I spent years watching banana peels turn into a science experiment in my backyard compost bin. You know the one, the tumbler that's supposed to break everything down in "just 6 weeks!" and instead produces a sad, half-rotted slurry you'd never put on a tomato plant. When I started feeding kitchen scraps to Black Soldier Fly Larvae instead, the difference wasn't subtle. It was like going from dial-up to fiber.
BSFL composting, or bioconversion, if you want the fancy term, is the process of letting larvae do what they evolved to do: eat organic waste, and eat it fast. I run Blue Grub Farms here in Aurora, Colorado, and this is the backbone of everything we do. Here's how it works, what you can throw in, and how to get rolling yourself.
What Is BSFL Composting?
You're using Black Soldier Fly larvae (Hermetia illucens) to break down organic material. During their growth phase, 3rd through 5th instar, these things are eating machines. We're talking 10–15% of their body weight per day in organic matter. A dense colony will visibly reduce a pile of food scraps overnight.
The difference between this and a traditional compost pile or worm bin: you don't turn it, it doesn't stink (when you manage it right), and it happens in days instead of months. My first time seeing a colony chew through a five-gallon bucket of coffee grounds in under a week, I was hooked.
And the adults aren't pests. They don't bite, they don't eat, and they couldn't care less about your kitchen. They look more like a skinny wasp than a housefly. In fact, a healthy BSFL bin actually repels houseflies, the larvae outcompete them for food.
What Can You Compost with BSFL?
This is where BSFL blow worm bins out of the water. Red wigglers are picky. BSFL are not. They'll happily eat:
- Fruit and vegetable scraps (their bread and butter)
- Coffee grounds and tea leaves
- Cooked food waste, yes, including some meat and dairy
- Spent grains (I get bags of this from a local brewery and the larvae destroy it)
- Manure, chicken, rabbit, cow, pig
- Pre-consumer food prep waste
- Cardboard and paper in small amounts mixed with wet stuff
What I'd avoid or limit:
- Big piles of straight meat or fish, pathogen risk goes up, and you'll attract things you don't want
- Super salty or acidic foods, throws off the microbial balance in the bin
- Onions and citrus in large volumes, I've noticed feeding slows when there's too much
- Pesticide-treated produce, larvae can bioaccumulate those toxins
- Anything that isn't organic matter, no plastic, no metal, no treated wood
Bottom line: if you're already producing kitchen scraps, coffee grounds, and food prep waste, you've got everything BSFL need. They eat what worms won't touch.
How Fast Does It Work?
When I keep my colonies at 80–86°F (27–30°C), which is their sweet spot, a batch of larvae in peak feeding stage will reduce organic waste by 50–75% in 10 to 14 days. What's left is called frass, a mix of larval castings, shed exoskeletons (that's chitin, and it's valuable), and partially processed substrate.
Here's what speeds it up or slows it down in my experience:
- Temperature: Below 75°F things slow noticeably. Below 68°F it almost stops. Here in Colorado winters, I run ceramic heat emitters on Inkbird controllers to keep bins warm.
- Moisture: You want 60–70% by weight. The squeeze test works, clumps in your hand but doesn't drip. Too wet and you get anaerobic stink. Too dry and larvae dehydrate.
- Stocking density: Not enough larvae for the food volume = slow processing. Too many = overcrowding, heat spikes, and larvae trying to evacuate. There's a sweet spot you'll learn to feel.
- Diet variety: A mix of wet and dry, carbs and protein, processes faster than any single material. I rotate between coffee grounds, produce scraps, and spent grain.
What Do You Get Out of It?
Two outputs, both useful: prepupae and frass.
Prepupae are larvae in their final stage before they pupate into flies. They turn dark brown, get plump, and are absolutely loaded with protein (~42%) and fat (~35%). If you keep reptiles, chickens, fish, or hedgehogs, these are premium live feed that you grew for free from your trash. That's the magic trick, waste in, animal feed out.
Frass is what's left in the bin after the larvae migrate out. I've been gardening with it for over a year now and the results are no joke. It contains:
- NPK from the breakdown process
- Chitin, which triggers plant immune responses and suppresses some soil pathogens
- A thriving beneficial microbial community
- Way lower pathogen load than raw manure
I top-dress it around tomatoes, peppers, and cannabis plants. I work it into potting mixes. My neighbor started calling it "bug dirt" and asking for bags of it. That's actually how the Frassquatch product line started, people kept wanting my frass.
How to Get Started
You don't need a lab or a barn. I started in a spare closet. Here's the short version:
1. Get a bin with a self-harvesting ramp.
When larvae hit the prepupae stage, they're hardwired to crawl away from the food and toward a dark, dry spot. A bin with a ramp exploits that instinct, they crawl up and drop into a collection bucket. You don't sort anything by hand. Our BSFL Bin Kit is designed around this exact principle. It's the same concept I use in my production bins, just scaled for a household.
2. Source your larvae or eggs.
You can buy starter larvae from us at Blue Grub Farms, or if you're in the continental US (May through September), you might attract wild BSF to an outdoor bin. In Colorado, I start seeing wild adults around late May when temps stay above 70°F consistently.
3. Set up your environment.
Larvae want warmth, 77–95°F is the range, with 80–86°F being ideal. Most homes in summer are fine. But if your bin is in an unheated garage or basement, you'll need a heat source in winter. I can't overstate this: cold kills colonies. It's the number one reason beginners in Colorado lose their first batch between November and March.
4. Start feeding, slowly.
Don't dump a week's worth of scraps in on day one. Start with a handful. As your colony population grows, scale up the food. If it starts to smell, you're overfeeding. If larvae are eating everything within hours and the bin looks dry, feed more. You'll develop a rhythm within a couple weeks.
5. Harvest prepupae and frass.
Dark larvae crawling up the ramp? Collect them. Feed them to your animals, dry them, or freeze them. What's left in the bin is frass. Let it air-dry, then bag it or spread it in the garden.
Frequently Asked Questions
Does it smell?
A well-run bin smells like damp earth, maybe slightly yeasty. If it reeks, something's wrong, usually too wet or overfed. Both are fixable in a day. I keep my bins in a spare room and my wife doesn't complain, which is the real benchmark.
Will it attract pests?
BSFL outcompete houseflies for food resources. A thriving colony is actually one of the better pest-fly deterrents you can have in a composting area. Keep a lid on the bin and you won't have issues.
Can I compost meat and dairy?
In moderation, yes. I toss in small amounts of leftover chicken, cheese rinds, that kind of thing. The larvae handle it fine. I wouldn't dump a pound of raw ground beef in there, that's asking for trouble with rodents and pathogens. But mixed in with other scraps? No problem.
How many larvae do I need?
For a typical household producing 1–2 lbs of food scraps per day, a colony of a few thousand larvae handles it easily. The bin kit comes with a starter culture sized for that volume. Within a month or two, you'll have more larvae than you know what to do with, that's the self-sustaining part.
Do I have to keep the colony going through winter?
Depends on your situation. If you're feeding animals year-round, you'll want supplemental heat to keep things active. If it's just composting and you don't mind pausing, you can let the colony wind down and restart in spring. I keep mine running year-round because I have reptiles that eat prepupae daily, but that's a personal call.
