Black Soldier Fly Composting Bin

Using a compost bin is an excellent way to reduce your garbage bill, keep waste out of landfills, and create beautifully rich soil for your gardens and planters, but with a few modifications we can turn a compost bin into a nutrient powerhouse that produces not only rich soil, but a rich "worm tea" as well as grubs that are a great protein source for chickens and fish in an aquaponics system.

The next step beyond a compost bin in a worm bin. By adding worms to your compost bin (typically red wigglers) you can speed up the composting process, and by adding a collection jar below the bin you can collect the nutrient rich "worm tea" that is produced from the decomposition process. The picture to the right shows some worm tea that was collected over a 24 hour period. This stuff works great as a fertilizer. This is your home made Miracle Grow. It can be diluted or sprayed directly on leaves or around roots, or it can be added directly to an aquaponics system. Beware, this stuff is really rich in nutrients. You want to be sure you do not overwhelm your aquaponics system with a lot of nutrients all at once, especially if you already have worms in your media beds or if your system is still young. As an aquaponics system matures, it will become more capable of processing large quantities of nutrients.
Worm bins are great, but you can still do better. With a few more modifications, you can convert your worm bin into a black soldier fly (BSF) bin. Black soldier flies have several advantages over worms, including the following:

• Black soldier flies are faster at composting than worms
• Black soldier flies excrete a pheromone that keeps away houseflies, maggots, bees, white flies, gnats, and other pests
• Black soldier flies are able to compost dairy products, meat, fats and oils, grains, and citrus while worms cannot. 
• It is generally recommended that your bury and turn a worm bin to prevent rodents and pests, while black soldier flies are so fast at composting that this is not necessary.
• Worms require a temperature range of 54-84, while black soldier flies are fine at any temperature between freezing and 100 F.
• Worms need to be harvested periodically because their castings are toxic to them.  Black soldier flies are self-harvesting!

Self harvesting you say? Yes! To understand how this works, let's take a look at the life cycle of a black soldier fly. Black soldier flies begin as as egg, that takes 102-105 hours to hatch. They emerge as a larvae that rapidly consumes all food available. A worm colony in a typical composting bin can consume about 15 lbs of food in a week. A black soldier fly colony in a similarly sized bin can consume 11 lbs PER DAY! After about 2 weeks the larvae reach maturity. If the temperature is not ideal or there is not enough food, they can take several months to mature. At this point, their mouth is replaced by an appendage that helps them move around better. Once they are mature, they do not eat again. This is why adult black soldier flies are not attracted to picnics, they won't try to fly into your home in search of food, and they don't spread disease. Unlike common houseflies, which live for 30 days and must consume food to survive, black soldier fly adults only live for a few days. However, before the larvae makes its transformation into a flying insect it tries to get away from the food source and tries to find a safe place to pupate. This is where the self harvesting comes in. The BSF bin has been designed such that as the larvae try to escape, there is only one way out... and this drops them into a jar where they can be collected and fed to the fish or chickens. In the picture above, you can see a washing machine hose with a cut-out milk jug handle that acts as a funnel. The washing machine hose is ribbed on the inside, so the black soldier flies can easily crawl up. Black soldier flies, however, cannot climb up the steep, slippery, and partially inverted walls of the barrel. Notice also that there is a vent installed in the bin. On the inside of the vent, there is a piece of cardboard wrapped around the pipe. This is done to encourage wild flying black soldier flies to lay their eggs in the bin. Adult black solder flies live for one purpose. They mate up in the trees, then find a place to lay their eggs. They are drawn to locations where black soldier fly larvae are thriving. When they are ready to lay their eggs, they prefer the tight pockets formed in the cardboard. Once the eggs are in the cardboard, this piece of cardboard can be added to the bin to expand the population or it can be used to seed another bin.

There are black soldier fly bins commercially available. A popular design is the BioPod. It costs $179 and has a collection bin for the larvae that crawl off and a section to collect the worm tea, however, you can build your own for a fraction of the cost and create something that you will probably be much happier with in the end. The design shown above has been very affordable and holds much more material than the commercial model. The design starts with a blue 55-gallon barrel (search http://www.craigslist.org for these), two cinder blocks (Home Depot), a two 2" UniSeals, three 2" PVC pipes (one for vent, two for drain), a piece of cardboard, 2 hinges (riveted to barrel), a 2" 90-degree bend PVC, a washing machine hose, a milk jug handle, a piece of screen material, a 2" PVC union, and a mason jar with lid. The picture to the right shows how a piece of screen material is placed in the union at the bottom of the barrel to keep un-composted material out of the worm tea. Links to most materials are shown below. The measurements are not precise, so I haven't included them here. Basically, just go with what looks good. I found a 1" hole in the barrel and the mason jar lid makes a perfect fit for the washing machine hose. When constructing your bin, be sure to make it high enough off the ground that you can place and access a jar below to collect the worm tea. You may also want to put Borax powder around the base of the cinder blocks to keep ants away while your bin is establishing itself. You can either purchase black soldier fly larvae locally or online, acquire from a friend, or wait for them to naturally colonize your bin.
To keep a sustainable population, you will need to either keep the bin in a greenhouse or live in an area where black soldier flies are native. Black soldier flies are native to zones 7-10, but can sometimes be found in zone 6. See image to the left to identify your climate zone.
Hopefully by now you are convinced that black soldier flies are the ideal way to do all your composting, while providing and excellent high-protein food source for your chickens or aquaponics fish. If you have any questions please speak up in the comments. I will be happy to address any concerns that were not fully addressed here. Thanks for reading and have fun on your new endeavor!








UPDATE 5/15/2012: Today I met with Dr. Radu Popa, professor of Biology at Portland State University. He was able to provide me with a bucket full of compost and Black Soldier Fly larvae, a container full of Black Soldier Fly pupae, and lots of great information. After meeting with him, I have a few corrections to make to the above information as well as some additional details.

1. The Black Soldier Flies do not excrete a pheromone or chemical repellent to keep away other insects. Instead, it is their voracious appetite and competitiveness for food that keeps away other insects. When a BSF bin is stocked to a high enough density, there will simply not be enough food for other insects to thrive.
2. I showed him my cardboard "welcome mat" for the BSF bin and he advised against this. The BSF bin can become very humid and cause the cardboard to rot. If a fungus takes hold on the cardboard it will immediately kill any and all BSF eggs on the cardboard. It is not known yet how this works, but Dr. Popa suspects that the fungi creates a form of antibiotic that is toxic to the eggs. Despite information I obtained from  http://blacksoldierflyblog.com/bsf-bucket-composter-version-2-1/ saying that cardboard makes an excellent addition to a BSF bin as an egg site, I am more inclined to believe Dr. Popa and avoid the fungi issues.
3. You BSF bin will probably not "winter over." You have several options: restart the bin with a new BSF colony each Spring, move the BSF bin indoors, add supplemental heating over the winter, or move a small colony indoors. I have heard online that the BSF generate enough heat throughout the winter, provided that you feed them enough, but I won't be able to say for sure until next Spring.
4. Your compost should be dripping wet. Add water if it looks too dry. The Black Soldier Flies need water. Previously, I had been under the impression that because vegetable matter is 70% water and the BSF generate a large amount of liquid that this would be enough, but after seeing Dr. Popa's BSF bins in action I can see that things are very wet. However, do not allow standing liquid. The Black Soldier Flies will not venture down into the liquid, and if it gets flooded they will crawl out. The perfect level of moisture is dripping wet, not underwater.
5. The leachate (worm tea, dark liquid collected from bin) should be heavily dilluted before applying to plants or aquaponics. Dr. Popa's recommendation for adding leachate to aquaponics was 10000:1 dilution applied daily. I have had decent results with 800:1 dilution (1 pint per 100 gallons) applied every 4 days. I cannot overemphasize the importance of diluting the liquid. It is so potent that it will burn any plants if applied directly, especially if the leachate is a very low pH or is high in amines. The leachate should not be stored for long periods because it will ferment.

Dr. Popa's website is http://www.dipterra.com/index.html, here he has links to two publications he has written on propagating black soldier flies and the life cycle and biology of Black Soldier Flies. He also told me he has a third book coming out on Black Soldier Fly gardening and composting.

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