Efficacy of Entomopathogenic Fungi for Sustainable Biocontrol of Fungus Gnat ( Bradysia difformis ) in Peat-Free Substrates: A Laboratory Study

Bradysia difformis Frey (Diptera: Sciaridae) is a fungus gnat that poses a significant threat to greenhouse cultures, and is attracted to soils devoid of peat. Fungal strains from the German Collection of Microorganisms and Cell Culture (DSMZ), such as Beauveria bassiana, Metarhizium flavoviride , Mucor hiemalis, and Niesslia tinuis, as well as Serendipita indica , were screened for entomopathogenic activity against B. difformis and their capacity to colonize Petunia hybrida cv . “Mitchell” and Ocimum basilicum plants. The survival rates of Bradysia difformis (three instar larvae) treated with Metarhizium flavoviride were 45.33% at 14 days following inoculation with 1 × 10 6 spores/mL of each fungal strain, when compared to others. We concluded that the fungal strain M. flavoviride could serve as an entomopathogenic fungus with the highest virulence against B . difformis larvae. Although M. flavoviride did not show a beneficial effect as an endophyte, interestingly, the strain Niesslia tinuis exhibited plant growth benefits in Petunia hybrida cv . “Mitchell” by enhancing its shoot length up to 13.18 ± 0.72 cm, whereas the control treatment had a shoot length up to 10.68 ± 0.39. Enzymatic assays confirmed the ability of M. flavoviride to produce cuticle-degrading enzymes such as chitinase and protease. Together, these findings highlight the potential of EMPF—particularly M. flavoviride —as a sustainable biocontrol tool well-suited for peat-free horticultural systems, offering an eco-friendly alternative to chemical insecticides where fungus gnat pressure is typically high.