How to Build a Black Soldier Fly Compost Bin
Building a black soldier fly compost bin will enable you to sustainably eliminate kitchen waste while providing tasty treats for your backyard chicken flock.
I’m constantly looking for ways to feed my livestock — chickens, rabbits, and pigs — at a minimal cost. Keeping overhead expenses down on the farm means I make fewer trips to the feed store, and I can pocket more money at the farmers market. But, for me, it’s more than that. It’s about closing the loop by keeping inputs within my farm and reducing waste.
I’ve found several ways to achieve a cost-free, closed-loop system to feed my farm animals. Using local spent brewery grains and picking up older produce from a nearby organic grocery store have lowered my feed costs. I also grow my own duckweed, which is high in protein, as a feed supplement.
Although, I think one of my favorite methods has been taking my kitchen scraps to build compost, which I use to grow black soldier fly larvae. During warmer months, these protein-packed grubs are a constant supply of food for my laying hens that free-range around the farm.
What Are Black Soldier Flies?
Black soldier flies aren’t your typical housefly; they aren’t vectors of disease, and they certainly aren’t pests. That’s because adult black soldier flies don’t have mouthparts to feed on waste, whereas houseflies are attracted to food and can spread illness as they flit between food sources. Instead, the soldier flies, which are more similar to mud daubers in appearance, are attracted to food that’s already beginning to decompose so they can lay eggs on it.
The insects lay their eggs on decomposing organic matter, such as kitchen waste, and the larvae soon emerge to feed. You’ve likely seen these whitish grubs before in your compost pile and maybe considered them repulsive maggots. However, the larvae are harmless, functioning much like earthworms in that they decompose organic matter quickly and efficiently. In fact, as the larvae feed on waste, they create an environment that’s inhospitable to pest flies seeking to lay eggs, which reduces their numbers, according to an article by the Texas Master Gardener Association.
In his essay “Black Soldier Fly & Red Worm Bioconversion,” Paul Olivier of the nonprofit Empowering the Poor through Waste Transformation writes that the larvae “can effect as much as a 20-fold reduction in the weight and volume of food waste in a period of less than 24 hours.” Olivier goes on to note that the larvae can eat as much as 40 grams of food waste per day in a square meter.
How Does It Work?
The idea behind a black soldier fly bin is simple: Fill a bin with kitchen waste, wait for black soldier flies to find the cache, and then you’ll eventually have grubs. You can also purchase live larvae, but I have yet to have a problem attracting bugs to my bin that deposit their eggs. And the beauty behind a bin is that, because of its construction, you won’t have to dig through decomposed food to find the grubs. The grubs will gladly crawl to you.
When the larvae turn into prepupae and have finished feeding, they’ll have the instinctual drive to climb from the decomposing organic matter. By installing a tube inside the bin, the grubs will take the high ground and fall out the other side, where a collection bin will be waiting. Along the way, most of the decomposing material will have been cleaned off, leaving you with snacks that are ready for your flock.
Black soldier flies are most active in warmer months, as are most insects. I’m lucky here in southern Louisiana, where warm weather abounds almost year-round. I can benefit from soldier fly larvae as much as eight months out of the year by just keeping the bin outside. However, even in a colder climate, the bin can still be a useful tool. Some farmers in frigid climates maintain their stock of soldier flies by keeping bins in a greenhouse or by insulating them.
Constructing the Bin
Tools and Materials
- Utility knife
- Power drill and 3/8-inch bit
- Tinsnips (optional)
- 18-gallon tote box with lid
- Cardboard scraps
- Small box of paper clips
- Roll off hook-and-loop tape
- 5-gallon bucket with lid
- Old aluminum gutter or PVC pipe
- Small metal screws or PVC elbow
There are plenty of ready-built soldier fly bins out there that you can purchase, and a host of designs that will, more or less, give you the same result. That being said, my soldier fly bin (see Photo A) isn’t the only way to do it, but I’ve found it a cheap, easy, and effective method. Most of my materials were already lying around the house or my farm, and the remainder cost less than $20. Be creative when looking for materials, and you could construct your bin for close to nothing too.
First, prepare the bin. Using a utility knife, cut a hole on one of the narrow sides of the tote box near the handles, preferably toward the top of the bin. This is where the gutter or pipe will sit. The pipe will need to be at a 45-degree angle, so test its slant before making the cut. The reason for this angle is that the prepupae will be able to climb up easily, while the larvae in the organic matter won’t, keeping them feeding below.
Using a drill, make holes along the top of the bin, about 2 inches below the lid. Space them every 4 or 5 inches. These holes will allow the soldier flies to enter the bin to lay their eggs, but will keep out larger insects or rodents.
Hang cardboard scraps with paper clips from the holes you’ve drilled. These should be suspended inside the bin, yet well above the bottom. The soldier flies will be encouraged to lay their eggs in the corrugated cardboard where it’s dry. When the larvae hatch, they’ll drop down into the organic matter below.
Attach the hook side of the roll of hook-and-loop tape around the inside of the bin, a few inches under the holes you’ve drilled (Photo D). Then, right under this first layer, attach another. This is a barrier for any prepupae that may climb on the walls of the bin looking for an escape. The expanse of tape will stop the grubs, but from time to time, the tape may need to be cleaned to keep it effective. Put a strip of hook-and-loop tape around the inside of the 5-gallon collection bucket too, just in case any prepupae attempt an escape.
If you’re using a section of gutter, use tinsnips to cut a V-shaped wedge out of it, about 6 inches from one end and far enough from the other to allow it to run into the bin. Bend the gutter to make an elbow, and use a small metal screw on either side to hold it in place (Photo E). Alternatively, if you’re using PVC pipe, use a PVC elbow to attach a 6-inch length to the longer piece that’ll run into the bin.
Position the gutter or pipe inside the bin at a 45-degree angle. Ensure that the bottom of the pipe makes contact with the bottom of the bin. I like to place wood chips, leaves, or some soil at the bottom for a base (Photo B). Then, seal any spaces between the gutter or pipe and the opening in the bin. Silicon will close up any gaps where the climbing prepupae might escape.
Stick the exposed end of the pipe over the edge of the bucket, and snap the bucket lid on as far as it will go (Photo C). Since there’s decomposing food in the bin, I keep it away from the house to avoid any smells.
Check on the progress for a few days, and eventually, you’ll see larvae eating the food scraps. Check the collection bin every day to confirm the grubs are indeed finding their way to the 5-gallon bucket and not escaping. Typically, you’ll be able to tell where the prepupae have escaped, because they’ll leave a muck trail.
Another issue I’ve noticed is the appearance of fire ants, which are invasive in the southeastern United States and can be attracted to the food scraps. At times, I’ve taken to placing the bin in a pan of water to ensure the ants can’t access the food inside the bin.
After a while, you’ll have a self-sustaining population of black soldier flies. If your chickens are anything like mine, they’ll be watching for the white bucket that holds the grubs every morning, waiting on their protein-packed snack.
Jonathan Olivier is an independent journalist who primarily writes about the environment and how humans interact with the natural world. His work has appeared in Outside, Backpacker, Mother Earth News, and other national publications.
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