Grubage bin

BSF are an ideal way to get rid of putrid wastes, like dairy products, meat, manure, and rotten fruits.  

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For serious composters, the black soldier fly (BSF), hermetia illucens, is a miracle insect. The BSF larvae (grubs) love putrid wastes, and are highly efficient at composting them. Grubs can reduce the mass of waste by 95%, leaving a fine compost behind. They don’t like competition, so they give off a distinctive odor to repel other flies. Adult BSF don’t eat these wastes, so they won’t be bothering you at the next backyard BBQ.

Once the grubs are ready to pupate, they climb out of the compost bin (as long as the ramp is 30-45 degrees), and drop themselves into a bucket, a meal fit for any number of small livestock, like chickens, fish, lizards, quail, and even pigs. They’ll convert 5 pounds of waste into 1 pound of high protein grubs, and they’ll do it faster than you would believe. BSF larvae like a nice, quiet place get their grub on, so we made them a bin in the shade. These guys help us process animal manures and some of the “trickier” wastes around the homestead.

Materials & Tools

Materials

55 gallon food grade plastic barrel
6 ft 1×8 lumber
8 ft 1×2 lumber
12” corner molding
6 ft x 3 ft sheet metal
1 1/2” wood screws
3/4” wood screws
1 1/2” 45 degree PVC elbow
Short piece of 1 1/2” PVC pipe
1 Gallon collection bucket with lid
Silicon caulk
Gravel and loose bedding (sawdust, chopped straw, shredded paper, etc)
1 ft by 1ft cardboard

Tools

Circular or jig saw
Cordless Drill
1/8” drill bit
2” hole saw
Tin snips
Tape measure
Stapler and staples

Step 1: Barrel

Start by measuring the length of your barrel. It should be about 34 inches. Now, mark the middle of the barrel at 17”. Slowly turn the barrel, and make marks around its center.
Using a circular saw, cut the barrel in half. These halves will be your grubage bins!

Step 2: Harvester

Mark the 6ft 1×8 board at 10” and 20”. Cut along these lines. These two 10” pieces will become the top of the ramp box.
Cut two 16” pieces from the remaining 1×8. The sides of the ramp will be made out of these.
Mark each of these 16” pieces at 8” on one long edge, and 14” on the other long edge. Make a line between these marks and cut the boards along the lines. You will now have 4 side pieces, 2 smaller and 2 larger. Pair up one small with one large piece for each side (the bottom of the sides should follow the angular cut).
Cut a 20” piece from the remaining 1×8. This is the bottom of the ramp.
Drill 1/8” holes along the slanted edges of the side boards. Each of the 4 boards should have 3 holes.
Lay out the side boards (see image). Using 1 1/2” wood screws attach the sides to the bottom board. The bottom should be arranged so that it follows the slanted edge of the side boards.
Drill 1/8” holes in the top boards. 4 holes per board should be sufficient. Using 1 1/2” wood screws, attach one top board only, line it up with the large side boards, so it spans across them. The top board should be arranged with it’s 10” length going perpendicular to the direction of the ramp/bottom board.

Step 3: Exit Hole

At the top edge of the bottom board, make a mark 2” from the edge, in the center of the bottom board. This will become the exit hole.
Using a 2” hole saw, make a hole centered on the mark.
Place the 1 1/2” 45 degree PVC elbow in the hole from the bottom. Rotate the elbow so that its unattached edge points vertically down towards the ground. Use a 3/4” wood screw to attach the elbow to the bottom of the ramp from the inside. You can use a piece of 1 1/2” PVC to connect the elbow to a collection bucket.
Cut 2 5” pieces of corner molding. With each of these pieces, line them up so that they go from the edge of the ramp to just above the center of the exit hole. Trim their ends so that they sit flush against the ramp’s side boards.
Drill 2 pilot holes in each molding piece, and attach them to the bottom boards. These will serve as guides to direct the BSFL towards the exit hole.

Step 4: Ramp

Find a seam line on the barrel half that runs vertically. Along this seam, make a mark at 1” from the cut edge of the barrel half, and another mark at 13”. Draw a line between these marks.
Make a mark at 8” from line, 1” from the cut edge of the barrel. Now, make a mark 8” from the line at 13” from the top. Draw a line between these 2 marks. Also, draw lines between the two 13” marks and then between the two 1” marks. You should now have a rectangle outlined on the side of the barrel.
Using the circular saw, cut out the rectangle opening.  4d. With a file or grinder, trim a slight curve on the bottom board where it will connect to the barrel. This allows for the slight curve of the barrel.
Take the ramp box and place it up against the barrel opening. Starting at the top, use 3/4” wood screws to attach the barrel to the box. Make sure the box is firmly attached to the barrel.
Cut a 26” piece of the 1×2 lumber. This will serve as a barrel support, helping the cut part of the barrel keep its shape. Insert one end of the board into the ramp box, and bring the other end tight against the opposite side of the barrel. Using 1 1/2” wood screws, attach this board to the side and top of the ramp box. Using 3/4” screws, attach the opposite side of the barrel to the end of the support board, position the board about 1 1/2’” below the top of the barrel.

Step 5: Roof

Cut a piece of sheet metal into a circle with a diameter of 30”. This will be the roof.
Cut the remaining 1×2 into one piece 22” long and 2 pieces 10” long. Make a cross out of these boards, centering the 2 shorter boards perpendicular to the 22” board. Mark the placement of the 2 shorter boards on the 22” piece. Drill 4 pilot holes in the 22” piece at the marks. Using 1/ 1/2” screws, assemble the roof cross.
Place the circle of sheet metal on the ground, and lay the roof cross on top. Draw a circle on the sheet metal that is about 2” larger than the cross. Every foot around the edge of the sheet metal, make a line towards the circle. Using tin snips, cut along these lines.
Create a lip around the edge of the roof by folding the strips, first towards the center, and then down to leave a safe edge on the roof. Periodically check the fitting on the roof on the barrel, and adjust your folds.
The lid should fit well, but allow a gap around the outside of the barrel so that adult female BSF can enter and lay eggs. It should keep the inside dry, but ventilated. Make a handle for the roof with a bit of hose or strips of rubber.

Step 6: Finishing Touches

Run a bead of silicon along the inside edges of the ramp. Make sure there are no gaps that grubs could get stuck in. Once your silicon is dry, place the other top board from Step 2a on the end of the self harvester.
Cut a 12” length of cardboard into 1.5” strips. Staple these cardboard strips to the side of the support board from Step 4f. These will serve as egg strips for female BSF.

Step 7: Setup

Your bin is ready to setup. Find a shady, calm spot to place your bin.
Drainage is essential to the proper function of your grubage bin. You can fill the bottom of the bin with a layer of gravel 3-4” deep, but this makes it heavy should you need to move it. We used old plastic bottles as our drainage filter.
On top of the gravel, place an absorbent material, like sawdust, chopped straw or grass to serve as bedding. This layer should be 4” deep. Create a depression in the center and place some fresh, rotting compost.
If you have a starter set of grubs from a friend, put them in the compost and close the lid. Check on them every few days and add compost and putrid wastes, starting with small amounts.
If you can’t find a source of grubs, then you can attract the female BSF to your bin using various baits, like sour corn, wet dog food, etc. If you live in climate zones 6-10, it is likely that you have a native population of BSF.

Other flies may inhabit your bin until your BSF population gets going. It helps to keep a cover material of chopped straw on top of the compost to prevent these other flies. Once your colony gets going, you can increase the amount of waste added. The barrel should be able to process up to 10 pounds of waste a day at maximum grub occupancy.

Initially, it is best to let the grubs pupate to increase the population of egg-laying females in your area. Once your bin has been running for while, and there are eggs in the cardboard strips, you may start harvesting your grubs for your animals. Set up a collection bucket at the drop pipe. The bucket should have a lid, and some bedding, like sawdust to keep the harvested grubs happy. Collect the grubs every day and give them to your animals.