There's a moment in this hobby where you stop thinking about the larvae and start thinking about the flies. Usually it happens when you realize you're buying grubs every spring and you'd rather not.
That's the breeding question. Can you get the adults to mate and lay for you, on purpose, in a space you control? And what does that setup actually look like?
I'll give you the honest version, which is a little less satisfying than the version you'll find on forums.
The two halves of the life cycle need opposite things
This is the thing that makes home breeding harder than home rearing, and it took me a while to see it clearly.
Larvae want warm, dark, damp, and crowded. That's the bin. Shade, always, because an active colony already generates its own heat and a bin sitting in direct sun cooks.
Adults want the exact opposite. They want light, space, and warmth in the air rather than in a substrate. Mating in black soldier flies is light-dependent. Females will not reliably mate without sufficient light intensity, and the research that has managed artificial-light mating used seriously bright lamps, not a shop light on a timer. Natural sunlight does the job, which is why outdoor and greenhouse breeding works and basement breeding usually doesn't.
They also want room. Adults mate in flight. The cages used in research settings are typically measured in cubic meters, not cubic feet, and there's a reasonably consistent finding that mating success drops as cage volume drops. A mesh laundry hamper is not going to cut it, and I've watched people try.
So a real breeding setup is: a large screened enclosure, in strong natural light, held somewhere around the mid 80s Fahrenheit, with humidity on the higher side, water available for the adults to drink, and a separate darker area near the food where females can lay. And then the bin itself, shaded, somewhere else.
That's two climates, one of which is a greenhouse.
Why most home keepers don't do this
Because there's a shortcut, and the shortcut is that the flies are already outside.
In most of the country during warm months, black soldier flies are a naturally occurring species doing this on their own. They find decomposing material by smell, they mate in whatever sunny spot they like, and the female comes to your bin only to lay. She does not need to live there. She needs to get in.
So the practical home version of a breeding setup is not a cage. It's an open bin with an entrance the flies can use and a lay surface waiting for them when they arrive. You are not breeding them. You are recruiting them, and letting the sun and the yard handle the part that's hard to replicate.
That's the design behind the BSFL Bin Kit, and it's why the bin is an open system rather than a sealed one. Wild adults come in through the entrance, lay on the egg tree, and the hatchlings drop into the scraps. You get free feed out of a bin you didn't have to stock.
What you can control on the laying side
Even without a cage, there's a lot you can do to make your bin the most attractive place in the neighborhood to lay eggs.
Smell. Females are drawn to actively decomposing material. A bin that's been running for a while and has some funk to it recruits better than a fresh one. Spent brewing grain, overripe fruit, and food scraps all work.
Lay surface geometry. Females lay in narrow dry gaps, roughly one to three millimeters, positioned near the food but not in it. Too wide and they skip it. Too wet and they skip it. Corrugated cardboard with the channels exposed is the classic free version. It also disintegrates, which is the whole reason I ended up designing Clippie and later The Coop. Clippie holds a lay surface at the right height over the substrate. The Coop is the full system for someone who wants to actually collect and move eggs rather than just let them hatch in place.
Position. Lay surfaces go above the substrate, in the drier zone, not touching the wet material. A clutch that ends up in the muck usually doesn't make it.
Timing. Adults live somewhere in the range of five days to two weeks, and females typically lay once, on the order of several hundred eggs. So the window is short and the traffic is episodic. If you check the lay surface once a week you will miss most of what's happening.
If you do want to build a real breeding enclosure
Some people have the space and the interest, and it's a legitimately fun project. A few things I'd tell you going in.
Go bigger than you think. Volume is the variable that kills small cages. If you're building, build it walk-in or close to it.
Use natural light. A greenhouse, a sunroom, a hoop house with shade cloth. Artificial lighting is possible but you're chasing intensity, not just presence, and the equipment gets expensive fast.
Give the adults water. They don't eat, but they do drink, and adult longevity drops noticeably without a water source. A damp sponge or wick works.
Stock it with pupae, not larvae. You want emergence happening inside the enclosure, and you want enough emerging at once that there are both sexes present in the same few days.
Accept a low hit rate at first. Nobody gets this working on the first build.
I'll be straight with you, I don't run a dedicated breeding cage for the farm right now. The open bin approach plus purchased stock when I need a specific volume covers what I need, and the cage build keeps getting pushed behind things that make more difference. Maybe that changes.
FAQ
Do black soldier flies need sunlight to mate? Yes, or something equivalent. Mating is light-dependent and requires fairly high intensity. Natural sunlight is the reliable option. Artificial lighting can work but needs to be much brighter than typical indoor lighting.
How big does a black soldier fly breeding cage need to be? Bigger than most home builds. Adults mate in flight and mating success drops as enclosure volume drops. Research cages are commonly several cubic meters. A small mesh cage will usually disappoint you.
Can I breed black soldier flies indoors? It's difficult. The light requirement is the main obstacle, and space is the second. A greenhouse or sunroom is a much better bet than a garage or basement.
How many eggs does one female lay? Several hundred, generally laid in a single clutch. Females typically lay once in their adult life, which lasts roughly five days to two weeks.
Do I need a breeding setup at all? Probably not. If black soldier flies are present in your area during warm months, an open bin with entrance holes and a good lay surface will recruit wild adults on its own. That's how most backyard keepers run.
Where do black soldier flies lay eggs? In narrow dry crevices near decomposing material, not in the material itself. The hatchlings drop down into the food after they emerge.
If you're deciding where to put your effort this season, I'd put it into making your bin easy to find and easy to lay in, before I'd put it into a cage. The sun is free and the flies are already out there.
