In recent years, the larval stage of Hermetia illucens, commonly known as the black soldier fly (BSFL), has been used to promote the circularity of the agri-food sector by bioconverting organic waste into larval biomass which has been used as a livestock feed. A secondary byproduct of this process is frass that can be used as an organic fertilizer. This study compared two different plant-based diets on frass characteristics as well as larval performance, nutritional composition, and waste reduction efficiency. A fruit/vegetable/bakery waste-based diet supplemented with brewery waste (FVBB) was compared to a control Gainesville (GV) reference diet and fed to BSFL under standard conditions. The results demonstrated that NPK and some of the macro and micronutrients in both frasses are comparable to commercially available organic fertilizers. It was shown that microorganisms present in frass from the two diets inhibit the mycelial growth of several plant pathogens through the production of antifungal and/or anti-oomycetes compound(s) (antibiosis). This diet also had a positive effect on individual larval mass (162.11 mg), bioconversion rate (13.32%), and larval crude lipid (35.99% of dry matter) content. The BSFL reared on this diet reduced feedstock dry matter by 67.76% in a very short time (10 days), which is a promising solution for food waste management.

Frass, the residual material resulting from the bioconversion of organic matter by black soldier fly larvae (BSFL), has gained attention as a sustainable alternative to conventional fertilizers due to its high nutrient content. Additionally, frass has been found to possess antifungal properties, which can help control plant pathogens affecting horticultural crops. In this study, frass from BSFL reared on the Gainesville diet, a universally employed reference/control diet, was investigated in vitro for its effect on the growth of seven important fungal/oomycete pathogens. Dual culture overlay assays clearly showed that fresh Gainesville diet extract, as well as BSFL frass extract derived from this diet, contained microorganisms producing compound(s) that strongly inhibit(s) the mycelial growth of fungal/oomycete plant pathogens. Fungi and bacteria were then isolated from the fresh Gainesville diet and BSFL frass using the serial dilution technique. Among the different fungi/bacteria isolated, the isolate GV1-11 in Gainesville diet and FGV15-6 in frass demonstrated strong antifungal/anti-oomycete activity. Both isolates were genetically identified by whole-genome sequencing as Bacillus velezensis, a bacterium used as a biocontrol agent, strongly suggesting that B. velezensis, which is present in the Gainesville diet, can survive the process of BSFL rearing and is one of the key factors contributing to the observed antifungal and anti-oomycete activity in the resulting frass. This work underlines the importance of the inherent microbial characteristics of feedstocks on the antifungal/anti-oomycete activity of frass and points out the possibility of exploiting frass to control plant pathogens affecting horticultural crops.

This study investigated the effect of black soldier fly larvae (BSFL) frass derived from BSFL reared on a diet composed of fruit/vegetable/bakery/brewery residues (FVBB diet) and on the Gainesville diet (GV diet) on the development of tomato (Solanum lycopersicum) Fusarium wilt caused by Fusarium oxysporum f. sp. lycopersici (FOL). Tomato plants were grown in a substrate inoculated with FOL that was amended (10%, v:v) or not (control) with either a commercial compost, pasteurized (70 °C for 1 h) frass from BSFL reared on a FVBB diet, non-pasteurized frass from BSFL reared on a FVBB diet, pasteurized frass from BSFL reared on the GV diet, or non-pasteurized frass from BSFL reared on the GV diet. The results show that frass from BSFL reared on the GV diet, irrespective of pasteurization, inhibited FOL root colonization and reduced the severity of tomato Fusarium wilt to a far greater extent than frass from BSFL reared on a FVBB diet and commercial compost made of peat, seaweed, and shrimps. This study suggests that BSFL frass, depending on the larval rearing diet, has the potential to serve as a pasteurized or non-pasteurized soil amendment with prophylactic properties against FOL in tomato plants, opening new avenues of research for the valorization of BSFL frass.