Activity at the cuticle: Yeasts antagonise infection of the entomopathogenic fungus Metarhizium anisopliae in Queensland fruit fly larvae

• Cuticular yeasts defend against Metarhizium anisopliae infection. • Cuticular yeasts can form symbiotic associations with Bactrocera tryoni . • Larval development requires yeast-derived nutrition. Entomopathogenic fungi in the genus Metarhizium have been shown to cause high mortality of Queensland fruit fly larvae, Bactrocera tryoni . In natural environments, however, these fungi interact with biotic and abiotic factors that may limit their biological control potential. Fungal microbes, particularly yeasts, are known to form mutualistic associations with insects, supporting development and potentially defending against pathogen infections when present on the insect cuticle. This study evaluated the antagonistic potential of yeasts isolated from the cuticle and gut of B. tryoni larvae against an isolate of Metarhizium anisopliae . Seven yeast isolates were evaluated through dual-culture assays and insect bioassays, which examined the impact of yeast isolates on the virulence of Metarhizium in pupating larvae. Larval development experiments were additionally performed to assess beneficial effects of yeasts on insect fitness. Yeast isolates demonstrated their capacity to impact the growth, spore production, and germination of M. anisopliae . The yeast genera Saprochaete, Hanseniaspora, and Pichia, when applied to the insect’s cuticle, reduced the virulence of Metarhizium . The results confirm that larval development depends on the presence of yeasts and that yeast-mediated differences in fitness can influence susceptibility to entomopathogenic fungal infection. Furthermore, yeast application to the larval cuticle reduced insect mortality, suggesting a protective barrier effect. These findings imply that the antagonistic properties of insect-associated yeasts may contribute to reduced efficacy of M. anisopliae in non-sterile soil. A better understanding of yeast activity at the cuticle may inform the development of more effective fungal entomopathogen-based mycoinsecticides.