The Ultimate Guide to Black Soldier Flies

Black soldier fly larvae are not the insect you're thinking of. They don't infest your food. They don't carry disease. They don't bite. And your chickens are going to lose their minds over them.

Hermetia illucens, the black soldier fly, is a saprophyte — meaning it thrives in decomposing organic matter and plays a natural role in breaking it down. What's remarkable isn't just what these larvae eat. It's what they leave behind: themselves (one of the most nutrient-dense protein sources available to backyard chicken keepers), and their frass (a soil amendment that puts commercial fertilizers to shame).

If you keep chickens, maintain a compost pile, or grow anything in the ground, this is worth understanding.

The Black Soldier Fly Life Cycle

The black soldier fly goes through complete metamorphosis — egg, larva, pupa, adult — over a total of about 38–45 days under optimal conditions. Here's what each stage actually means for you.

Eggs

A female lays 200–600 eggs per clutch, sometimes more. She doesn't lay them in your substrate — she lays them in dry crevices above it, specifically seeking out narrow gaps and corrugated surfaces near a food source. This is why oviposition tools like the BSFL Coop use a layered corrugated design: it's mimicking exactly what the fly is looking for. Eggs hatch in about 4 days at 27–30°C (80–86°F).

Larvae (Instars 1–5): The Growth Phase

This is where the magic happens. Larvae go through five instars over roughly 14–21 days under good conditions (25–35°C, 60–75% humidity). During this phase they're doing two things simultaneously: consuming organic waste at an impressive rate and packing on protein, fat, and calcium. A healthy larval mass will generate its own heat, and in dense populations, substrate temperature can exceed ambient by several degrees — worth monitoring.

The larval stage can technically stretch to 30 days or compress slightly under exceptional conditions, but if you're managing your bin well, 14–21 days is the realistic window.

Prepupae (Instar 6): The Harvest Stage

This stage is often treated as part of the pupal stage. It's not. The prepupa is still a larva — 6th instar — and it's the stage with the highest protein content of any point in the life cycle, along with high accumulated fat. It's also the stage designed for self-harvesting.

When larvae darken from cream to brown to near-black and stop feeding, they've entered this phase. They're programmed to migrate toward dark, cool, dry areas to pupate. A ramp at the right angle in your rearing bin lets them walk themselves right into a collection container. No digging, no sorting.

If you want the most nutritional punch for your chickens, prepupae are what you're after.

Pupae: The Transition

After migration, the prepupa forms a hardened outer casing called a puparium and becomes a true pupa. This is a non-feeding, non-moving stage lasting about 14 days at 24–28°C. If you're running a breeding colony, you'll transfer pupae to an emergence container before they hatch — they can chew through a surprising variety of materials when ready to emerge.

Adults: The Breeding Stage

Adult black soldier flies live 8–14 days, with moisture and food supplementation pushing toward the longer end. They have reduced mouthparts and no functional gut, so they don't feed on your food, don't bite, and don't transmit pathogens. Their only job is mating and egg-laying, and they're quite good at it.

Mating is aerial and requires two things almost every indoor colony struggles with: space (minimum 1 cubic meter for a breeding cage) and serious light intensity. Standard shop lights and plant grow lights don't cut it. Adults need full-spectrum light with a UV component at intensities well above what most household fixtures produce — research suggests somewhere in the range of 40–200+ µmol/m²/s depending on spectrum. Below ~23°C, mating largely stops. If your indoor colony isn't producing eggs, inadequate light is almost always the reason.

What BSFL Eat

BSFL are impressively broad in what they'll consume. Fruit and vegetable scraps, cooked grains, coffee grounds, and spent brewing grains are all excellent. They handle meat and fish in moderate quantities. They will eat dairy, though it should be introduced carefully — too much too fast creates anaerobic conditions and odor. Large bones, highly processed foods, and citrus in large quantities are best avoided, especially with a young colony.

The rule of thumb: if it was once food, BSFL will probably eat it. Start with easy-to-process materials and let the colony grow into harder substrates over time.

What BSFL Are Used For

Live Feed for Chickens, Reptiles, and Fish

Prepupae are roughly 40–44% protein and 30–35% fat on a dry matter basis, with a calcium:phosphorus ratio that makes them one of the best naturally balanced feeder insects available. Chickens in particular respond to them with what can only be described as excitement. Reptiles — especially bearded dragons, leopard geckos, and blue-tongued skinks — take them readily. Fish do too.

Composting and Waste Reduction

A thriving bin of BSFL can process kitchen scraps faster than any worm bin, and they do it without the odor problems associated with standard composting when managed correctly. They're not a replacement for a compost pile — they're the first stage of one.

Frass: The Byproduct That's Worth Keeping

BSFL frass — the castings and shed exoskeletons left behind after a processing cycle — is a high-value soil amendment. It contains chitin (which stimulates plant immune responses and suppresses soil pathogens), beneficial microorganisms, and a balanced NPK profile. It's different from traditional compost and behaves differently in the soil. If you're growing anything, don't throw the frass away.

Getting Started

The fastest path to a working colony is a self-harvesting bin setup with a small starter culture of larvae and a consistent food source. You don't need to source eggs or manage breeding until you're ready to scale — starter larvae placed in a well-configured bin will self-regulate, and adults will eventually find the bin on their own in warm months.

The BSFL Bin Kit is designed exactly for this. It turns any plastic storage tote into a self-harvesting composter with built-in oviposition support — no experience required.

Ready to set one up? Read the setup guide.

Common Questions

Do BSFL smell?

A well-managed bin shouldn't. Odor is almost always a sign of overfeeding or too much moisture. Keep the substrate moist but not wet, don't add more food until the previous batch is mostly processed, and make sure you have airflow. Fix those three things and the smell goes away.

Can I keep them indoors?

Larvae — yes, easily. Adults are another story. Breeding indoors requires enough light intensity and space to trigger mating behavior, which most standard indoor setups don't provide. Many hobbyists run larvae indoors and let adults find the bin naturally in warmer months.

What temperature do they need?

Larvae thrive between 25–35°C (77–95°F). Below 15°C they go dormant. Above 40°C they'll die. If you're in a cold climate, you'll need to manage heat during winter months or plan around a seasonal production window.

How do I know when to harvest?

Watch for larvae that darken in color (cream to brown to near-black) and start migrating up and out of the substrate. That's the self-harvesting instinct kicking in. A ramp in your bin channels them directly into a collection container. No intervention needed.