Spent Mushroom Substrate Feeds Black Soldier Flies to Produce Fertiliser and Animal Feed

Spent mushroom substrate (SMS), the leftover material from mushroom cultivation, is often discarded as waste. Yet, new research reveals its potential as a key ingredient for sustainable agriculture. A study published in PeerJ demonstrates how SMS can be repurposed to rear black soldier fly larvae (BSFL), serving subsequently as fertiliser and animal feed quality.

Image Credits: Catherine Terrell and Laura Ingwell

Reusing SMS Waste

Global mushroom consumption has reached 34 million tonnes annually, producing five kilograms of SMS waste for every kilogram of mushrooms harvested. Traditionally, this byproduct is discarded post harvest. However, SMS still contains valuable organic matter, unused nutrients, and enzymes, making it a prime candidate for circular economy solutions.

Image credits: Funj Shrooming Co.

Black Soldier Fly Larvae: Nature’s Waste Converters

Black soldier flies, Hermetia illucens, are exceptional at breaking down organic waste. Their larvae convert low-nutrient substrates into protein-rich biomass and nutrient-dense frass (excrement), which can be used as animal feed and biofertiliser, respectively. The study tested three types of SMS—Agrocybe cylindracea, Lentinus polychrous, and Pleurotus pulmonarius—mixed with vegetables at varying ratios (0%, 15%, and 30%) and subjected to fermentation.

Research Findings

1. Optimal Supplementation: At 15% SMS supplementation, BSFL growth performance matched that of the control (vegetable-only diet). Increasing SMS to 30% reduced larval, pre-pupal, and pupal weights, suggesting diminishing returns at higher concentrations.

2. Fermentation Impact: Self-fermenting SMS for two months did not significantly alter nutrient composition or larval growth but improved the nutritional quality of BSFL biomass and frass.

3. Nutrient Boost: BSFL digestion increased protein content in larval biomass by 6- to 10-fold compared to raw SMS. Frass from digested SMS showed higher nitrogen, phosphorus, and potassium (NPK) levels, meeting organic fertiliser standards.

4. Survival Rates: BSFL thrived on all SMS types, with survival rates exceeding 99%, outperforming the vegetable-only control (91.4%).

   
   

Future Exploration

The research highlights SMS as a viable, low-cost substrate for BSFL rearing, aligning with circular economy principles. By converting waste into high-protein feed and biofertiliser, this approach addresses two critical challenges: reducing agricultural waste and improving resource efficiency.

While the study confirms SMS’s potential, further optimisation is needed. Anaerobic fermentation or bacterial inoculation could enhance substrate quality, and blending SMS with higher-nutrient wastes may improve larval growth. Scaling this method could revolutionise waste management in the mushroom industry while supporting sustainable agriculture.