1 Biology Department, Concordia University, 7141 Sherbrooke St. W., Montreal, QC, H4B 1R6, Canada. Electronic address: carlos.lopezmanzano@mail.concordia.ca.
2 Biology Department, Concordia University, 7141 Sherbrooke St. W., Montreal, QC, H4B 1R6, Canada.
3 Biology Department, Concordia University, 7141 Sherbrooke St. W., Montreal, QC, H4B 1R6, Canada. Electronic address: rassim.khelifa@concordia.ca.

Urban environments accumulate large volumes of fecal waste from superabundant wildlife, yet few circular strategies exist to convert this biomass into useful products. We assessed the potential of black soldier fly (BSF) larvae to bioconvert feces from the Canada goose, a common bird that creates significant fecal pollution in parks and green spaces. Field surveys across 11 urban sites in Southern Quebec and Ontario showed a significant positive correlation between flock size and number of droppings, indicating that larger groups generate high fecal deposition. In laboratory trials, BSF larvae successfully survived and completed development on goose feces. In raw (non-autoclaved) treatments, larvae consumed 56% of fecal dry matter in the 100% feces-only diet (F100) and 64% in the control diet, while waste reduction index values were highest in the 50% mixture of feces-Gainesville diet. Autoclaving reduced consumption across feces-based diets and reduced larval growth in F100. Survival remained high in control (96.4%) and mixed diets (95.4%) but declined in F100 (80.6%). Larvae fed on F100 exhibited slower development, lower adult body mass, and shorter adult lifespan. We further tested whether goose-derived frass could fertilize duckweed. Frass supported sustained and enhanced plant growth, increasing yields by 32% at 10 g L^-1 relative to Hoagland medium while outperforming both fresh feces and nutrient-rich controls. Root length was reduced in frass treatments (up to 83% shorter) consistent with plant responses in high-nutrient environments. Overall, BSF larvae converted goose feces into insect biomass and a nutrient-rich fertilizer, providing a practical circular approach for managing wildlife-derived fecal waste.