Black Soldier Fly Frass Harbors Thermotolerant Growth-Promoting Microbes — And That Could Reshape How We Think About Biofertilizers
A peer-reviewed study just published in ACS Agricultural Science & Technology has done something the insect agriculture and biofertilizer industries have been waiting for: it systematically identified, characterized, and confirmed the activity of plant growth-promoting bacteria (PGPB) living inside Black Soldier Fly (BSF) frass — and critically, demonstrated that many of these beneficial microbes survive the mandatory heat treatment required before frass can be sold as a fertilizer.
The Science
Researchers from the University of Basilicata (Italy) and KU Leuven (Belgium) conducted a comprehensive screen of frass from BSF larvae (Hermetia illucens) reared on 10 different organic substrates — ranging from broccoli and artichoke to spent barley and olive pomace (Lomonaco et al., ACS Agric. Sci. Technol. 2026, 6, 489–504. DOI: 10.1021/acsagscitech.5c00811).
Using a rhizosphere-mimicking agar (RMA) — a selective medium designed to favor microbes capable of colonizing plant root systems — the team isolated 149 bacterial strains and screened them for key plant growth-promoting (PGP) traits:
Phosphate solubilization — the ability to liberate locked soil phosphorus for plant uptake
Ammonia production — a direct nitrogen source for plant roots
Auxin (IAA) production — the primary hormone governing root architecture and development
Gibberellin production — growth hormones that regulate shoot elongation, germination, and stress responses
Compatibility with humic acids — essential for function in real soil environments rich in organic matter
Six bacterial genera emerged as the highest performers: Serratia, Peribacillus, Acinetobacter, Pseudocitrobacter, Bacillus, and Enterobacter. These isolates were then validated in living Arabidopsis thaliana plants — the gold standard model organism of plant biology.
What the Plant Bioassays Showed
The Arabidopsis experiments revealed something nuanced and practically important: not all PGP bacteria work the same way, and the "best" strain depends on what you want to achieve.
Some strains — particularly Serratia, Peribacillus, and Bacillus — promoted both root elongation and root hair density simultaneously, a combination that translates directly to increased surface area for nutrient and water uptake. This is the root architecture profile you want in a biofertilizer targeting yield under normal conditions.
Other strains — including Acinetobacter and Pseudocitrobacter — produced the most prolific root hair formation of any treatment, even while limiting overall root length. The authors interpret this as a possible strategy for maximizing nutrient scavenging efficiency under resource-limited conditions: a denser, more compact root system packed with absorptive hairs rather than a long root that grows into depleted soil zones.
This trade-off between elongation and hair density has profound implications for precision biofertilizer design: matching microbial strain to agronomic context, not simply applying any "PGPB product" and expecting uniform outcomes.
The Heat Treatment Finding — Why It Matters
Here is where this research becomes commercially transformative. Under EU Regulation 2021/1925, all frass sold as a fertilizer must be heat-treated at 70°C for at least 60 minutes to ensure pathogen elimination. The longstanding concern in the industry has been that this treatment would simultaneously destroy the beneficial microorganisms that give frass much of its biological value.
The data tell a different story. Rhizobacterial populations across all frass types declined only marginally after heat treatment — from an average of 6.52 log CFU/g in untreated frass to 6.23 log CFU/g post-treatment. That represents approximately 95.6% retention of viable rhizobacteria through a 70°C, 1-hour kill step.
Multiple genera responsible for the strongest PGP activity — Bacillus, Acinetobacter, Enterobacter, Cronobacter, and Klebsiella — were detected in both untreated and heat-treated frass. While Bacillus's thermotolerance is well-explained by its spore-forming ability, the survival of non-spore-forming genera like Acinetobacter and Enterobacter through the same treatment is a remarkable finding that demands further mechanistic investigation.
The practical implication: compliant, heat-treated BSF frass may retain meaningful biofertilizer function — potentially justifying premium positioning for frass-based products derived from specific larval diets optimized for PGPM content.
Larval Diet Is the Hidden Variable
Perhaps the most actionable finding for agricultural practitioners and insect farming operators is the strong influence of larval feeding substrate on the microbial profile of the resulting frass.
The study found that frass from larvae fed artichoke, broccoli, and turnip greens consistently harbored the highest loads and diversity of PGP microorganisms. The researchers attribute this, at least in part, to the biochemical composition of these substrates: artichoke is rich in soluble fiber (notably inulin), while Brassicaceae vegetables accumulate sulfur-rich glucosinolates — compounds that may selectively enrich microbial communities adapted to metabolize or tolerate them.
By contrast, frass from larvae fed olive pomace or the whey/wheat seed substrate contained few or none of the six target PGPM genera. The same insect, on a different diet, produces a functionally different biofertilizer — a distinction that current commodity frass markets entirely ignore.
This finding aligns directly with what agronomists and soil biologists have long understood about the soil microbiome: substrate drives community composition, community composition drives function, and function drives agronomic outcomes. The same principle now extends upstream into insect frass production.
The Bigger Picture: Circular Agriculture at the Microbial Level
Black Soldier Fly bioconversion already represents one of the most compelling models of circular agriculture: organic waste streams enter as larval feed, high-quality protein and lipid exit as animal feed ingredients, and nutrient-rich frass exits as a soil amendment. This research adds a biological dimension to that circular model that has not previously been quantified.
If frass is not just an organic fertilizer but a delivery vehicle for a functional microbial community — one that can be engineered upstream by substrate selection and preserved downstream through properly managed heat treatment — then the value proposition of BSF frass products shifts fundamentally.
The path forward involves:
Frass characterization standards that go beyond nutrient analysis (N-P-K) to include microbial community profiling
Diet optimization protocols for insect farms seeking to maximize PGPM content in their frass
Strain co-application studies to identify synergistic combinations across the Bacillus, Enterobacter, and Serratia genera
Field validation across economically relevant crop species beyond Arabidopsis
Integration with soil health management programs that already account for native rhizosphere biology
🌿 From Waste Stream to Weed Suppression: This Is What Regenerative Agriculture Looks Like
At Higher Ground Plant Consulting, we work at the intersection of soil microbiology, plant science, and sustainable agricultural systems — helping producers, agribusinesses, and biotechnology companies turn emerging research into practical farm-level protocols.
Whether you are evaluating insect frass as a fertility input, developing biofertilizer products for regulatory submission, writing grant proposals around circular agricultural systems, or building soil health monitoring programs for your operation, we bring 20+ years of scientific expertise and real-world implementation experience to the table.
The science is moving fast. Is your operation keeping up?
📩 Contact us: brian@highergroundplantconsulting.com 🌐 Read more on our blog: highergroundplantconsulting.com/blog 💼 Connect: linkedin.com/in/brian-king-phd
Full Citation
Lomonaco, G.; De Smet, J.; IJdema, F.; Ceusters, J.; Iannielli, F.; Salvia, R.; Amato, M.; Scieuzo, C.; Falabella, P. (2026). Selection and In Vitro Assessment of Plant Growth-Promoting Bacteria from Black Soldier Fly (Hermetia illucens) Frass. ACS Agricultural Science & Technology, 6, 489–504. https://doi.org/10.1021/acsagscitech.5c00811
#SoilHealth #Biofertilizer #RegenerativeAgriculture #CircularAgriculture #BlackSoldierFly #InsectAgriculture #PlantScience #SoilMicrobiology #SustainableAg #AgriculturalBiotechnology #HigherGroundPlantConsulting #BiologicalFarming #SoilBiology
Dr. Brian C. King, PhD, MBA | CEO & Principal Investigator | Higher Ground Plant Consulting LLC | Lexington, KY
