Composting with black soldier fly larvae (BSFL) means letting a colony of grubs eat your food scraps and turn them into two useful things: a nutrient-rich soil amendment called frass, and a pile of protein-packed larvae you can feed to chickens, reptiles, or fish. It's faster than a traditional compost pile and faster than worm composting. BSFL can work through soft food waste in a matter of days instead of months.
I came to this the long way around. I was trying to deal with dog waste, tried regular composting, it took way too long, tried earthworms, they didn't touch the stuff. Then I read about black soldier fly larvae and gave them a bin of scraps to see what would happen. Two days later the food was gone and all that was left was the larvae and the frass. That's the part nobody tells you up front: how fast these grubs actually move.
How BSFL Composting Is Different From Regular Composting
A traditional compost pile works by microbial decomposition. Bacteria and fungi slowly break material down, and you turn the pile to keep it aerated. It works, but it's slow, usually two to six months for finished compost, and it doesn't handle wet food waste well. Meat, dairy, and oily scraps tend to go rancid and attract pests before they break down.
BSFL composting works by digestion instead of decomposition. The larvae physically eat the waste. A dense population of grubs can process soft food scraps extremely fast, and because they're consuming the material rather than waiting for it to rot, the bin stays far less smelly than a stalled compost pile. The larvae also generate metabolic heat as they feed, which helps dry the substrate from the inside.
The other big difference is what you get out of it. Regular composting gives you compost. BSFL composting gives you two products: frass, the larvae's castings, which work as a soil amendment, and the larvae themselves, which are a live feed source. So you're getting feed out of your food waste. A regular compost pile never gives you that.
What BSFL Will and Won't Eat
BSFL handle most pre-consumer and kitchen food waste well: fruit and vegetable scraps, spent brewing grain, coffee grounds, bread, cooked grains, and poultry mash. They'll also take on things a worm bin or compost pile struggles with, including small amounts of meat and dairy, because the larvae process material fast enough that it doesn't sit and rot. For a full breakdown of safe and unsafe inputs, see what black soldier fly larvae eat.
What to go easy on: large quantities of meat (high pathogen load if it sits), heavily salted or pickled food, citrus and acidic scraps in bulk, and anything sprayed with pesticides. Bones, eggshells, and fibrous yard waste like sticks and woody stems won't get eaten. The larvae just leave them behind.
Moisture is what trips most people up. Wet, fruit-heavy scraps spike the moisture level in a bin fast, and a waterlogged bin is the most common way BSFL composting goes wrong. If your bin is already swampy, here's how to fix a too-wet BSFL bin. Balancing wet inputs with a dry carbon material keeps the substrate workable. Dry, edible carbon like oats, wheat bran, or spent grain is better than inert material like plain cardboard, because the larvae will actually eat it and convert it into frass rather than leaving it sitting in the bin.
What You Get Out of It: Frass, Reduced Waste, and Feed
Frass. As the larvae feed, they leave behind a dry, crumbly material called frass, a mix of castings and shed exoskeleton. It contains nitrogen, phosphorus, and potassium along with chitin, and it's used as a soil amendment and fertilizer. The exact nutrient ratios depend heavily on what you fed the colony, so frass from a bin running on spent grain will look different from frass run on mixed kitchen scraps.
Waste reduction. A working colony dramatically cuts the volume of food waste you'd otherwise throw out. Scraps that would take months to break down in a pile, or get bagged and sent to a landfill, get processed in days. For anyone trying to reduce what they send to the trash, that's the whole point.
Live feed. The larvae you grow are worth money on their own. BSFL run roughly 40–50% protein and 25–35% fat on a dry-weight basis, and live larvae carry a naturally favorable calcium-to-phosphorus ratio that most feeder insects don't. Backyard chicken keepers, reptile owners, and fishkeepers all use them as feed. So the same bin that handles your food waste also produces something your animals will go wild for.
How to Start a BSFL Compost Bin
The basic setup is simple: a container, some starter larvae, food waste, and warmth. You don't need much to begin. A dark plastic storage bin works fine, and that's exactly what I started with. If you'd rather start with the parts already figured out, our BSFL Bin Kit drops a self-harvesting ramp, collection insert, entrance pod, and Egg Tree into any tote.
- A bin. A dark plastic tote keeps light levels low, which the larvae prefer. It needs ventilation and a way for the larvae to stay above the wettest part of the substrate.
- Starter larvae. You begin a colony by adding young BSFL to the bin. From there you can either keep buying starter batches or set up an adult breeding cage to produce your own eggs and run a self-sustaining cycle — that's what The Coop is built for.
- Warmth. BSFL feed and grow best at about 27–30°C (80–86°F). They slow down as it gets cooler, nearly stop developing below about 19°C (66°F), and face heavy die-off below roughly 15°C (59°F). In a place with real winters, and I'm in Colorado, that means a heated indoor space, not an outdoor bin you forget about in November.
- Food waste. Start small and watch how fast it disappears. If scraps are gone in under 12 hours, you can feed more. If food is still sitting there after a day and a half, ease off. Overfeeding is what pushes a bin toward going sour.
When the larvae reach the prepupal stage they darken and stop eating, and they instinctively crawl away from the substrate looking for somewhere dry to pupate. You can use that. A ramp angled out of the bin lets them self-harvest into a collection container, so you're not digging through the muck by hand. That's exactly what our Grub Escape ramp does.
Common Problems and How to Fix Them
The bin smells bad. A healthy BSFL bin smells earthy, not foul. A sharp ammonia or rotten smell means the substrate has gone anaerobic, usually too wet, too packed down, or overfed. Add dry carbon material, stop feeding for a day or two, and aerate it.
Larvae are crawling out in large numbers. Mass evacuation almost always means the substrate has gone anaerobic and the grubs are abandoning it. Same fix: dry carbon, stop feeding, turn the material to restore airflow.
The waste isn't breaking down fast. Usually temperature. If the bin is cool, the larvae slow way down. Check that you're holding the substrate in the high-20s Celsius, and make sure your colony is large enough for the volume of scraps you're adding.
You're getting other flies. An open, rotting bin can attract houseflies. A well-managed BSFL bin actually discourages them. The larvae outcompete other fly species for the food, and adult black soldier flies don't hang around kitchens or land on food. Keeping the bin covered and not overfeeding keeps nuisance flies down.
Is BSFL Composting Right for You?
It depends on what matters to you. If you want to cut what you send to the landfill, feed your chickens or reptiles for free, or make a frass soil amendment fast, BSFL composting does all three at once. It asks for more attention than a compost pile you ignore in the corner of the yard, mostly around temperature and moisture. But once it's dialed in, the grubs do the work.
Frequently Asked Questions
Q: What is black soldier fly composting?
A: It's a method of processing food waste using black soldier fly larvae (BSFL) instead of microbes or earthworms. The larvae eat the scraps directly, producing a soil amendment called frass and a crop of protein-rich larvae that can be used as animal feed. It's faster than traditional composting because the waste is being eaten rather than left to decompose.
Q: How fast do black soldier fly larvae compost food waste?
A: A dense, healthy colony can work through soft food scraps in a matter of days, compared to the two to six months a traditional compost pile typically needs. Speed depends on colony size, temperature, and the type of waste. Soft, wet scraps go fastest.
Q: Does a BSFL compost bin smell?
A: A well-managed bin has a mild, earthy smell. A strong ammonia or rotten odor is a warning sign that the substrate has gone anaerobic from being too wet or overfed. The fix is to add dry carbon material, stop feeding briefly, and aerate the bin.
Q: Can you compost meat and dairy with black soldier fly larvae?
A: In small amounts, yes. BSFL handle some meat and dairy that a worm bin or compost pile can't, because they process material fast enough that it doesn't sit and rot. Large quantities of meat aren't recommended due to pathogen load, and bones won't be eaten.
Q: What's the difference between BSFL frass and regular compost?
A: Frass is the castings left behind by the larvae, a dry, crumbly material containing nitrogen, phosphorus, potassium, and chitin, used as a soil amendment. Regular compost is fully decomposed organic matter. Frass is a byproduct of the larvae digesting waste, while compost is the result of microbial breakdown. Exact frass nutrient ratios vary with what the colony was fed.
Q: Do I need to buy new larvae every time, or can the colony sustain itself?
A: You can run it either way. Many people start by buying batches of starter larvae. To make it self-sustaining, you set up a separate adult breeding cage so the flies lay eggs that restock your bin, closing the loop without repeat purchases.
