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BSF Metrics #5 — Larval Density and Survival: More Larvae per Tray Is Not More Output
BSF Metrics That Actually Matter #5 — Larval density and survival.
01 · The metric
Why density decides the batch
Density is the metric most operators never test. And it is one of the few where being wrong does not cost you 10 % — it costs you the whole batch.
Outside the window, you do not see small inefficiency. You see large-scale failure: substrate that never gets eaten, hotspots that kill part of the batch, cycles that drift toward 12+ days.
02 · The discipline
Measure before you change
You cannot optimise what you do not measure:
Before finding the right density, you need the discipline to dose accurately and log the result. Weight-based dosing — count 100 larvae once, weigh them, then weigh every dose — gives a real input number. Weighing harvest per tray gives a real output number. Without both, every recipe change is guesswork.
A scale is the single cheapest piece of equipment in BSF production, and the most often skipped.
03 · The window
The output window
The window: 8,000 to 15,000 larvae per 8 kg of substrate.
Below ~8,000 on 8 kg: larvae underperform. Substrate often does not get fully processed before it spoils or the cycle runs out. We consistently see longer cycles and substrate left over. Whatever the exact mechanism, the threshold is real.
Above ~15,000 on 8 kg: more larvae stop adding output. Total mass per tray plateaus, individuals shrink, metabolic heat climbs (M4 last week — local hotspots above 40 °C that the room sensor never shows), and you lose part of the batch.
Both ends look like operator problems. Neither is. Both are dosing.
04 · The trade-off
Three positions inside it
Inside the window, you trade three things:
Closer to 8,000 — larger individuals, longer cycle, less margin if substrate quality slips. A bad week of substrate can leave food in the trays.
Closer to 15,000 — faster cycle, more robust against recipe variation, smaller individuals at harvest. More larvae chewing the same substrate finishes it predictably even when conditions slip.
Middle — a compromise on both. This is where most production should sit.
There is no single right number. There is the right trade-off for your substrate, your buyer, and how stable your conditions are.
05 · The starting point
Finding your own recipe
Where to start, and how to move:
Load 8 kg substrate, dose 10,000 larvae, run a 10-day cycle. Watch the result and iterate one variable at a time, keeping the cycle at 10 days for the best economics.
The flowchart below shows the full recipe-finding loop we use. The principle is the same in one line: if everything was eaten in 10 days you have margin to test; if not, wait 4 more days before concluding the recipe is too rich. Stay between 8,000 and 15,000 larvae per 8 kg.
What is your count per tray — and have you ever moved it deliberately to find your right recipe?
Keep exploring
Related topics and pages
- BSF Metrics #4 — Temperature: Substrate Is the Target, Air Is the Medium — the previous metric in the same seven-post series
- BSF Metrics #2 — Rearing Cycle Length: Why Extra Days Kill Your Margins — the density band and the cycle length it trades against
- Production capacity in BSF farming: Realistic output metrics that actually hold — why maximum density is a trap and the 60–75% planning rule
Read next
- BSF Metrics #4 — Temperature: Substrate Is the Target, Air Is the Medium — the previous metric in the same seven-post series
- BSF Metrics #2 — Rearing Cycle Length: Why Extra Days Kill Your Margins — the density band and the cycle length it trades against
- Larvae Counting / Dosing Tabletop Prototype Demand-Test — the counting and dosing routine that makes density testable
- KALARVO R1 — Preliminary report — the screening round where recipe and carrier choice, not density alone, decided the outcome
Next step
Check density on your own material
Manna can help set a larval density target and the dosing and logging routine that holds it on a specific material.