Plant biostimulants in Hermetia illucens frass: insights into fresh and recirculated frass

Abstract This is the first and extensive study investigating the bioactive composition of black soldier fly larvae (BSFL) frass through liquid chromatography tandem mass spectrometry (LC-MS/MS), with specific focus on phytohormones and their links to frass microbial composition. Two types of frass were evaluated: fresh frass (Fr-Cycle 1), obtained after one larval bioconversion cycle of food waste; and recirculated frass (Fr-Cycle 2), produced by reintroducing fresh frass into a subsequent bioconversion cycle. Frass recirculation did not affect overall nutrient composition but resulted in marked reductions in humic (80%) and fulvic (65%) acid concentrations, likely due to further decomposition of organic matter during recirculation. In parallel, recirculation significantly altered the bacterial community, decreasing the relative abundance of Actinobacteria while increasing Proteobacteria. Saliently, this is the first mass spectrometric evidence for the presence of cytokinins (both cZ and tZ, as well as their precursors) in BSFL frass, but several other phytohormones were identified in frass at nanomolar levels, including auxins (IAA, PAA, IAM, among others), abscisic acid, salicylates, indoleamines and an ethylene precursor. Interestingly, specific microbial genera were found to be positively correlated with phytohormones, including Proteus , Providencia and Morganella . The potential connections between microorganisms, phytohormones and physico-chemical characteristics of frass might help explain the results reported in the current literature exploring frass as a biofertilizer or soil amendment. The results presented herein open possibilities for approaching BSFL frass research from a new perspective, paving the way for future studies aiming at a more mechanistic understanding of its interactions with soil, plants and microorganisms.