Binary Mixtures of Essential Oils: Potent Housefly Adulticides That Are Safe Against Non-Target Species

In this study, we investigated the insecticidal potential of <i>Eucalyptus globulus</i> Labill. and <i>Cymbopogon citratus</i> Stapf essential oils (EOs), both alone and in synergistic blends with their primary active compounds, against adult houseflies (<i>Musca domestica</i> L.). Toxicity assessments were also conducted on non-target organisms-dwarf honeybees (<i>Apis florea</i> Fabricius) and guppies (<i>Poecilia reticulata</i> Peters)-to evaluate environmental safety. All binary EO mixtures demonstrated superior efficacy compared to individual EOs and the synthetic pyrethroid <i>α</i>-cypermethrin (1% positive control). The most potent formulation, combining 2.5% (<i>v</i>/<i>v</i>) geranial with 2.5% (<i>v</i>/<i>v</i>) <i>E. globulus</i> EO, exhibited a synergistic effect, achieving complete fly mortality (LT₅₀: 0.06 h). This mixture's mortality index significantly exceeded those of single-component formulations, with a mortality index of 0.22, confirming greater toxicity to flies than <i>α</i>-cypermethrin. Importantly, all the tested EOs and their blends were non-toxic to honeybees and guppies; in comparison, <i>α</i>-cypermethrin caused significant harm. These findings highlight the 2.5% (<i>v</i>/<i>v</i>) geranial + 2.5% (<i>v</i>/<i>v</i>) <i>E. globulus</i> EO blend as a highly effective and environmentally friendly alternative to conventional insecticides. Further research is recommended to optimize its formulation for practical use in sustainable fly management.