A self-harvesting BSFL bin is a bin that uses a behavior black soldier fly larvae already have. When a larva is done eating and ready to pupate, it stops feeding, turns dark, and goes looking for somewhere dry to crawl off to. That stage is called the prepupa. In a bare tub, prepupae climb the walls and end up on your patio or in the grass. In a self-harvesting bin, you give them a path, usually a ramp, that leads out of the substrate and into a container. They harvest themselves. You just empty the container.
That's the whole idea, and it's a good one. I wrote a longer post on harvesting if you want the biology. This one is about the part the ramp doesn't solve.
The ramp works, until a batch goes all at once
Prepupae will find a ramp. Most of them. The trouble is that larvae from the same egg clutch mature at about the same time, so you don't get a trickle, you get a wave. When a few hundred of them decide to leave on the same warm evening, a lot of them don't bother looking for the ramp. They go straight up whatever wall is closest. With a plain tote and a ramp, I'd find prepupae on the floor around the bin the next morning and I'd have no idea how many I'd lost.
I didn't want to give up the ramps, they work and they're the right idea. I wanted something that caught the wall crawlers too.
The Grub Track
In the Grubhouse, the colony tub sits inside a track that runs the whole way around its outside edge. Two ramps run out of the tub onto the track. A prepupa that climbs a ramp lands on the track. A prepupa that climbs the tub wall and goes over the rim also lands on the track. Either way, it follows the track around to a single drop hole with a deflector on it and falls into the collection tub on the shelf underneath.
I verified the wall-crawler part on the prototype before I'd write it down. The ramp climbers were never the question. It was whether the ones going over the rim would actually end up on the track and keep walking instead of dropping off the outside. They do. Everybody ends up in the same tub.
Why it's a separate tub underneath
The collection tub is the second tier of the Grubhouse, on the shelf below the colony. Keeping the prepupae separate from the colony matters for a couple of reasons. They're dry and done eating, so they don't want to be in the wet substrate anyway, and if they're left in there they'll pupate in the corners and you'll be fishing out adults later. In their own tub you can scoop out what you need for the chickens, or let a batch pupate to start the next generation.
What this doesn't fix
A self-harvesting bin only harvests larvae that are healthy enough to mature. If the bin's too wet, too hot, or going anaerobic, you won't get a wave of prepupae, you'll get a die-off or a mass exodus of larvae that aren't ready yet, which is a different problem. I've got posts on why larvae leave early and what to do about a wet bin. The Grubhouse's shade-cloth walls are my answer to the moisture side of that, but that's its own post.
If you've got a tote and a ramp and it mostly works, that's fine, keep going. Just count what ends up on the floor some morning after a warm night. That number is what the track is for.
