Cultivating Soldier Grubs to Feed the Flock
Note that I have duplicated this article in the Grow It!: Composting section under the original title, “Black Soldier Fly, White Magic.” I include it here to suggest that cultivating soldier grubs offers the homestead or farm flockster a superior live poultry feed—one of the best examples of “waste to resource.”
Photos courtesy of Bonnie Long except as noted.
Trash to Treasure
I have always been fascinated by the transformation of something “yucky” into something prized. When my father brought home a bushel basket of manure from my grandmother’s chicken coop in the trunk of our car, and worked it into our garden beds, I was filled with wonder: That stuff they warn us barefoot boys not to step in, it’s going to make our vegetables grow? Wow! When I started growing gardens of my own, I had many reasons to avoid “fertility in a bag,” but first and foremost I shunned chemical fertilizers because they would deny me a role in composting—the miracle of turning manures and rotting vegetation into harvests.
And I’ve been a sucker for the idea that I could make an alliance with another animal species to work this magic myself: I was enough of a kid to jump enthusiastically into composting with worms, turning “pony poop” into fertilizer and chicken feed; and enough of a madman to turn carrion from a friend’s beaver trapping service into live protein for the flock. [Do note that I’ve had some second thoughts on the latter idea. See the important advisory at that link.]
Now I’m excited about yet another species that magically transforms “wastes” into resources. Let me introduce you.
Meet the Genie: Hermetia illucens
Most of us are annoyed and repelled by “flies.” House flies buzz into the house and onto our food, possibly carrying disease-causing microbes. Horse flies bite. Blow flies lay their eggs in carrion, and the larvae rid the world of dead carcasses—an essential ecological service for which we are grateful, even if we are repelled by the process as “just too gross!”
But none of us within its range are either annoyed or repelled by the Black Soldier Fly, Hermetia illucens—indeed, it is unlikely most of us have ever even noticed this innocuous flying insect. Why would we? They look nothing like the flies we find annoying. They do not “buzz” us or come inside the house. They do not bite. A resting adult looks like a slender black wasp (but without the sting)—quite pretty, actually.
The life cycle of the Black Soldier could be a textbook example of the most common insect life cycle: egg, larva, pupa, adult.
Like countless other essential decomposers, the work of Hermetia illucens in the world is the conversion of organic debris into residues that feed the soil food web, and into new individuals who carry that work into the future. More poetically, decomposers turn death back into life. In the case of the Black Soldier, the cycle starts with eggs laid by the female near a concentrated deposit of moist, nutrient-dense organic refuse such as succulent vegetable matter and manures.
The eggs incubate for four days to three weeks, then hatch into larvae, legless and wingless grubs that are “all throughput”: busily feeding mouth on one end, extraction of all nutrients usable for vigorous growth in the digestive system, and ejection of undigestible feed components out the other end. The grubs molt their skins, to allow for their rapid growth, in five successive phases called instars.
Under ideal conditions, the larvae mature in 10 days. Their tissues are now developed enough, and they have stored energy reserves enough, to support the next phase: the miracle of metamorphosis, the transformation into a completely different insect form. Pupation does not take place within the feeding medium, however. When they are ready, the prepupal grubs have the instinct to leave it and find a place to burrow into the earth and initiate metamorphosis. After ten days or so, they emerge as winged adults.
As is the case with many species of butterfly, the adult phase is exclusively sexual: The winged phase is solely about mating, and, for the female, finding the best possible place to lay her eggs. They do not feed at all in this phase, which lasts only five to eight days. Death quickly ensues for both male and female adults, once fertile eggs have been laid to start the cycle anew.
An Alliance with the Soldier
Useful ideas emerge from alert observation. In the case of the Black Soldier life cycle, we should note particularly: They are specialists in a critically important role—the breakdown of organic debris that would otherwise choke the ecology. Their high level of feeding activity and rapid growth in the larval stage imply the concentration of considerable nutrients. Since the adults do not feed—indeed, they do not even have functioning mouth parts—they do not bite, nor do they come buzzing around us or our houses, looking for something to eat. Of special interest is their habit of crawling out of the feeding medium when it is time to pupate.
These facets of the life cycle hint at an opportunity to make an alliance with Hermetia illucens to: responsibly manage organic residues such as spoiled or unused food, manures, culled fruits and vegetables, etc.; and to reclaim the residual energy in such materials as additions to soil fertility, or even high-quality feed (the mature grubs themselves) which we can offer to livestock species such as chickens, pigs, or farmed fish. By channeling the “crawl-off” of the grubs, we can even cause them to self-harvest into a collection bucket! And because the vigorous activity of the larvae keeps the feeding medium constantly aerated (most pathogenic microbes prefer anaerobic conditions), and because the adults do not feed at all, this species is not a vector for diseases. Happily, soldier grubs tend to inhibit development of larvae of all other fly species—house flies, fruit flies, blow flies—so cultivating the Black Soldier can actually reduce populations of flies with a higher “nuisance profile.”
Much creative work has in fact been done toward making exactly such an alliance. Dr. Paul Olivier has designed systems for grub composting in Vietnam and other developing countries, to reduce the load of food wastes on streets and in landfills, while providing needed employment. Academic researchers such as Sophie St-Hilaire and Craig Sheppard have experimented with soldier grubs to manage fish offal, and manures in commercial poultry and swine houses; and to yield high-protein feed supplements for various livestock species, including commercially raised carnivorous fish. Dr. Olivier and his son Robert have founded a company (ESR International, LLC) which sells the well-designed BioPod™, a molded high-density plastic bin similar in some ways to, but more efficient than, home scale “worms eat my garbage” vermicomposting units.
Parameters for Black Soldier Composting
I am working on a soldier grub colony in a BioPod™, and in a future article will detail its management; as well as explore some ideas for more low-tech, less expensive, do-it-yourself projects for soldier grub composting. My goal is to replace 25 percent of purchased feed I offer my chickens with nutrient-dense soldier grubs (42 percent protein, 35 percent fat, dry weight).
For now, these are some of the parameters that govern the design and management of a successful operation.
If you live in climate zones 7 through 10, there is almost certainly a native Black Soldier population ready to work for you. (I’m in Zone 6b, and I found soldier grubs in my vermicomposting bins for years before I started cultivating them.) All you have to do to start a working colony is to set up feeds and protected conditions that fit their life cycle; and the gravid females (mated, ready to lay eggs) will come. Soldiers can survive considerably farther north than Zone 7. If there is no wild population, you can purchase “starter” grubs through the mail. How easy it will be to keep a local population going will depend on just how far north of their natural range you live.
Bin design is quite flexible—the essentials are that the bin: protect the colony from predation and rain; give ready access to gravid females; ensure compatible living conditions; and provide for “crawl-off” of the mature grubs. The unit should be set up in the shade: The high metabolic level of the grubs generates a good deal of heat—additional heat supplied by direct sunlight could be disastrous for the colony. Otherwise, placement of the bin depends on your own convenience. It does not have to be stuck off in “the back forty”: Aerobic (oxygen-rich) conditions in a well-managed colony prevent unpleasant odors.
The most efficient conversion to biomass occurs in typical food wastes. But if yours is a frugal household such as ours, there is no edible food wasted, other than castoffs like coffee grounds (which grubs love), tea leaves, peelings, and trimmings. (It might be possible in a larger project to receive food scraps from institutions like schools or restaurants.) In the absence of such “food residuals,” any mix of succulent vegetable and fruit matter works well, such as over-mature and cull fruits and vegetables (the lurking Monster Zuchinni perhaps). Since grubs love the big outer wrapper leaves of cabbage (and still-succulent spent broccoli plants), I plan to grow more rape (a close relative of kale, thus of cabbage)—which I grow as a cover crop anyway—to harvest as feed. I’m experimenting with comfrey as well—its succulent large leaves are packed with protein and minerals. Perhaps next year I’ll plant more of the big pumpkins and Hubbard squashes for feeding grubs.
Manures can make good feed. Low-fiber pig and chicken manures are best—there may be no better way to deal with these manures than using energy-hungry grubs. The larvae will work horse, rabbit, and ruminant manures, but the higher fiber content of these manures reduces feeding efficiency (since the larvae cannot digest the cellulosic residues of plant stems). Of course, if the manure available to you for processing is a high-fiber type (horse manure in my case), you could simply increase bin capacity to meet your production goals, and plan on cleaning out the bin a little more frequently.
Meat and fish scraps, and dairy products, can be fed to active colonies, but in limited amounts—the grubs will not thrive with too much high-protein feed. Such foods should not exceed 5 percent of total feed offered.
Do not use as feed materials that are dry, fibrous, high-cellulose, tough—such as weeds, grass, leaves, stalks, paper, cardboard.
Look again at the list of appropriate feeds. Note that all have high moisture content; and that grubs do not thrive in anaerobic conditions. Conclusion: Efficient drainage out of the bin is essential, and will likely be your major management challenge. If conditions in the colony do become too wet, either cease feeding for a while or add a moisture-absorbing material such as shredded office paper (not newspaper), coir (the granular residue of coconut husk fiber extraction), and the like.
Note that the effluent from the bin can be used exactly like a “manure tea.” Some sources advise against using it on garden crops, just to play it safe, but you can use it to encourage cover crops and other non-food plants.
An essential feature of any bin design is provision of some sort of “ramp” the mature grubs can use to exit the feeding medium. If the incline is no greater than 45 degrees, they will have no problem wriggling up the ramp, which might end in some sort of gutter to direct their further migration. The grubs have no hesitation crawling off the end of the gutter and launching themselves into space, assuming they’re going to land on earth, where they can burrow and pupate. Uh-oh!—wily you have placed the spout over a collection bucket. Next morning you only have to pick up the bucket and go feed the chickens. (They love soldier grubs.)
Do remember, though, to scatter a few grubs where they can burrow, pupate, and emerge as adults ready to carry on the cycle.
A colony of soldier grubs is like a chicken flock: A well-managed colony is a lot more productive than a wild one—that is the essence of the almost symbiotic relationship between Homo sapiens and another species which we call “domestication.”
What levels of production might we expect? That depends on many factors—bin design, feeds offered, ambient temperatures, management experience and skills (the “your mileage may vary” factor). But the equation, at whatever level of production, is all positive: If we offer the grubs 100 pounds of food wastes, for example, they will reduce it to 5 pounds of residue usable as a superior soil amendment; in the process generating 10 and possibly up to 20 pounds of live grubs that can be fed to livestock; in addition to liquid effluent (how much depends on the moisture content of the feeding materials) which can be used to feed crops. Hey, wait a minute—what happened to the “wastes”? There is absolutely no waste remaining after this conversion—it has all been transformed into resource.
In the future, widespread farm-scale conversion by soldier grubs of organic residues like manures could reduce dependence on purchased feeds for poultry, pigs, and fish. Or the grubs could be dried and sold for addition to livestock feeds—as a replacement for the thousands of tons of ocean fish harvested annually for fish meal (at today’s prices, approaching $1,000 per ton and rising).
This species is enormously adaptive in response to changing environmental conditions. If food supply runs out, the grubs go dormant until more food is available. During winter, they delay maturation for several months before resuming development. This adaptability gives the operator great flexibility in managing the colony.
Because of the enormous reduction in volume of the feedstocks offered to the colony, cleaning out the substrate—the undigestible residue at the bottom of the bin—need not be frequent. This residue makes a great soil amendment. Or it can be added to vermicomposting bins for even better natural fertilizer production. (See below.)
Because of the heat generated by their high metabolic rate, it is possible to keep a colony active through the winter. However, over-wintering a soldier colony depends on too many factors to consider here. I will return to this topic in a following article.
Partners in Vermicomposting
Those who compost with worms for managing organic “wastes” may enjoy expanding their repertoire to include a soldier grub colony. There are important differences between the two: Redworms live and feed in a large, undifferentiated feeding mass; grubs should be fed only what they will consume in a day. The population in a worm bin is self-sustaining; whereas a soldier grub colony requires renewal from an ambient wild population—otherwise, there will be no egg-laying females to ensure a new supply of grubs in the bin.
In some ways, soldier grubs are superior to redworms for composting: They are more active, yielding greater production of live biomass; and grubs so conveniently self-harvest into our collection buckets.
However, we needn’t choose between the two on the basis of which is “better”—actually, the two species make wonderful partners. Grubs digest fresh putrescent matter in a hurry (worms wait until bacteria are consuming them, then feed on the bacteria); while redworms convert the cellulosic residues grubs are unable to digest. Studies in Asia demonstrate that redworms grow three to four times faster on the residues from soldier grubs, than on food wastes.
The Big Picture
To paraphrase Wendell Berry, our modern habit is to take a beautiful natural solution—the recycling by decomposer organisms of residual energy in “wastes” into food for other members of the ecology, including those in the soil food web—and neatly divide it into several hideous problems. Nature does not know the meaning of the term “waste”—we have created “waste,” the loss of reclaimable energies in organic residues, as a new phenomenon inherent in the human economy. The refuse in landfills consists of up to 12 percent uneaten or unused food, whose breakdown there not only squanders its resource potential, but generates methane, an even more potent climate-altering gas than carbon dioxide. Manures from livestock operations are seldom fully utilized for soil improvement, and run off to natural water systems, where they function more as toxin than fertilizer.
It is easy to despair of making an individual difference where some of our Big Problems are concerned. But with regard to the organic residues from our tables, gardens and orchards, and backyard livestock operations, we can—with forethought and dedication, and in ways that are fun—help end the inexcusable and unsustainable sluicing of un-reclaimed resources down a black hole from which they do not return. Hermetia illucens is ready to help.
For More Information on the Black Soldier
- “Jerry aka GW,” who has been cultivating soldier grubs to feed pond fish for several years, maintains the informative blacksoldierflyblog.com. GW also serves as a distributor of the well-designed BioPod™ (see next item), which I am currently using to house my grub colony. Quoting GW: “If you like gardening, traditional composting, or vermiculture, then you’re a likely candidate for black soldier fly culturing. Likewise, if you watch nature and science shows you will probably find BSF as fascinating as I do.”
- theBioPod.com is the website for ESR International, LLC, the company that designed and sells the BioPod™ for soldier grub composting. Lots of useful information about managing a working colony, and about many other bioconversion/waste management issues. (BioPod™is a trademark of ESR International, LLC.)
- ESR International, LLC, sponsors an online discussion forum for enthusiasts working with this fascinating species.
- Introduction to bioconversion of putrescent “wastes,” including the evolution of the major BioPod™ designs to date.
- A useful one-page summary of facts about Hermetia illucens and what it has to offer.
- An interesting article by Paul Olivier, “Utilizing Lower Life Forms for the Bioconversion of Putrescent Waste (and how this could dramatically reduce carbon emissions)” (pdf format).