Effects of black soldier fly larval frass addition on the winter wheat ecosystem: a mesocosm experiment

Black Soldier Fly (Hermetia illucens) larval (BSFL) frass demonstrated a pronounced inhibitory effect in vitro on isolates of the soil-borne phytopathogenic fungus Fusarium oxysporum, the causal agent of Fusarium wilt. No studies have examined the influence of BSFL frass on Fusarium wilt in crops. To address this gap, we conducted a mesocosm experiment to assess the efficacy of BSFL frass (200 g m⁻²) added during two different stages of winter wheat seedling growth (simultaneously with sowing or at the tillering stage) on four parameters (study outcomes) that describe the development of Fusarium wilt: plant dry mass, Fusarium wilt incidence, number of Fusarium spp. gene copies in the soil, and biomass of Fusarium spp. in the soil. Soil chemical properties and abundance of key soil invertebrates (Nematoda, Acari and Collembola) were measured to describe mechanisms. Fusarium spp. biomass increased 132 % in the later BSFL frass addition treatment compared to the control (soil without BSFL frass amendment). Fusarium wilt incidence decreased non-significantly in both early (3 %) and later (8 %) addition treatments. Plant dry mass increased 27 % in the early addition treatment, while soil phosphorus concentration increased 21 % in the later addition treatment. Bacterial feeding and predatory nematodes increased 106 % and 65 %, respectively, in the later addition treatment. Mycophagous Acari increased 106 % in the early addition treatment, while predatory Acari showed five-fold growth in later addition treatments. The main mechanism of BSFL frass influence appears to be promoting saprotrophic mycobiota and increasing soluble phosphorus availability for plants.