If you found a big dark fly hanging around your compost pile or your garden, and it looked a little like a wasp but moved kind of slow and clumsy, there's a decent chance you were looking at a black soldier fly. They turn up where things are breaking down, and if you keep backyard chickens, compost your kitchen scraps, or just spend time outside in the warm months, you've probably crossed paths with one without knowing it.
So here's the plain version of what it actually is.
The short answer
The black soldier fly is a true fly, species name Hermetia illucens. It's in the family Stratiomyidae, which is the soldier fly group, and it's native to the Americas. Over the last century it spread to most of the warmer parts of the world, partly on its own and partly riding along with people. The adult is the part most folks see. It's the larvae, the fat little grubs, that do all the work everyone in this niche gets excited about.
The adult fly is maybe 15 to 20 millimeters long, dark, almost black, with a slightly metallic look in the right light. It has two clear little windows on the first segment of its abdomen, which is one of the easiest ways to tell it apart from a wasp once you know to look. It does not bite. It does not sting. And here's the part that still kind of gets me, the adult doesn't really eat at all.
Wait, it doesn't eat?
Right. This is the thing I was floored by when I first started reading about them.
When a black soldier fly is in its larval stage, it eats constantly. That's its whole job, it packs on fat and protein from decaying organic matter. By the time it becomes an adult fly, it has stored up enough energy to live the rest of its life without another meal. Adults have vestigial mouthparts, which is a fancy way of saying the mouth is basically there for decoration. They live off what they banked as larvae.
That short adult life, usually somewhere around 5 to 14 days, is spent doing one thing, finding a mate and laying eggs. The females, the mamas, lay their eggs in dry cracks and crevices near a food source, not in the food itself. Then they're done.
The life cycle, start to finish
The whole thing runs through four stages.
First, the egg. A female lays a clutch of eggs, often several hundred, tucked into a dry gap close to something decomposing. Those hatch in roughly 4 days.
Second, the larva. This is the long, hungry stretch. The tiny larvae drop onto the food and start eating, and they keep eating and growing through several molts. How long this takes depends a lot on temperature and how much food is around. In warm conditions with steady feed it can be a couple of weeks, in cooler conditions it stretches out. They mostly stop developing once it gets down around 12 degrees Celsius, which is about 54 Fahrenheit, so cold really slows them down.
Third, the prepupa. When a larva is done eating, it changes color, empties its gut, and crawls away from the food looking for somewhere dry to finish up. This wandering instinct is actually really useful, it's what lets a good bin design let them harvest themselves.
Fourth, the pupa and then the adult. It hardens into a pupa, sits still for a while, and eventually the adult fly emerges to start the whole thing over.
Is the black soldier fly dangerous?
Short version, no, and this is one of the best things about them.
Houseflies are a problem because the adults land on garbage and then land on your food, which is how they move germs around. The black soldier fly adult doesn't do that, because it isn't feeding. It's not crawling around on rotting stuff looking for a meal, it already ate, as a larva, and now it just wants to mate. It is not considered a disease carrier the way houseflies are. It doesn't bite people or animals, it has no stinger, and it isn't aggressive. Honestly the adults are kind of slow and easy to catch.
There's a longer write up on the safety question coming, but that's the gist. If anything, having black soldier flies show up is usually a sign your compost is healthy and active.
Why anyone cares about a fly
Here's where it stops being a fun fact and starts being useful.
The larvae are one of the most efficient organic recyclers we know of. They'll eat food scraps, spent grains, overripe produce, and a lot of other organic waste, and they do it fast. What they leave behind is the larvae themselves, which are roughly 40 to 50 percent protein and 25 to 35 percent fat on a dry weight basis, plus a crumbly leftover called frass that works as a soil amendment.
So depending on what matters to you, this one creature checks a lot of different boxes. If cutting down on the waste you send to the landfill is important, or if feeding your chickens or reptiles without buying bags of feed is important, or if making your own compost and frass fast is important, the black soldier fly is doing all three at once. That's the part that pulled me in and never really let go.
If you want to actually raise some yourself, the easiest way to start is a contained setup like our BSFL Bin Kit, which gives the larvae a place to work and gives you a way to collect what they produce without digging around in the muck.
Frequently asked questions
Is a black soldier fly the same as a wasp?
No. It's a fly, not a wasp, it just mimics that look a little, which probably helps keep predators away. It has no stinger and it won't sting you.
Are black soldier fly larvae the same as the ones you find on garbage?
No. They're fly larvae, but they're a completely different animal from the housefly larvae most people picture. They don't infest your food, they don't carry disease, and they're raised on purpose for feed and composting.
Where do black soldier flies come from?
They're native to the Americas and are now found across most warm regions of the world. They tend to show up on their own wherever there's decomposing organic matter in the warmer months.
How long does a black soldier fly live?
The adult lives roughly 5 to 14 days. Almost the entire life span before that is spent as a larva, which is the stage that does the eating.
Do black soldier flies bite or carry disease?
No on both. Adults don't feed, so they aren't moving germs around like houseflies, and they have no biting or stinging parts.
Can I raise black soldier fly larvae at home?
Yes, and you don't need much. A bin, some kitchen scraps, and a starter colony will get you going. If you'd rather not improvise the whole setup, a BSFL Bin Kit covers the parts that are annoying to build yourself.
That's the black soldier fly. A slow, harmless, kind of charming fly that spends its short adult life not eating, after spending its larval life eating just about everything you'd otherwise throw away.
