Environmental Frass Remodels the Larval Gut Microbiome to Enable Black Soldier Fly (Hermetia illucens) Bioconversion of Livestock Manure

Black Soldier Fly larvae (BSFL) consume nearly any organic material and show promise for the remediation of agricultural waste, including livestock manure. BSFL feeding generates frass, a mixture of unconsumed substrate, larval feces, and microorganisms. We identified a wild population of BSFL (Yuba strain) thriving on rabbit manure, a challenging substrate due to its high lignocellulose content. We investigated if frass extract from this source was necessary and sufficient to support bioconversion of manure and if such effects are altered by captive husbandry. Larvae supplemented with domestic frass failed to grow on rabbit manure but grew well when supplemented with wild-sourced frass. Microbiome analysis revealed that wild-sourced frass contributes a cohort of lignocellulose-degrading microorganisms to the BSFL gut, particularly Rummeliibacillus bacteria and Scedosporium fungi. These growth-promoting effects extended to cattle manure. Following initial seeding with wild-sourced frass, lab-cultured frass ultimately failed to support growth on rabbit manure and the gut microbiome shifted toward bacteria commonly found in domestic BSFL. This study reveals that frass extract derived from environmental sources can improve the bioconversion ability of BSFL and that beneficial microbes are lost during captive-rearing.