How to Start a Black Soldier Fly Compost Bin Indoors
Once you get started composting with black soldier fly larvae (BSFL), you’ll wish you had done it sooner. These little guys are the ultimate composting machines. They take about as much upkeep as a normal compost bin and they can compost way more of your kitchen scraps. Plus, they’re stuffed to the gills on carrot peelings, you can feed them to your pets!
In this article, we’ll talk about how to go micro with an indoor black soldier fly compost setup that’s perfect for small living or winter composting. Let’s get into it.
Building a Home for Black Soldier Flies
The classic design for a black soldier fly compost bin involves a big plastic storage bin where you put your compost. The bin has a ramp inside that allows the pupating larvae to climb up out of the compost and through an opening. Once out in the world, they fall into a bucket, where you can collect them.
Indoor BSFL compost setups usually use a modified design of open plastic bins that are long, wide, and shallow. You can slide these bins into shelves in your house, which will contain the larvae and help keep their humidity up.
You’ll need at least two bins: one to hold your larvae and another to sift out their frass. More on this in a minute.
As the larvae grow, you’ll continue sifting them through larger and larger sieves. When they’ve reached full size and start to slow down, you can either harvest them or separate them out to let them pupate into flies.
Conditions Inside a BSFL Compost Bin
Unlike a lot of feeder insects, black soldier flies prefer slightly warm, humid (though not swampy) conditions. They are drawn to the heat and moisture of compost. They also like things dark. Inside the compost bin, you’re shooting for 70-90°F and 60-70% humidity.
How to Keep Black Soldier Flies Indoors
So you’ve got your indoor BSFL bins, you’ve got some compost, and you’re ready to start bug farming. Let’s start at the beginning and walk through every stage in a black soldier fly’s life cycle.
Hatching BSFL Eggs
In total, black soldier flies live 40-45 days. They begin life as an egg, laid by a full-grown fly into an “eggy”, which is any structure you make that looks like where the adult wants to lay its eggs. The easiest way to make an eggy is to stack several sheets of cardboard together and bind them between two pieces of wood.
Then, you suspend the eggy over a compost bin. The tiny crevices in the ends of the cardboard, paired with the sweet stink of compost, will draw in adult female flies, who will deposit their eggs. There are some great tutorials out there with more detail on how to do this step right.
Black soldier flies can detect rotting food waste from up to a mile away, and will seek out spots to place their eggs where the larvae will have plenty to eat. You can actually start your colony like this by attracting wild females to donate some eggs.
The Larval Stage
After the female sets her eggs up in the eggy, they fall into the compost. They’re too small to see at this point, but don’t fret. They’re already doing their thing. In four days, the eggs will hatch.
Once they do, you’ll start noticing tons of tiny larvae eating like crazy. The larvae grow quickly, with a major milestone at the 10-day mark, when they have almost reached full size. Their bodies will darken as they grow until they’re a dark brown color. This is the stage where you’ll most likely start your colony, with a shipment of ready-grown BSFL.
Sifting Frass and Keeping Odors Out
As your larvae grow, their main job is to eat food and make frass. Frass is basically bug poop, which comes in the form of highly aerated, nutrient-rich powder that works like magic at making plants grow. Frass also contains ammonia, which is the source of “bug odor”.
So the key step for you at this stage is to sift your larvae every day or so. You will need three sieves, made from different size mesh and window screen frames. You’ll use a 1mm mesh sieve for the first ten days to separate out your young larvae.
As they get larger, you’ll move on to sifting out their frass with a 3mm sieve. This is just big enough to let frass pass through without the larvae falling into it. When they leave behind uneaten food, you can use a third sieve with 5mm mesh to separate your larvae from big uneaten pieces. Removing these regularly will help keep the smell down further.
Keep sifting your larvae using the 3mm and 5mm screens, and you’ll be surprised how little smell they produce. As they start to pupate, you can harvest them while sifting their frass and either use them for bearded dragon food, or let them turn into flies.
Starting the Process Over
Once they start to pupate, they will turn a dark color and move away from the heat of the compost. Their bodies will harden and they’ll undergo metamorphosis into full-blown flies.
After pupating, the flies emerge and are ready to reproduce. Black soldier flies don’t look much like a common house fly, in fact, they look more like bees with black bodies. If you’re trying to repeat the process and collect more eggs, make sure the flies emerge inside a mesh lepidtarium.
Inside the lepidtarium, you want a container of compost and an eggy. The flies will lay eggs, which you can then collect. After they’ve done their job and donated some more offspring for you to compost with, you can release them. Black soldier flies do a great job of keeping common house flies away from your home, and they’re good neighbors to have around. And just like that, the cycle begins all over again!