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Black soldier fly frass: organic fertilizer from insect systems.
Black soldier fly (BSF) larvae eat organic waste fast and turn it into protein for animal feed. What they leave behind, called frass, is a nitrogen-rich organic fertilizer. Kenyan trials show it can perform well, but it behaves differently from ordinary compost and needs handling with care.
The numbers that matter
BSF is a feed business first and a fertilizer business second. Here is what the research says about the fertilizer side.
From waste to larvae to frass
- Feed the larvaeYoung larvae are fed organic waste such as fruit and vegetable waste, brewers’ grain, market waste or manure. A single larva can eat 25 to 500 mg of waste a day, and larvae can eat up to four times their body weight daily.
- Wait about 10 to 14 daysFeeding time depends on the waste, temperature, moisture and how many larvae are in the container. Some food-waste and vegetable substrates reached the prepupal stage in 7 to 8 days.
- Harvest the larvaeThe larvae are separated out and used as animal feed. Under good conditions the waste volume falls by 50 to 80 percent.
- Collect the frassWhat is left is frass: the larvae’s castings mixed with undigested waste. In one review, frass made up about 45 percent of the dry matter of food waste input and about 40 percent for poultry manure.
- Stabilise itDry it, compost it, or both before it goes on a crop. The next two sections explain why.
SOURCE: “BLACK SOLDIER FLY (HERMETIA ILLUCENS) LARVAE AND FRASS: SUSTAINABLE ORGANIC WASTE CONVERSION, CIRCULAR BIOECONOMY BENEFITS, AND NUTRITIONAL VALORIZATION”, AGRICULTURE (MDPI), 2026
Nutrients in frass
| A Kenyan frass sample | Typical range in the literature (dry matter) | |
|---|---|---|
| Nitrogen (N) | 2.1% | about 2 to 4% |
| Phosphorus (P) | not reported here | about 1 to 5%, varies with the waste |
| Potassium (K) | not reported here | about 0.5 to 4.1%, varies with the waste |
| Organic carbon | 35.2% | about 37 to 45% |
| Carbon-to-nitrogen ratio | 16.8 | about 15 |
| pH | 7.7 | about 7.5 |
| Moisture | 30.1% | not given |
Two points matter in practice. First, the phosphorus and potassium vary most with what the larvae ate, so test your own frass. Second, one review puts about 98 percent of the mineral nitrogen in frass in the form of ammonium. That makes it fast-acting, and it is also why fresh frass can harm seedlings.
SOURCES: KENYATTA UNIVERSITY / ICIPE MAIZE TRIAL, FRONTIERS IN PLANT SCIENCE (2020); AGRICULTURE (MDPI) REVIEW (2026); FRONTIERS OF AGRICULTURAL SCIENCE AND ENGINEERING REVIEW
Don’t put fresh frass on seedlings
A preliminary maize trial in southeastern Madagascar compared fresh frass, frass composted for five weeks, and cattle manure. All were applied at the same nitrogen rate.
| Treatment | Seed germination | Ammonium-N in the material |
|---|---|---|
| Cattle manure | 95.8% | – |
| Frass composted for 5 weeks | 91.7% | 1,750 mg/kg |
| Fresh frass, sun-dried for 4 days | 19.4% | 4,700 mg/kg |
The authors blame the high ammonium in fresh frass for the poor germination. Composting cut it by about 63 percent. They advise applying fresh frass only after the crop has germinated, and with caution. The trial was one season at one site with three replicates, so treat it as a warning rather than a rule.
What to do
- Mature it first. Stack the frass in a heap and let it compost. A review reports frass mixed with more than 10 percent biochar maturing in five weeks, and frass adjusted to a carbon-to-nitrogen ratio of 15 doing the same. See heap composting for turning and monitoring.
- Keep rates moderate. Very high rates may temporarily inhibit seed germination. A review suggests limiting sensitive crops such as tomato to 150 to 200 kg of nitrogen per hectare.
- Mix it into the soil and watch soil pH and salinity if you use it repeatedly, especially in dry areas.
SOURCES: FRONTIERS IN SUSTAINABLE FOOD SYSTEMS (2025), MADAGASCAR MAIZE TRIAL; FRONTIERS OF AGRICULTURAL SCIENCE AND ENGINEERING REVIEW; AGRICULTURE (MDPI) REVIEW (2026)
Frass on maize and vegetables in Nairobi
Two field studies by Kenyatta University and icipe, the International Centre of Insect Physiology and Ecology, tested frass on the university’s farm in Nairobi.
Maize (two seasons, 2019 to 2020)
- Frass raised maize grain yield over unfertilised plots by 71 to 96 percent in the short rains and 49 to 101 percent in the long rains, depending on the rate.
- Frass at 2.5 t per hectare gave an improvement that a commercial organic fertilizer (SAFI) needed double that rate to match.
- At 100 kg of nitrogen per hectare, frass gave 27 percent more grain than the commercial organic fertilizer and 7 percent more than urea.
Tomatoes, kale and French beans
- Frass combined with half the nitrogen as NPK fertilizer outyielded NPK alone by 22 to 135 percent for tomatoes, 20 to 27 percent for kale and 38 to 50 percent for French beans.
- Used on its own, frass gave tomato yields 58 to 75 percent higher than brewers’ grain compost.
- The authors recommend 1.24 t of frass plus 322 kg of NPK per hectare for these vegetables.
Read these results with care.
Each study ran at one site over two seasons, with frass from one producer. The vegetable trial’s full nitrogen dose, 371 kg per hectare, is well above the rates in the maize trial. And the authors call frass a relatively new product whose performance in other conditions is less established.
SOURCES: “EXPLORING BLACK SOLDIER FLY FRASS AS NOVEL FERTILIZER FOR IMPROVED GROWTH, YIELD, AND NITROGEN USE EFFICIENCY OF MAIZE UNDER FIELD CONDITIONS”, FRONTIERS IN PLANT SCIENCE (2020); BSF FRASS FERTILIZER IN VEGETABLE PRODUCTION, NAIROBI, FRONTIERS IN PLANT SCIENCE (2021)
Calculate by nitrogen, not by the bag
Frass varies, so work from its nitrogen content on your own lab report.
Tonnes of frass per hectare = target kg of N ÷ (frass N% × 10).
For frass with 2.1 percent nitrogen, 60 kg of nitrogen needs about 2.9 t of frass per hectare. That matches the Nairobi maize trial, which used about 1.4, 2.9 and 4.8 t per hectare for 30, 60 and 100 kg of nitrogen. Treat the result as a starting point and adjust it with a soil test and local advice.
Use the trial rates as a reference, not a prescription. Ask your extension officer, or us, to match a rate to your soil and crop. And keep the maize guide’s lesson in mind: on poor soil, organic inputs often work best alongside some fertilizer. See our maize guide.
SOURCES: KENYATTA UNIVERSITY / ICIPE MAIZE TRIAL (2020); ARITHMETIC BY ESSENTIA KANAN.
The waste the larvae eat decides what the frass is
Frass is only as clean as its feedstock. The research on BSF safety in Africa is mostly about the larvae, not the frass, but the warnings carry over.
- Pathogens. A study found E. coli, Salmonella and Staphylococcus aureus in larvae reared on urban organic waste. The larvae’s natural antimicrobial compounds do not fully remove pathogens from a contaminated feed.
- Heavy metals. In a controlled study, larvae accumulated cadmium at about nine times and lead at about twice the level in their feed.
- Residues. Crop residues can carry pesticide and herbicide residues, and livestock waste can carry veterinary drug residues.
- Risky feedstocks. Human or animal faecal sludge, industrial and processing waste, and municipal and market waste each need screening.
- Crops eaten raw. A review advises extra composting or heat treatment for crops eaten raw, especially with certain feedstocks. It reports heavy metals usually below regulatory limits when clean feedstocks are used.
Regulation.
One review calls the regulatory framework for insects as food and feed “almost non-existent” across Africa. We did not find a Kenyan standard written specifically for frass. Frass sold as a fertilizer is likely to be judged against the same rules as other organic fertilizers, such as the draft KS 2290 limits for E. coli, Salmonella and heavy metals (see selling compost). That is our inference, so confirm with KEBS and the relevant regulator before selling frass.
SOURCES: “CRITICAL SAFETY CONCERNS IN THE PRODUCTION OF BLACK SOLDIER FLY LARVAE IN AFRICA”, DISCOVER FOOD (2025); FRONTIERS OF AGRICULTURAL SCIENCE AND ENGINEERING REVIEW
Where BSF fits, and where it doesn’t
It suits
- Producers with a steady supply of wet organic waste, such as fruit processors, breweries, markets and abattoirs.
- Anyone who wants animal feed as well as fertilizer from the same waste.
- Operators who can test their waste and their frass.
It doesn’t suit
- Small households. A bin or bokashi bucket is simpler.
- Anyone with an unreliable or contaminated waste supply.
- Anyone who expects finished, ready-to-use compost straight from the larvae.
For a worked example, see our case study on avocado oil waste in Murang’a and Kiambu.
Questions people ask
Is frass the same as compost?
No. Frass is the larvae’s castings mixed with undigested waste. It is nitrogen-rich, and most of its mineral nitrogen is ammonium. Composting it first makes it a stabler product.
Can I use fresh frass?
It is risky. In a Madagascar trial, fresh frass cut seed germination to about 19 percent, against about 92 percent for composted frass. Mature it first.
How much frass per acre?
Work from your frass’s nitrogen content and your crop’s need. In the Nairobi maize trial, 2.5 t per hectare (about 1 t per acre) already improved yields.
Is it safe for vegetables?
It depends on what the larvae ate. Use clean feedstocks, test the frass, and mature or heat-treat it, especially for crops eaten raw.
Can I replace chemical fertilizer with it?
Frass matched or beat urea at the same nitrogen rate in a Nairobi maize trial. The best vegetable results came from frass plus half the NPK, not from frass alone.
Where can I buy it?
Ask Kenyan BSF producers, and check any product for a lab analysis before you buy. See our Trust page for how to choose safely.
Thinking of turning waste into larvae and frass?
We help processors, farms and institutions assess their waste, plan a safe system and test the fertilizer it produces.