Towards Eco-Friendly Pest Control: Synergistic Potential of Entomopathogenic Fungi and Plant Essential Oils Against Galleria mellonella and Tenebrio molitor

Entomopathogenic fungi such as Beauveria bassiana and Metarhizium brunneum are important biological control agents, but their relatively slow action can limit efficiency. Essential oils, rich in bioactive compounds, represent potential complementary tools due to their insecticidal and repellent properties. The present study evaluated the individual and combined effects of B. bassiana (BMAUM-M6-4) and M. brunneum (V275) with thyme, rosemary, and eucalyptus essential oils against Galleria mellonella and Tenebrio molitor larvae. Essential oils were applied individually, simultaneously with fungal suspensions, or sequentially with a 24-hour interval to assess interactions. In addition, in vitro assays were conducted to determine the potential inhibitory effects of essential oils on fungal mycelial growth and conidial germination.The results revealed that thyme oil produced the strongest synergistic effect with both fungal isolates. Simultaneous application of thyme oil with B. bassiana or M. brunneum significantly increased larval mortality compared to single treatments. Rosemary oil showed moderate compatibility, although pre-exposure of larvae to rosemary reduced fungal efficacy. Eucalyptus oil contributed little to mortality under laboratory conditions. Importantly, in vitro assays showed that essential oils at 1% concentration did not significantly inhibit fungal spore germination, and only minor reductions in mycelial growth were observed, particularly with eucalyptus oil showing the least effect.Overall, combining entomopathogenic fungi with selected essential oils, particularly thyme and rosemary, enhanced pest mortality and demonstrated potential as an environmentally friendly alternative within integrated pest management strategies. Nevertheless, the effectiveness of these treatments depends on application sequence and host species, underscoring the need for optimization and validation under semi-field and field conditions.