies of the combinations were compared against those of single-component formulations and DEET. Safety bioassays of all formulations and DEET on non-target species—guppy, molly, dwarf honeybee, and stingless bee—were conducted. GC–MS analysis showed that the main constituent of lemongrass EO was geranial (46.83%) and that of star anise EO was trans-anethole (92.88%). All combinations were highly synergistic compared to single-component formulations, with an increased repellent value (IR) of 34.6 to 51.2%. The greatest synergistic effect was achieved by 1.0% lemongrass EO + 1.0% trans-anethole combination, with an IR of 51.2%. The strongest, 100% repellent rate at 6 h was achieved by 1.0% geranial + 1.0% trans-anethole. They were twice as effective as DEET and caused obvious damage to housefly antennae under microscopic observation. All single-component formulations and combinations were benign to the four tested non-target species. In contrast, DEET was highly toxic to them. The synergistic repellency and biosafety of these two combinations are compellingly strong support for developing them into an effective green repellent. [ABSTRACT FROM AUTHOR]