Composting Getting You Down? Let Black Soldier Flies Do the Job! : The Life and Times of BSF (Black Soldier Flies)
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Composting Getting You Down? Let Black Soldier Flies Do the Job!

by Terry Green on 05/03/13

Maintaining a well-run aerobic composting pile takes attention to detail and manual labor to ensure a stable end-product. The goal in aerobic composting is to create conditions suitable for aerobic microorganisms to grow and flourish in oxidizing and reducing the mass of waste left behind. Done correctly, stabilized compost (the residue left behind in the pile no longer readily susceptible to oxidation and decomposition) is rich in humic substances. It can be added to soil to improve the physical structure of the soil, its water retention and its nutrient binding capacity. Here’s a neat trick you can do to accelerate the decomposition process – let Black Soldier flies (BSF) take over the job. Save yourself a lot of work.

BSF are not regarded as pests and have been quietly and efficiently recycling organic wastes around the world for thousands of years (The Life of a Black Soldier Fly | A Children's book | Fleeting Moments?).  You can jump start the process in your compost pile or a bin by seeding it with BSF at the beginning of the compost season (usually after winter passes and as the weather warms up and organic debris begins to accumulate). Seeding can be done starting with young larvae added directly to your pile or bin. Or place pre-pupae purchased through the internet near your compost pile or bin. Adult BSF on emerging from pre-pupae seeded near your pile or bin will mate and lay eggs nearby which will soon hatch into a new generation of larvae ready to chomp away on the waste. Alternatively,  simply let indigenous native BSF find your compost pile or bin and start processing the waste. BSF will accelerate its decomposition providing, of course, you let them do the work (don’t kill off the larvae you find in your compost thinking that they are some sort of gross pest that has by happenstance contaminated your pile or bin!).

The general rule of thumb in carrying out aerobic composting is to mix 2 parts of brown stuff (autumn leaves, paper, peat moss, wood chips, cornstalks, hay and straw, dried out grass, etc. -waste residues rich in carbon) with 1 part of green stuff (green grass clippings, weed stalks, green vegetal debris, food scrap – wastes rich in nitrogen) together before adding them to your pile or bin, or to place each in alternating layers not more than about 2 to 6 inches thick in building up your pile. The pile must be kept moist (about as wet as a wrung out sponge), not soggy, and it needs to be aerated as you add material to it. Too much water in the pile, or simply not getting enough air mixed into the pile creates anaerobic pockets resulting in decomposition of the waste under anaerobic fermentation conditions. This can lead in short order to a stinking, rotting pile of waste (Recycling Food Scrap? | What’s with the Stink?).

If you let BSF help out in getting the job done, you won’t need to be nearly as fussy about layering and mixing waste into your compost pile, or aeration, in breaking down the waste. You won’t need to turn it over at all while BSF larvae feed on the waste. They aerate and mix it as they burrow through it while feeding on it and microbes growing off the waste. Depending upon the temperature and weather conditions where you live, you may need to add water from time to time to keep your compost pile moist. But dispense with turning it over. Turning it over with larvae present in the compost pile will cause the larvae to migrate to the edges of your pile as your pile overheats inside its center section.

With larvae feeding on the waste, the pile temperature is cooler than that seen under conventional aerobic compositing conditions. It should not exceed a temperature of  ~ 113 F (45 C). The cooler operating temperature combined with a healthy population of larvae feeding on the waste results in less odor volatilizing off the pile. If the pile begins to overheat, cool it down by adding more water to the pile.
image of compost bin with BSF processing food scrap waste
Fig. 1. Example of a composting bin used in combination with Black Soldier flies (BSF) in accelerating the decomposition of organic waste added to the bin. Left, view of a common residential composting bin of which many versions are available at most hardware and garden stores; Right, cross sectional view of produce, garden and vegetal matter undergoing decomposition inside the bin with the help of BSF larvae processing the waste. Copyright © 2013, Terry Green, All rights reserved.

Fig. 1 shows a residential compost bin (Fig. 1a) which works very well with BSF, and a cross sectional view of food scrap and vegetal matter undergoing decomposition with the help of BSF larvae infesting the bin (Fig. 1b). Many variations of this type of composting bin will suffice and can be obtained at most garden and hardware stores. Adult BSF, out of sight and outdoors, spontaneously seek out decaying organic matter as the weather warms up in late Spring, and will lay eggs after mating from which new larvae emerge in a matter of a few days of egg deposition. Young larvae seek out and begin feeding on the organic waste they encounter, breaking it down as they burrow through and grow off of the waste. Holes around the walls of the bin (Fig. 1a) provide easy access and egress sites for BSF entering and leaving the bin while excluding rodents and other pests from getting at the decaying matter held inside the bin.

Because larvae are photophobic (light fearing), they generally stay hidden from sight. Pre-pupae self-harvest at night and quickly crawl to safe places under rocks, bark mulch, or any other cover they can find nearby. This behavior makes it simple to create a clean compost site free of visible larvae crawling around for those uncomfortable with viewing larvae (also referred to as grubs or maggots and, to the uninitiated, sometimes confused with maggots of the common housefly). Simply house your compost bin over bark mulch, pebbles, decorative bricks, or other cover.

In temperate climates, as Fall sets in and cooler temperatures prevail, BSF activity will fall off. At this time of the year, residue still left in the compost pile or bin can be turned over and aerated as in the traditional method to finish off the remaining residues, or simply buried in the soil to yield a rich and fertile soil ready for planting the following Spring.

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Comments (2)

1. RV said on 11/10/13 - 04:34PM
In your last paragraph, you suggest to bury the residue in the soil, won't that make the soil acidic? I've got 7 gallons of residue. Do you suggest a cup of residue per square foot?
2. Terry Green said on 11/11/13 - 08:01AM
Burying the leftover residue should not make the soil acidic. The pH of the residue varies depending on when the bin is emptied out, the population of larvae processing the residue waste at that time, and the amount of food scrap still remaining in your recycling bin (see our blog, “Recycling Food Scrap? I What’s with the Stink?”). Microbes and microfauna in the soil will easily take over processing of the residue left in the soil. Dig a trough 1 to 2 feet wide and not more than about 1 foot deep. Fill the trough half full with residue while mixing some soil into with the residue as you bury it, and back fill the last four to six inches with the remaining soil removed from the trough. The soil will easily process up to about a gallon (~ 8 lbs) per square foot. Wait until spring before planting again in the soil where you buried the residue.

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