The metabolic interactions between the black soldier fly and fungi

Bacteria are known to colonize the insect gut and determine a positive effect on their host’s fitness by providing essential nutrients or improving digestion efficiency. However, information on the nutritional contribution to their insect hosts is still scarce and fragmentary. Our research model is the black soldier fly (Hermetia illucens, BSF), a cosmopolitan species that thrives in rotten organic matter. Therefore, it is hypothesized to interact with the microorganism community in its surroundings. The most common fungi in the BSF environment (household compost bins) was found to be Candida tropicalis, which when supplemented to the larvae feeding substrate, increased the larval weight. This research further investigated whether the metabolic effect of the Candida on the BSF is directly, through consumption, or indirectly, through metabolic changes in the substrate. The larvae that were supplemented with C. tropicalis displayed an increase in the fatty acid biosynthesis pathway, due to an increase in the palmitic and myristic acids that are abundant in the fungi. In addition, the presence of C. tropicalis in the substrate caused an increase in threonine, leucine, and isoleucine in the larvae and may affect larval weight through the Target of Rapamycin (TOR) pathway. This study suggests two mechanisms underlying the nutritional effect of fungi on BSF larvae. The first involves direct digestion of fungi or nutrients that are extracted by fungi in the insect gut, while the other implies an indirect fungal effect due to the digestion of the feeding substrate and extraction of nutrients in the substrate.