The Science Behind Mealworm Frass Fertiliser

Mealworm frass is attracting attention because it combines plant nutrients, carbon-rich organic matter and insect-derived compounds in a compact soil amendment. The research is promising—but the strongest evidence is more specific than some of the sweeping claims often made about insect frass online.

This guide separates what has been demonstrated in published mealworm-frass studies from what remains dependent on crop, dose, soil, frass source and growing conditions.

What is mealworm frass?

Mealworm frass is the fine material produced during the rearing of Tenebrio molitor larvae. It consists primarily of insect excreta, together with small amounts of shed insect material and residual feed particles. It contains nitrogen, phosphorus, potassium, carbon and other nutrients, but the exact composition is not universal.

What is actually in RichRoots frass?

Rather than use a generic industry NPK figure, we publish the current independent analysis of our own material:

Natural products vary, so these figures should be treated as the analysis of the tested RichRoots material rather than an immutable specification for every future batch.

1. Evidence for plant nutrition

A frequently cited mealworm-frass study by Houben and colleagues tested frass in a greenhouse barley experiment. At the application rate and nutrient conditions used in that study, mealworm frass produced crop biomass and nitrogen, phosphorus and potassium uptake comparable with a mineral NPK treatment.

That is useful evidence that mealworm frass can function as a fertiliser. It is not the same as proving that frass will outperform mineral fertiliser in every crop, soil or field condition. The authors themselves noted the need for further in-situ research.

2. Evidence for soil biological activity

Houben et al. also reported increased soil microbial activity after frass addition. This supports the idea that frass can interact with the biological component of soil, not simply act as a bag of mineral nutrients.

However, “frass contains beneficial microbes” should not be treated as the whole explanation for its fertilising effect. In a later controlled study, Blakstad and colleagues concluded that the mealworm-frass microbiota did not play a role in the fertilising capacity they observed. In other words, the organic matter, nutrients and other components of frass matter too.

3. Chitin and plant defence: promising, but not a guarantee

Chitin is associated with insect exoskeletons and is one reason frass is scientifically interesting. Plant defence responses to chitin and chitin-containing materials are an active research area.

In the 2023 Blakstad study, mealworm frass did not induce the local root callose response the researchers tested, but it did induce systemic resistance against Botrytis cinerea in their experimental system. That is a much more precise statement than saying “frass boosts immunity” in every plant.

For home gardeners, the responsible interpretation is that plant-defence effects are plausible and experimentally supported in particular systems, but should not be marketed as guaranteed pest or disease control.

4. Why frass composition varies

Amorim and colleagues compared yellow-mealworm frass with poultry litter and found meaningful differences in nutrient composition between frass sources. Their work also emphasised that rearing substrate and supplements can influence the chemical and microbial characteristics of the final material.

In one comparison at the same 5 Mg/ha application rate, one mealworm-frass source could supply 159 kg of nitrogen per hectare versus 78 kg from one poultry-litter source. The important lesson is not that “all frass has twice the nitrogen of poultry manure”; it is that the source matters.

The same caution applies to statements about pathogens or heavy metals. Very low concentrations in a particular research sample do not prove that every frass product is universally free of those contaminants. Product-specific testing is more meaningful than a generic category claim.

5. What can we reasonably say about mealworm frass?

The current evidence supports using mealworm frass as a nutrient-containing organic fertiliser and soil amendment. Published studies show that it can support plant growth and nutrient uptake, affect soil microbial activity, and influence plant defence responses under certain controlled conditions.

What the evidence does not justify is treating every possible benefit as automatic. We do not assume that mealworm frass cannot be overapplied, that it will outperform conventional fertiliser in every setting, that live microbes are always responsible for its effects, or that it guarantees resistance to pests and diseases.

6. The practical takeaway for gardeners

You do not need to reproduce a greenhouse experiment to use frass effectively. For ordinary garden use, our recommendation is simple: apply it to soil at modest rates, lightly incorporate it where practical, and water it in.

See our step-by-step application guide for pot and garden-bed starting rates. For quick questions, read our Mealworm Frass Fertiliser FAQ.

For repeated feeding across several plants, veggie beds or larger pots, explore the 360g Large Garden Pouch.

Shop the 360g Large Garden Pouch →

References

1. Houben, D. et al. (2020). Potential use of mealworm frass as a fertilizer: Impact on crop growth and soil properties. Scientific Reports. doi:10.1038/s41598-020-61765-x
2. Blakstad, A. et al. (2023). Frass from yellow mealworm (Tenebrio molitor) as plant fertilizer and defense priming agent. Biocatalysis and Agricultural Biotechnology. doi:10.1016/j.bcab.2023.102862
3. Amorim, H. C. S. et al. (2024). Insect frass composition and potential use as an organic fertilizer in circular economies. Journal of Economic Entomology. doi:10.1093/jee/toad234