Last year we built an outdoor bin for composting by worms and black soldier flies. Both can be fed to fish and chickens and, in our case, would be fed to the tilapia in our aquaponics system as well as our chickens.
The bin was constructed of cedar boards we had from the old grow bed in the greenhouse before we converted the greenhouse to an aquaponics system. I wouldn't have used fresh cedar boards since the oils in cedar repel insects. But, in this case, the boards had been used for almost two years in the greenhouse and then set outside for another year or more before they were used.
The bin is designed with two equal sections to allow for composting in one side at a time. We put hardware cloth on the bottom and in between the two sections. We then used a layer of shade cloth (something we had on hand) at the bottom to keep the finished compost from falling out the bottom.
We cut a piece of PVC pipe in half lengthwise and then using heat, bent the top as seen in the pictures. This allowed the pipe to run down into the compost and be secured at the top of the bin. A small cutout was made in the side for the pipe to set down into. The lid is enough longer than the main bin to allow for buckets to set at the ends of the bins to catch the BSF larvae as they climb to the top.
Having used it for a season, the following are the design changes I'd make.
- The pipe should be set further down into the side wall by another inch or two. Many of the BSF larvae went into the bucket but a number of others climbed onto the sides and were crushed by the lid. I have no idea how many simply crawled to freedom. The other possibility would be to have a 45 degree angle elbow and have the top portion of the pipe be complete so it forms an actual tube for he BSF to climb into, forcing them to go into the bucket.
- The lid should be water impervious. We did try to seal between the boards, however, water still gets into the bins and the buckets. This makes it harder to control the overall environment.
- The entire bin is built so it is up off the ground by about 4" (10 cm). This may have contributed to the death of the BSF larvae over the winter. We had temperatures around 17 degrees F (-8C) for almost a week. This spring all of the BSF larvae were dead. I'm assuming in a more protected environment they would have survived the winter and resumed activity when the weather warmed up.