Black Soldier Fly Digester Build Workshop

“Black Soldier Fly” — the name resonates with fear and dread, and perhaps even conjures an image of winged, facet-eyed soldiers wielding guns. In reality, black soldier flies (Hermetia illucens) are useful native critters that chew through organic remnants, helping turn organic material into compost while producing tasty treats for chickens.

The black soldier fly is a non-pest tropical and warm-temperate region insect useful for managing small and large amounts of biosolids and animal manure. They are native to this region but do not like to come indoors — so you won’t find them buzzing around the dinner table. They do not feed as adults or spread disease like other flies. Although large and potentially scary-looking, since the females can be about the size of a large wasp, they do not bite humans or livestock. After black soldier fly residue is vermicomposted, it can be used as a soil amendment.

The total life cycle of a black soldier fly lasts just over a month. Black soldier flies lay 600 to 1200 eggs at a time, in dry crevices above or around moist waste material. After five days, the eggs hatch and white larva drop into the waste material and begin to consume it, growing to about ¾ inch over two weeks. Between day 19 – 33 of the life cycle, the larva turn into gray pupae and quit consuming material; this begins the migratory stage, when they crawl up and out of the bin to burrow. These pupae contain essential amino and fatty acids, which make them great food sources for pigs, chicken and fish.

Black soldier flies can reduce organic waste material by as much as 95%, depending on temperature and content. A medium-sized digester can process about 80 lbs. of bio material in a day. The Southern Appalachian Highlands Conservancy recently hosted a Digester Build Workshop on its Community Farm, to demonstrate how to construct a digester for organic material using black soldier flies.

Building a digester

For a regular household – for example, if you’re going to be feeding kitchen scraps, garden waste or small livestock manure – a 20-gallon tub is a good size to start. For a small vegetable farm, a 100+ gallon size would be best. During SAHC’s Digester Build workshop, we cut an olive oil tank in half and used it to construct a medium-sized digester. Both small and medium-sized digesters are modular, so you can add as many as you need over time. The basic construction is the same for each: a tub or container to hold the organic material; ramp for the pupa to crawl up and out; collection bucket to hold the pupa that crawl out; cardboard or similar medium for oviposition by the female black soldier fly; and lid or cover if the digester is not placed under a roof, to keep rain out.

Although you can build a digester with ramps that feed into a collection bucket located outside the digester, our design incorporates the collection bucket and ramps within the digester, which works well at a medium scale. The ramps should have a trough and or small sides so the pupa do not crawl off, and they must also be flexible so they can adjust to new organic material being added without becoming buried. Ramps could be made of PVC pipe, wood, old gutter, siding, etc. Place cardboard on the inside walls of the digester, so that the eggs laid by the female will be above the organic material. Locate your digester under an open-sided shelter, or place something over top to keep water out. When covering the digester be sure to leave enough room for the female black soldier fly to get to the cardboard to lay her eggs.

Starting your own colony

Because the Black Soldier Fly is a naturally occurring insect in our region, you can attract the female to lay eggs near a food source with a strong odor. Start a compost bin with a mix of kitchen scraps that are a couple of days old. The females will detect the chemical signal of a larval food source. It is important to give the female black soldier fly a location to deposit her eggs, so place a stack of corrugated cardboard on the inside wall of the container. Within two weeks, you should have black soldier fly eggs in the cardboard, which you can then transfer to the wall of your digester. The larva will hatch and fall into the organic material and start growing.

For more info, or to visit SAHC’s Community Farm and see a black soldier fly digester in action, contact chris@appalachian.org or 828.253.0095 ext 203.