Hermetia illucens breeding by-products in plant protection: chitosan and frass effects on fungal phytopathogens

The transition toward sustainable agriculture necessitates reducing reliance on synthetic fertilizers and pesticides, driving the search for eco-friendly alternatives. This doctoral research investigates the valorization of Hermetia illucens (Black Soldier Fly – BSF) breeding by-products – i.e. pupal exuviae and frass – as a source for innovative tools for plant growth promotion and protection against fungal pathogens within a circular economy framework. The first phase of the current thesis focuses on the extraction and characterization of chitosan from BSF pupal exuviae (PE). Two formulations, decolorized (PEDEC) and non-decolorized (PEND), were evaluated against commercial crustacean chitosan for the control of Botrytis cinerea on strawberries. In vitro assays revealed that PEND chitosan exhibited superior direct fungitoxic activity, inhibiting mycelial growth significantly more than its decolored (PEDEC) counterpart. In vivo experiments confirmed that edible coatings based on these chitosans extended the shelf life of strawberries. Notably, the mechanisms of action differed: the non-decolorized form (PEND), likely containing bioactive chitosan-melanin complexes, acted directly against the fungus, whereas the decolorized form (PEDEC) primarily elicited host biochemical defenses, such as increased total phenolic and flavonoid content. The second phase explored the potential of BSF frass as a soil amendment and s seed priming agent. First, the dosage of a frass liquid extract was optimized and applied to durum wheat (Triticum durum) to assess its biostimulant effects and suppressiveness against Fusarium sporotrichioides. Seed priming with 10% frass extract, particularly when combined with the well-known biocontrol agent Trichoderma afroharzianum T22, significantly reduced damping-off disease incidence (up to 78% reduction) and enhanced seedling growth. This protective effect was correlated with a rapid modulation of the plant antioxidant system, specifically superoxide dismutase (SOD) activity. In addition, microbiological analyses of the frass extract led to the isolation of Paenibacillus polymyxa, a bacterium demonstrating strong in vitro antagonism against Fusarium spp., suggesting that the efficacy of frass is largely microbiome-mediated. Further investigations on tomato (Solanum lycopersicum) demonstrated that soil amendment with frass effectively reduced the severity of Fusarium wilt (F. oxysporum f.sp. lycopersici). This suppression was linked to the elicitation of enzymatic and non-enzymatic antioxidant responses in the plant. Finally, experiments on the model plant Arabidopsis thaliana provided deeper insights into the frass microbiome. Several plant growth-promoting rhizobacteria (PGPR) were isolated, including Bacillus velezensis, B. licheniformis, and Paenibacillus sp.. These isolates- and particularly B. velezensis- not only promoted root development but also demonstrated significant antagonism against foliar pathogens such as B. cinerea and Alternaria brassicicola. In conclusion, this research demonstrates that H. illucens by-products are not merely waste but valuable bio-resources. Both insect-derived chitosan and frass proved effective in controlling economically relevant phytopathogens and promoting plant health. These findings support the scaling-up and integration of BSF by-products into Integrated Pest Management (IPM), offering a sustainable, cost-effective solution for modern agriculture.