Using BSF Frass to Grow Better Mushrooms

Turning Mealworm Frass into a Win for Mushrooms—and Fish

A recent MDPI paper titled “From Waste to Value: Investigating Mushroom Stems from Pleurotus ostreatus Grown on Mealworm Frass as a Nutritional Source for Aquaculture Feed” sheds light on ways to valorize two kinds of underutilized byproducts: insect frass and mushroom stems (mdpi.com). Conducted in 2023 by researchers from Celabor, CTICH, CIIMAR, Tebrio, and other European institutions, the study explores how adding 0–15% mealworm frass (from Tenebrio molitor) to oyster mushroom substrate affects yield, nutritional content in both caps (for human consumption) and stems (typically discarded), as well as the net potential for aquaculture feed use (mdpi.com).

They found that modest frass inclusion—in the neighborhood of 2.5%—actually boosted yield, while higher percentages (10–15%) began to diminish it versus the control. Protein in stems jumped from ~7.8% (dry matter) up to ~22.3% at 15% mealworm frass; for fruiting bodies, from ~24.7% to ~31.0% across the same range (mdpi.com). Essential amino acids, especially in stems, also improved significantly, though β-glucan content declined with rising frass levels. Notably, only the non-polar dichloromethane extracts from stems grown on 12.5–15% frass showed antimicrobial activity against Tenacibaculum maritimum, a bacterial fish pathogen, with inhibition zones around 10–11 mm (mdpi.com).

What Does THIS Mean for BSF Frass?

If you’re using—or considering using—Black Soldier Fly (Hermetia illucens) systems, mealworm frass isn’t a perfect stand-in for BSF frass, but there are many overlaps and fertile areas for investigation.

Frass Composition: Species Matters

• Recent comparative work shows that BSF frass often has higher levels of plant-available nitrogen (especially ammonium) than many other insect frasses. It also tends to carry more potassium in useful forms (sciencedirect.com).
• One study found that BSF frass fertilizer had significantly higher N (by 20–130%) and K (17–193%) concentrations relative to other frass types, including mealworm frass, with corresponding stronger nutrient release profiles (nature.com).
• Microbial safety matters: fresh BSF frass often fails to meet strict EU microbiological standards unless heat-treated (about 70 °C for 1 hour) to reduce bacterial load. Heat-treated BSF frass typically meets guidelines; mealworm frass shows similar needs in certain cases but sometimes passes even without treatment, depending on source and contamination (pubmed.ncbi.nlm.nih.gov).

Implications Based on the MDPI Outcomes

Because BSF frass often has a richer nitrogen content and potentially different micro- and macronutrient balance compared to mealworm frass, it could amplify the nutritional boosts seen in the MDPI study (e.g., protein in stems and bodies). However:

• High frass levels might also depress yield or delay colonization, due to changes in substrate structure, C:N ratio, moisture retention, etc.
• Functional properties like antimicrobial activity—seen only in stems at high mealworm-frass inclusion—may or may not replicate with BSF frass, depending on secondary metabolites and substrate interaction.

Similar Research with BSF, Spent Mushroom Substrate, and SMS Dual Systems

While MDPI’s study focused on mealworm frass, there are some useful studies involving BSF frass or mushroom substrate involving BSF larvae:

• One paper tested feeding spent mushroom substrate (SMS) to BSF larvae (mixed with vegetables), showing that the resulting frass was high in NPK and that larval protein content increased, making the frass itself useful and nutrient-rich (mdpi.com).
• Another study found that replacing up to 20% of chicken feed with SMS in BSF larval diets yielded comparable growth; however higher levels led to reduced yields—similar to how mealworm frass beyond ~10–12.5% reduced mushroom yield in the MDPI work (mdpi.com).

These results begin to sketch how insect and fungal systems can be paired in circular ways.

Opportunities and What to Try If You Grow Mushrooms + BSF

Based on MDPI’s findings and what we know of BSF frass, here are directions worth exploring:

1. Substrate amendment trials using BSF frass
   Start with low inclusion (2–5%) and increment toward 10–15%. Monitor mycelial colonization speed, fruit body yield, stem:caps ratio, and how substrate structure behaves (aeration, compaction, moisture retention).

2. Nutritional profiling of stems and caps
   Like MDPI, measure protein, amino acid profile (essential vs. non-essential), fatty acid classes, β-glucans, chitin levels, minerals, heavy metals. See whether BSF frass boosts protein but depresses something else (e.g. β-glucans) as mealworm frass did.

3. Safety & regulatory compliance Heat treatment or curing of frass may be needed, especially with BSF systems. Use studies like Heat Treatment and Storage of Frass from BSF Larvae etc. to guide protocols to ensure compliance and safety (pubmed.ncbi.nlm.nih.gov).

4. Functional bioactivity screening Test stems or whole mushrooms grown on frass-amended substrates for antimicrobial activity against aquaculture pathogens, perhaps Tenacibaculum maritimum or others prevalent in your region. Extraction methods (polar vs. non-polar) may yield different results as in the MDPI paper (mdpi.com).

5. Circular systems integration

   • Grow BSF larvae on mushroom byproducts (e.g. SMS or spent straw).
   • Use resulting BSF frass in substrate mixes for mushrooms.
   • Harvest caps for food, stems for feed, perhaps fish feed or animal feed, depending on nutrition and safety.
   • Use residues (spent substrate, non-feed parts) as compost or further insect feed.

6. Address market & perception issues
   Educate local fish farmers, feed formulators, consumers about novel ingredients. Investment in demonstration trials, cost-benefit analysis, feed trials in aquaculture to show performance (growth, health, feed conversion ratios).

Limitations, Caveats, Things to Check Closely

• Nutrient variability: BSF frass’s composition can shift significantly depending on what the larvae are fed, how frass is processed (drying, heat treating), and how “fresh” or decomposed it is.
• Substrate physicality: Frass inclusion may affect substrate aeration, moisture retention, and compactness—key for mycelium growth and fruiting.
• Functional loss vs. gain: As shown in the MDPI study, β-glucans dropped with high frass levels—so higher protein may come at cost of other bioactives like immunomodulators.
• Regulatory limits: For feed use (aquaculture especially), heavy metal thresholds, microbiological safety, toxin presence (if substrate was contaminated), will be scrutinized.
• Economic feasibility: Cost of frass collection, processing (heat treatment, drying, milling), and integration into substrate must make economic sense compared to conventional supplements.