Thus far overshadowed by its cousin the black soldier fly, the house fly (Musca domestica) offers similar potential for recycling waste generated in livestock production, a new study shows. House fly larvae are chock full of protein to juice up feed and fat to produce biodiesel, while their droppings, called frass, can be used as a soil amendment. Manure digested by house fly larvae yields biomass comprised of 55 percent to 63 percent protein and 15 percent to 23 percent fat, and the fat from larvae can be processed into biodiesel. (Photo by Nikolai Vladimirov via iNaturalist, CC BY-NC 4.0)

By Ed Ricciuti

Ed Ricciuti

The sight of fly maggots noshing on manure has an undeniably high yuck quotient—but not for everyone. Some environmental scientists see it as a thing of beauty. The same buzzy pests that swarm over manure in livestock facilities can be economically sound agents for cleaning up poop while going on to feed the animals that produced it in the first place, suggests research published this month in the Journal of Economic Entomology.

Specifically, house flies (Musca domestica) should be viewed as waste management agents, not just as noxious pests, says Chelsea D. Miranda, Ph.D., of Texas A&M University (TAMU), lead author on the study. The maggots—fly larvae—can recycle the manure into livestock feed, soil amendments, and even biodiesel fuel. Manure draws house flies like, well, flies to manure. They eat it and breed in it, making them abominable pests of barns, coops, and pens. However, if managed on an industrialized basis, house flies can be super waste recyclers, says TAMU’s Jeffery K. Tomberlin, Ph.D., senior author on the study.

Chelsea D. Miranda, Ph.D.

A few countries, notably China, already recycle manure by feeding it to house fly larvae and then using the larvae as an ingredient of animal feed. Because they can spread pathogens, however, house flies have been avoided most everywhere else.

The black soldier fly (Hermetia illucens), on the other hand, is not a disease threat and so is widely approved as a recycling agent. The U.S. Food and Drug Administration approved its use in 2018. The house fly deserves the same status as a tool for recycling animal waste, Miranda and Tomberlin say.

Both species have pros and cons. The primary virtue of the soldier fly is that adults do not feed and thus do not spread pathogens. A downside is that the reproduction of the soldier fly is highly dependent on light, specifically the range of wavelengths that are visible to the human eye, which we see as sunlight. Soldier flies are often bred in glass greenhouses but, where seasonality and climatic conditions reduce the supply of sunlight, it gets trickier. Artificial lighting can be complex and expensive. “House flies share some of the same qualities as the black soldier fly, and, in areas where it is difficult to rear the black soldier fly, the house fly should be considered,” says Miranda.

Adds Tomberlin, “The house fly can have more protein than black soldier fly. The black soldier fly can digest a wider range of organic side streams. It really comes down to the objective of a company and the resources available to them.”

A few countries recycle manure by feeding it to larvae of the house fly (Musca domestica, adult shown here) and then using the larvae as an ingredient of animal feed. Because they can spread pathogens, however, house flies have been avoided most everywhere else. The black soldier fly (Hermetia illucens), on the other hand, is not a disease threat and so is widely approved as a recycling agent. The U.S. Food and Drug Administration approved its use in 2018. But the house fly deserves the same status as a tool for recycling animal waste, some researchers say. (Photo by Chelsea D. Miranda, Ph.D.)

Using fly larvae to recycle livestock manure reflects the concept of a circular economy, in effect creating revenue by manufacturing products through the reuse of a resource that otherwise would be thrown away. It is tailor-made for areas where livestock and poultry production are concentrated and manure disposal an immense task. (Photo by Chelsea D. Miranda, Ph.D.)

Indeed, house fly larvae are chock full of protein to juice up feed and fat to produce biodiesel while their droppings, called frass, can be used as a soil amendment. Manure digested by house fly larvae yields biomass comprised of 55 percent to 63 percent protein and 15 percent to 23 percent fat, which can be processed into biodiesel. In the recycling process, the flies reduce contaminating pathogens and heavy metals, while eliminating more than half of manure waste, a major cause of eutrophication that causes oxygen-dead zones in water bodies.

Not surprisingly, the scientists found that large-scale production involving of thousands of house fly larvae is much more efficient than experimental production using hundreds of them, done in previous research. Recycling manure reflects the concept of a circular economy, in effect creating revenue by manufacturing products through the reuse of a resource that otherwise would be thrown away. It is tailor-made for areas where livestock and poultry production are concentrated and manure disposal an immense task.

A key part of the research tested the impact of manure from pigs, poultry, and dairy cows fed to house flies on an industrial scale of production. The survival rate of larvae reaching the stage at which they stopped moving and started to form their pupal case was 73 percent for those fed pig manure, 67 percent for poultry manure, and 50 percent for dairy manure. Each experimental sample involved 4,000 larvae. The rate of development, however, differed little between manure types.

When compared with the results of feeding experiments on a small-scale, with a few hundred rather than thousands of larvae, industrial-size production was much more efficient. “This study really demonstrates the importance of doing industrial scale work- to verify the economic model in terms of waste conversion to protein,” says Tomberlin.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.