Flies may be new swine waste management tool
Perspectives On Line: The Magazine of the College of Agriculture and Life Sciences
NC State University

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Flies may be new swine
waste management tool

Dr. Wes Watson holds a soldier fly, the focus of his research at N.C. State's Lake Wheeler Road Field Laboratory, where Watson and his colleagues are determining optimal growing conditions for the fly's larvae. (Photo by Sheri D. Thomas)

The idea may seem to fly in the face of reason, but researchers at North Carolina State University and the University of Georgia think a type of fly may play a pivotal role in the development of new waste management technologies for hog farmers.

Flies are usually considered pests, and farmers typically try to get rid of them, but scientists think some hog farmers may eventually embrace black soldier flies.

Researchers from the two universities think that by feeding swine manure to the larvae, or maggots, of black soldier flies they can not only reduce significantly the amount of manure but produce two valuable byproducts. They’ve developed a system that uses the fly’s instincts to eliminate much of the human labor that might otherwise be involved in harvesting one of these by-products.

Dr. David W. “Wes” Watson, an entomologist in N.C. State’s College of Agriculture and Life Sciences, is working with Drs. Craig Sheppard, Gary Burtle and Larry Newton from the University of Georgia to refine and optimize a system that Sheppard developed to treat poultry farm waste.

The work is funded by agreements between the N.C. Attorney General and Smithfield Foods and Premium Standard Farms. Through the agreements, $17.1 million is being used to construct and test 18 alternatives to the lagoon-and-spray-field technology used on most N.C. hog farms.

The lagoon-and-spray-field system is attractive to farmers because it’s relatively inexpensive. Researchers think that the key to making more expensive alternative waste treatment technologies economically feasible is to have them turn the nutrient-rich swine waste into value-added products that farmers can sell.

The N.C. State and Georgia researchers believe black soldier flies could be an important component of several alternative technologies that separate the nutrient-rich solid portion of the waste stream from the liquid portion. Black soldier fly larvae could be used to treat the solid portion.

“The black soldier fly is a common species in southern states,” says Watson. The flies are not considered pests, Watson adds, so people generally don’t notice them. The flies aren’t usually found in residential areas, although they may be seen around backyard compost bins. The flies are slender, about an inch long and are often mistaken for wasps. Like the larvae of many other flies, the larvae of black soldier flies feed on decaying organic matter, including manure, but the black soldier fly is different.

“There’s a huge number of insects that are decomposers of feces,” says Watson. “But most are also pests.” He adds that black soldier fly larvae seem to be particularly voracious when it comes to decomposing manure.

In the self-harvesting system that Sheppard, the Georgia entomologist has designed, manure and fly larvae are put into shallow concrete pits, where the larvae feed on the manure. The edges of the pit are sloped up at 45 degree angles. When the larvae turn into pre-pupae, their natural tendency is to move up and away from the larvae. The pre-pupae are about an inch long. They have no discernible features, with the exception of tiny ridges, and move by inching along.

Sheppard has found that the 45-degree angle is steep enough to keep larvae in the pit but not so steep that the pre-pupae can’t crawl out. Pre-pupae crawl up the 45-degree slope, fall into a gutter at the top of the pit wall, crawl along the gutter to a pail, then fall into the pail, where they can be collected. In other words, the pre-pupae harvest themselves.

The flat shape of the soldier fly pre-pupae appears to help them crawl up the slope, Watson says. The rounder pre-pupae of other flies tend to roll back down the slope.

The researchers believe that the pre-pupae, which are rich in protein and fat, may have value as an animal feed ingredient, perhaps for aquaculture. Indeed, Watson points out that adult black soldier flies are nourished their entire lives — a period of about two weeks — by the protein and fat accumulated by the larvae. The adult flies never eat, which is why they aren’t considered pests.

At the same time, the scientists say the material that remains after the larvae have digested manure should have value. This material, which looks almost powdery, should have value as a soil amendment. It has little odor — it smells like soil — and can be transported easily.

The researchers are working to develop a simple greenhouse that farmers may use to raise larvae to digest manure. A greenhouse is necessary to encourage adult soldier flies to lay eggs to replenish the basins in the winter.

— Dave Caldwell


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