Fungal-bacterial interactions leading to increased insect mortality: Trichoderma xixiacum enhances the pathogenicity of Serratia marcescens in Protaetia brevitarsis larvae

Edible insect farming presents significant potential as a sustainable solution to meet the increasing demand for protein and biowaste recycling. However, microbial contamination in rearing substrates poses a serious threat to insect populations. This study investigates the pathogenicity of Serratia marcescens YSMM-S1, isolated from dead Protaetia brevitarsis larvae, and explores its interaction with Trichoderma xixiacum, a fungus co-isolated from the same rearing environment. The identified S. marcescens YSMM-S1 was confirmed as a highly virulent strain, causing 100 % larval mortality within 24 h. In addition, T. xixiacum identified via extensive molecular and morphological characterization, is reported here for the first time in Korea. Co-cultivation and mycophagy assays revealed a significant increase in S. marcescens proliferation in the presence of T. xixiacum, likely through fungal mycelium consumption. Moreover, preinoculation with T. xixiacum in oak-fermented sawdust significantly enhanced S. marcescens population growth and colonization. In vivo assays further demonstrated that co-inoculation of the substrate with both microorganisms resulted in significantly higher larval mortality compared to single inoculations. This study provides novel insights into the complex microbial dynamics in insect-rearing systems, emphasizing the role of T. xixiacum in facilitating bacterial pathogenicity. The findings highlight the need for careful microbial management to prevent outbreaks in mass-rearing environments. This work also contributes to the understanding of fungal-bacterial interactions in insect farming and offers new perspectives on the ecological roles of Trichoderma species in rearing substrates.