Bioactive molecules derived from insects and their applications in agriculture

The black soldier fly (BSF; Hermetia illucens, Diptera: Stratiomyidae) is among the most promising species for the bioconversion of organic waste. A key co-product of BSF farming is larval frass, a mixture of larval excreta, exuviae, and residual substrate used as an organic fertilizer that delivers organic matter, micro- and macro- nutrient and potentially plant growth-promoting microorganisms (PGPM). Despite growing interest, a systematic framework linking larval diet and post-processing treatment to frass physico-chemical properties and crop responses remains limited. This thesis combines a critical synthesis of the literature on frass characteristics (macro-/micronutrients, pH, organic matter, electrical conductivity, moisture, dry matter) and PGPM occurrence with experimental investigations that evaluate agronomic benefits and constraints. Different experiments were performed in order to analyze all these aspects: 1) Pot trial to test how BSF larval frass influences establishment of barley (Hordeum vulgare L., cv. Laureate) at 21°C, including as variables larval diet (Gainesville vs whey–seed), frass thermal treatment (untreated vs 70 °C for 1 h), and dose (5, 12, 19, 26 g pot⁻¹). The measured parameters included shoot biometry, chlorophyll content, root morphology, root/shoot ratio, and water-use efficiency. Gainesville-derived frass more consistently improved early shoot and root growth, whereas high application rates depressed performance, underscoring strong dose dependence and the role of frass chemical-physical profiles. 2) PGPM were isolated and characterized from a panel of ten frass types (derived from BSF larvae fed on varying vegetable waste) using a rhizosphere-mimicking agar. A total of 149 isolates were screened for hallmark traits (e.g compatibility with humic acids, phosphate solubilization, ammonia production, indole-3-acetic acid and gibberellin synthesis). Six promising isolates (Serratia sp., Peribacillus sp., Acinetobacter sp., Pseudocitrobacter sp., Bacillus sp., Enterobacter sp.) were tested in vivo on Arabidopsis thaliana, showing variable effects on germination, root elongation, and root hair development, often aligned with phytohormone profiles; several bacteria persisted after heat treatment, indicating a thermotolerant PGPM fraction. 7 3) The impact of the 70 °C/1 h heat treatment was assessed on physical parameters (water activity, pH, electrical conductivity, dry matter) and on the microbiological profile (Total Viable Count, E. coli, Enterococcaceae, Enterobacteriaceae, Bacterial endospores, Salmonella), of the same frass used in experiment n°2, to determine how processing shifts these features and whether pathogen levels meet EU regulatory limits (Reg. EU 1925/2021). For untreated frass that exceeded the regulatory limits, heat treatment restored compliance. 4) Finally, phytotoxicity and agronomic dose response were evaluated using lettuce (Lactuca sativa) seed germination and seedling growth, identifying dose windows that maximize benefit while minimizing inhibitory effects. Overall, the work positions BSF frass as a tunable input whose efficacy depends on diet, processing, and application rate, providing an evidence base for optimized BSF-derived biofertilizers and practical, crop-specific guidelines.