Insect feces and exoskeletons can make agriculture more sustainable and produce less waste, scientists say.
Insects excrete, just as we do, but their feces have a more pleasing name: frass. Unlike us, insects molt as they grow up, producing a series of crinkly silhouettes of their outgrown exoskeletons, also called exuviae.
This waste may be great news for plants. Exuviae and frass contain polymers and nutrients that promote plant growth when mixed into soil.
In an opinion paper published on Wednesday in the journal Trends in Plant Science, several scientists argue that this kind of insect residue should be used to grow sustainable crops. Insects are increasingly being farmed for food (for humans) or feed (for animals), producing a growing amount of waste. The scientists propose collecting this waste and mixing it into soil in order to stimulate microbes that promote plant growth. Then, the farmed insects would feed on organic waste from crop production, creating a circular food system.
“Finally somebody has made that connection,” said Esther Ngumbi, an entomologist at the University of Illinois Urbana-Champaign who was not involved with the research. Dr. Ngumbi works with insect frass and beneficial microbes in her lab. “I appreciated their entire thinking of all these other benefits that could come along by using this never-used asset,” Dr. Ngumbi said.
Marcel Dicke, an ecologist at Wageningen University & Research in the Netherlands and one of the paper’s authors, said: “It turns out we can kill two flies with one stone.” He added, “You can produce insects for food and then still also use the residual stream to promote sustainable crop production.”
Dr. Dicke, a longtime proponent of eating insects, first learned of the benefits of insect waste in a conversation with Wietse de Boer, a microbiologist at the Netherlands Institute of Ecology and a co-author of the paper. Dr. de Boer knew that adding molted insect skins to soil enriched its existing bacteria, promoting plant growth. Dr. Dicke, who happened to be studying how bacteria could increase plants’ resilience to pests, wanted to form a team to research these ideas.
With colleagues, they surveyed scarce research on how insect residue could encourage plant health. Much of this research focused on frass, which provides soil with nitrogen and other nutrients that are often added to cropland in the form of animal manure or petroleum-based fertilizers.
In the new paper, the researchers argue that both insect frass and exuviae can promote plant health above and below ground in myriad ways: by increasing the abundance of beneficial soil microbes, by enhancing plant growth, by triggering plant defenses against herbivores, by stimulating native soil microbes that combat plant pathogens and by helping plant reproduction.
When insects molt, their exuviae contain chitin, a hard polymer that also makes up the shells of crustaceans like shrimp. Microbes, such as the bacteria Bacilli, help plants break down chitin into usable forms. And when insect exoskeletons decompose, they spur the growth of Bacilli and other microbes that are already in the soil.
The researchers focused on exuviae and frass from crickets, mealworms and black soldier flies, said Katherine Barragán-Fonseca, a doctoral student at Wageningen University & Research and an author of the paper.
While the new paper proposes how this circular system might work in theory, the researchers have begun to run experiments in the lab and in the field to determine how it might work in practice. “This is very exciting, but how much poop do I need?” Dr. Ngumbi said, as an example.
After experimenting with different ratios of frass and exuviae from different insects, Ms. Barragán-Fonseca finalized a powdered mixture. She then conducted experiments in which she mixed a few grams of it into the soil before planting mustard. She said she found the mixture could increase plant reproduction by increasing the number of flowers, attracting even more pollinators. These results are unpublished.
“It’s great to see the power that these insects have,” Ms. Barragán-Fonseca said. “Trash for someone can be a treasure for other purposes.”
Insect farming is a growing industry, meaning more insect waste will be produced. This waste used to be discarded, but some companies are beginning to sell it as fertilizer, Dr. Dicke said. Though one insect’s frass may seem negligibly small, it balloons on an industrial scale; a mealworm farmer in Nebraska produces approximately two pounds of frass for every pound of mealworms.
Recycling this waste would make insect farming — which is already more efficient than farming larger livestock such as cows and pigs — even more sustainable. “We’re dealing with climate change, which brings a lot of stressors,” Dr. Ngumbi said. “Anything to boost plant productivity is always a plus.”
Dr. Dicke has contacted farmers and applied for funding to experiment with this sustainable circular system of insect waste on a wider scale. “Insects have been seen as organisms we should get rid of,” he said. “But we are really dependent on them.”
Insect feces, too, are often seen as something to get rid of. But Dr. Ngumbi said the new paper might change her disposal practices in her own lab. “I work with insects and they poop and poop and poop,” she said. “Now I’m like, I’ve got to collect this poop.” She added, “I’ve got to take it into my summer plants.”
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