DIY Compost Bin with Composting Grubs

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DIY Black Soldier Fly Compost Bin

What if you could have a compost bin that could do way more than just process your kitchen scraps? What if, for example, it also produced liquid fertilizer and feed for your chickens, reptiles, and other pets? If that sounds too good to be true, we’ve got great news. You can accomplish all that with hardly any effort by making a compost bin designed specifically for black soldier fly larvae. It’s cheap, easy, and does the hard work itself. Let’s find out how.

What are Your Composting Goals?

With our black soldier fly compost bin, or “soldier fly farm,” we want to accomplish a few things. We need to:

  • Lure in adult soldier flies. This means our compost bin needs to be protected from the weather. It also needs to tell the flies that the bin is where they should lay their eggs.
  • Create a place for adult black soldier flies to lay their eggs. This can be done with very cheap materials.
  • Use black soldier fly larvae to decompose kitchen scraps (including meat and dairy products) and yard waste. These scraps are the fuel that enables our soldier fly farm to work.
  • Collect large soldier fly “prepupae” to use for chicken or bearded dragon food feed
  • Collect worm “juice” to use as liquid fertilizer.
  • Maintain warm, moist conditions in the bin. We’re looking for 81-99 degrees to promote egg laying by the females and 70% humidity to encourage the larvae to emerge.

Parts to Build the Compost Bin

To build your black soldier fly compost bin, you’ll need a few cheap materials. First, we need a container. This can be a dark plastic storage tub, a bucket from the hardware store, or even a homemade wooden frame. Inside this, we need to build both a trap for the prepupae and a drain for liquid fertilizer. We also need to provide a way in for the queen. We can accomplish both these tasks with some PVC piping. Sizes 1 1/2” or 2” should work fine. We’ll also need some T pieces and elbows.

Once inside, the female black soldier fly will look for a place to lay her eggs. A piece of corrugated cardboard will work perfectly for this. We’ll also need some wire to hang it. When the prepupae migrate away from the compost, they’ll travel up the ramp and out of the bin. They just need a place to go. You can use a milk jug or a bucket to catch them.

For tools, we’ll keep it pretty simple. A drill, 1/4” bit, and a hole saw the same size as your PVC pipe. We’ll also need a hand saw and a file. Lastly, you’ll need some black soldier fly larvae to get your farm started. 

How to Build the DIY Compost Bin

Container

How you assemble your compost bin is up to you. There are lots of approaches to building one. We’ll use an example design that can be easily modified, or you can take the basic principle and do it differently. We’re going to build ours inside a black plastic storage tub with PVC ramping. This will help to maintain the temperature and make our bin portable.

Using the hole saw, cut a hole in the lid. Then, cut two holes in the body of the bin, one in the center of one side just under the lid and another on the opposite side near the bottom. Fit small pieces of PVC into a T piece and insert it into the hole on top. This is where the females will come in. Directly under this, use wire to poke a hole in a square of cardboard roughly 4x4”. Bend the wire so the cardboard doesn’t fall off and suspend it 4” below the entrance.

Tubing

Next, cut some holes in your PVC piping with your hand saw and file down the edges. The holes should be shallow, but big enough for larvae to get into. We want the holes to be facing up when we lay the PVC into the bin. This is how the prepupae will enter the ramp and migrate when they’re ready. Arrange the PVC pipe using elbows and T pieces so that one pipe goes up either side of the bin, follows the corner, and meets back up at the exits. It should look like a rectangle sitting on an angle in the bin.

Where the PVC meets up on either side, the T pieces will pass through the holes you drilled. On the juice (bottom) end, use an elbow piece facing up to catch liquid fertilizer. On the larvae (top) end, use an elbow piece pointing down into your larvae catcher.

Grubs To Gold 

And that’s about it! Now you just need larvae and compost. The larvae will eat like crazy, grow, and migrate away up the PVC tubes into your trap. You can use them as feed. They’ll create liquid fertilizer that you can use in a spray bottle. Adult flies will return, lay their eggs, and repeat the cycle. All you have to do is keep them fed and reap the benefits.

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