Fly Larvae Eat Through Waste and Destroy Most Human Viruses in the Process

Eight days. That is all it takes for black soldier fly larvae to consume a pile of sewage sludge and gain weight while doing it. What researchers publishing in ACS Environmental Science & Technology Letters wanted to know was what happens to the viruses hiding inside.

The answer is mostly encouraging - and partly sobering.

Three waste streams, one hungry insect

Gang Luo and colleagues fed three separate groups of Hermetia illucens larvae different organic waste types: food scraps, sewage sludge, and pig manure. After eight days, every group had put on weight. Larvae fed food waste grew fastest, followed by those on pig manure, then those on sewage sludge. The insects were doing exactly what bioconversion advocates hope for - turning waste into biomass.

The more revealing part came when the team analyzed what happened to the RNA viruses present in each feedstock. The original waste materials harbored a wide array of viral types capable of infecting bacteria, fungi, plants, animals, and humans. Noroviruses, the bug responsible for outbreaks on cruise ships and in schools, were among them.

Most viruses destroyed, but frass from fecal waste stays risky

Larval digestion significantly reduced the abundance of most human-pathogenic viruses in fecal waste streams. The less comfortable finding: larvae fed sewage sludge or pig manure harbored higher viral diversity overall, and their frass - the small nutrient-rich pellets the insects excrete - still contained RNA viruses capable of infecting humans. Picobirnaviruses, which can cause digestive symptoms, persisted in both the larvae themselves and their frass.

Larvae that ate food waste showed a different profile. Their bodies contained only low levels of insect-specific viruses, which the researchers consider minimal ecological or human health risk. This distinction matters enormously if the end goal is using larvae or their frass commercially as animal feed or fertilizer.

Circular economy potential - with conditions attached

Black soldier fly larvae have attracted genuine interest from waste management researchers for years. They convert organic material quickly and have been proposed as both animal feed and a protein source for aquaculture. Their frass works as a soil amendment. The potential for a circular system - waste in, useful biomass out - is real.

But circularity only works if the outputs are safe. "Viruses in organic wastes have rarely been studied in a systematic way," said corresponding author Gang Luo. "Our research shows that black soldier fly larvae can help reduce potential viral risks, highlighting the promise of this approach for future waste treatment."

The caveat is important. A reduction in viral load is not the same as elimination. The study was conducted in controlled laboratory conditions, not at the scale or temperature variability of real operations. The experiments measured viral RNA presence rather than live viral infectivity - meaning some detected genetic material may belong to non-infectious particles. Whether surviving viruses in frass can actually cause infections is the next question the team plans to investigate.

The gap between lab bench and farm gate

The practical implication is clear even before those future studies arrive: larvae consuming fecal wastes should not feed directly to animals or have their frass spread on fields without additional treatment steps. This is not fatal to the technology - many waste treatment systems use sequential steps. It does mean that black soldier fly bioconversion is not yet a plug-and-play solution for sewage or manure with known viral contamination.

Luo described viral activity in the final larvae and frass as "key to safely reusing them in a circular waste management system." Compared to the alternatives - landfilling, incineration, or lagoon storage, all carrying their own environmental costs - insect bioconversion still looks worth pursuing. The study appeared in ACS Environmental Science & Technology Letters, with funding from the National Natural Science Foundation of China.