Why Feed Per Larva Decides How Safely BSFL Clean Waste

Anupa Velusamy

17 hours ago

4 min read

Packing more larvae onto less feed makes a BSFL unit more efficient, but on protein-rich waste it can leave more germs alive. A study from the Swedish University of Agricultural Sciences, published September 23, 2026, is the clearest test of this trade-off yet (Frontiers in Microbiology).

Having the right larvae density is one of the important aspect of BSFL farming. Not only it helps in optimum time frame for waste reduction, but also helps to keep good health of the larvae in itseld. Control over odour and effect output on the fully properly processed compost (Frass) also depends on the processing larval density. It directly relates to the heat generation in the compost and moisture retention in the substrate.

Adding to the above concerns, the recent study states that the BSFL density is directly related to the pathogen mitigation in the substrate.

Why Pathogens Matter in BSFL Waste Treatment

BSFL are fed on exactly the kinds of waste that can carry disease: food scraps, abattoir waste, manure. Two of the most common culprits are Salmonella and harmful strains of E. coli, both major causes of food-borne illness.

Whatever survives in the waste can end up in two products:

Frass, the leftover mix of larval droppings and residue, which is sold as fertilizer and goes onto crops.
Larvae, which become animal feed.

The good news is that larvae kill many pathogens as they feed. Earlier work cited by the authors recorded a 6-log (99.9999%) drop in Salmonella Typhimurium within 8 days. The open question was how farm settings, such as how crowded the larvae are and how much feed each gets, change that effect.

The Study

Evans Were, Björn Vinnerås and Cecilia Lalander of the Swedish University of Agricultural Sciences in Uppsala ran three lab experiments.

Two kinds of waste.

Carbohydrate-rich: 40% bread waste and 60% cabbage waste (15.4% crude protein).
Protein-rich: brewers' spent grains, pig abattoir waste and a little bovine blood (31.9% crude protein).

Three feed rates. Larvae were stocked at 1, 3 or 6 larvae per cm² on the same amount of waste. That gave each larva roughly 0.6 g, 0.2 g or 0.1 g of feed (as volatile solids). The more crowded the tray, the less food per larva.

Six test germs. The waste was spiked with two strains of E. coli and four types of Salmonella (Abony, Dublin, Senftenberg and Typhimurium). In one experiment, germs were added once at the start. In another, they were added on days 0, 3 and 6, to mimic waste that keeps arriving contaminated.

Conditions. Trays were kept dark at about 30°C for 10 to 14 days, with larva-free trays as controls.

The Results

On protein-rich waste, less feed per larva meant fewer germs killed. On carbohydrate-rich waste the pattern was weaker and mixed. The numbers below are log reductions over 14 days after a single contamination at the start. Higher numbers mean more germs killed.

Protein-rich waste

High feed (1 larva/cm²): Salmonella 5.1 · E. coli 4.6
Medium feed (3 larvae/cm²): Salmonella 5.1 · E. coli 2.4
Low feed (6 larvae/cm²): Salmonella 4.5 · E. coli 2.3
No larvae (control): Salmonella 1.5 · E. coli 0.2

Carbohydrate-rich waste

High feed (1 larva/cm²): Salmonella 1.7 · E. coli 2.2
Medium feed (3 larvae/cm²): Salmonella 2.9 · E. coli 2.7
Low feed (6 larvae/cm²): Salmonella 2.9 · E. coli 2.5
No larvae (control): Salmonella 1.8 · E. coli 2.7

Three things stand out.

Protein-rich waste plus well-fed larvae gave the strongest clean-up. Salmonella fell 5.1 log and E. coli 4.6 log, far beyond the larva-free control.
E. coli was most sensitive to crowding. On protein-rich waste, its reduction halved from 4.6 to 2.3 log as feed per larva fell.
Carbohydrate-rich waste gave weak kill at every density. Results there stayed close to the larva-free control, so the waste type mattered as much as the feed rate.

Repeated contamination made things worse. When germs were added on days 0, 3 and 6, pathogen reduction was lower overall. At some low-feed settings, Salmonella reduction was no better than in trays with no larvae at all.

The efficiency side of the trade-off. Crowded trays did look better on paper. Bioconversion efficiency on carbohydrate waste rose from 15.1% to 28.2% between the high and medium feed rates, and more of the waste was consumed. But larvae grew much smaller (about 100 mg against about 250 mg), fewer survived, and less frass was produced.

What a Log Reduction Means

Each "log" is a tenfold drop in live germs. The scale sounds small, but the gaps between numbers are huge.

2 log: 99% killed, 10,000 left from every 1 million
3 log: 99.9% killed, 1,000 left
4 log: 99.99% killed, 100 left
5 log: 99.999% killed, 10 left

So when E. coli reduction fell from 4.6 to 2.3 log, it was not a small slip. Roughly 25 of every million germs survived at the high feed rate. At the low feed rate, about 5,000 survived, some 200 times more.

Why Less Feed Per Larva Weakens Pathogen Kill

Fighting germs costs larvae energy, and hungry, crowded larvae have less to spare. The authors did not prove a single cause, but they put forward four likely reasons.

Immune defences need fuel. Larvae make natural antimicrobial peptides, small proteins that kill bacteria. Building them is "nutritionally and energetically costly." When feed runs short, the authors suggest this defence weakens. Their protein-rich waste was also fairly low in fat (11.6% of dry matter), so energy may have run out fast at high density.
Crowding without food backfires. Insects in dense groups often raise their immune defences in advance, expecting disease. That only works if they have the energy to follow through.
Waste build-up. Crowded larvae on little feed may be exposed to more ammonia and other metabolic waste, which stresses them. The authors note they did not measure ammonia, so this remains a hypothesis.
Heat. Dense larvae generate heat and can push the substrate well above room temperature. Heat stress can slow growth and weaken immune responses. Temperature was not measured either.

The common thread: pathogen kill is partly a living process in the larvae, not just a side effect of the waste being eaten.