- Supplementing soil with insects’ cast-off outer skin after a molt can help increase plant biomass, the number of flowers, pollinator attraction, seed production, and even resilience to insect herbivore attacks, according to researchers.
- Farmers are already using insects, in particular the black soldier fly, for livestock feed and waste reduction, and this new use could help the transition to a more sustainable and circular agricultural system, scientists say.
- Along with further investments in research and development, a higher uptake in insect farming practices, by both small and industrial farmers, will improve for boosting crop productivity within circular agriculture.
The latest recipe is here: you’ll need a cup of minerals, two tablespoons of air and water, a few billion microorganisms, a teaspoon of organic matter and … a pinch of old bug skin. No need to bake, your soil is ready to be sown.
A study published in Agriculture, Ecosystems & Environment last year found that adding insect exuviae — the remains of exoskeletons that arthropods cast off after molting — to soils positively impacts plant reproduction. The researchers found that after treatments of black soldier flies (Hermetia illucens) exuviae to soils, plants grew hardier and more resilient.
The black soldier fly strikes again
I decompose your trash, feed your animals, and gives your plants a boost. Who am I? The black soldier fly, or the BSF.
With rising concerns over sustainability, insects have gained momentum in the agricultural sector because of their capacity to reduce organic waste and convert it into a high-quality source of protein for livestock. Currently, BSFs are the most widely used insects produced for animal feed, namely for fish, poultry and pigs.
But what if BSF could also improve plant health? With this in mind, Katherine Barragán-Fonseca, a researcher with Wageningen University & Research (WUR) in the Netherlands and lead author of the 2022 paper, investigated the benefits of the sleek-looking fly beyond its current uses: her team mixed powdered BSF exuviae into soil planted with black mustard seeds (Brassica nigra).
Native to Europe, black mustard is an annual plant similar to field mustard (Brassica rapa), an important vegetable and oil source.
“I selected Brassica nigra because it is a good system to make ecological experiments,” Barragán-Fonseca said, citing its fast growth cycle. She added, “and we have a deep knowledge in our laboratory on this system.”
During their life, plants are in continual interaction with insects and microbes above and below the ground. And exuviae are rich in chitin, a natural biomolecule that can boost beneficial microorganisms like rhizobacteria, as well as potentially hike plant defense against attackers and cause changes in plant traits exploited by insects.
Higher, better, fuller, stronger
After a three-week experiment in a common garden, the plants grown on soil supplemented with BSF exuviae “were significantly taller than plants grown on field soil alone, with significantly larger plant width and leaf length than control plants,” the scientists write in their study.
The plants grown in exuviae-infused soil also produced more seeds and flowers. The flowers, a sign that the tested plants were able to invest more resources in reproductive tissues, were also more frequently visited by insect pollinators, like syrphid flies and honey bees. And during each visit, the pollinators contacted more flowers than they did on the control plants.
But pollinators aren’t the only insects attracted to black mustard. Herbivores, like the caterpillars of cabbage butterflies (Pieris brassicae), often feed on black mustard leaves. But the researchers found that plants grown with BSF exuviae managed to maintain their larger size and higher seed production even in the face of predation, suggesting that the plants better resisted attackers.
Additionally, using BSF skin benefited plant-pollinator mutualism — the interaction between different species leading to positive effects — and plant fitness, even during herbivore attacks.
After five weeks, the scientists even found fewer aphids (Brevicoryne brassicae) on plants that were grown in supplemented soil.
They finally observed no significant shift in the pollinator community, reinforcing the potential of exuviae as a biofertilizer.
“Insect production is a good strategy for fighting some of [our] environmental problems,” Barragán-Fonseca said. “But it has to be adopted by big companies and also small farmers.”
Keeping insects in the food loop
The BSF thrives on consuming organic matter, which it converts into larval biomass. The protein-rich larvae can be fed to livestock, reducing waste and land use while feeding animals at the same time. And now with the ability of its exuviae to boost plant growth as a third use, the insect could help develop circular crop production systems.
“Insect farmers can use organic residues produced in their farms to feed the larvae of the insect rearing,” Barragán-Fonseca said. “The use of insect-derived products represents a great opportunity to improve crop productivity within circular agriculture.”
She added she’s confident that producers will soon develop products from insect residual streams.
Earlier this year, another study by the same research center confirmed that like exuviae, BSF frass — a mixture of feces, substrate residues and shed exoskeletons — also had the potential to be used as an organic fertilizer and increase circular systems in agriculture.
Maya Zaken, head of newly founded food company Philafeed in South Africa, is testing some of these methods as a way to lower demand for the resource-intensive soy-based feed that’s currently widely used.
“We collect waste from Bao Wow, for example, a restaurant in Knysna town,” she said. “We process the waste [with BSF] and are left with two products: larvae and frass. Valley Organics [a poultry company] buys our feed for their chickens, which are later sold back to Bao Wow to be served in one of their baos, where the frass is sold to farmers in the area to grow food. Then, the cycle continues.”
Philafeed said it hopes to process 14 metric tons of waste a month with BSF in an upcoming facility.
“By turning waste into a resource, and creating closed food loop systems, we are meeting local farmers’ needs in a way that is not invasive on the environment,” said Zaken, adding she plans to market exuviae in the future. “Increasing the sustainable inputs of protein and fertilizer and, subsequently, the demand for these sustainable alternatives…can have a massive, multiple-effect impact for the environment and food security.”
A study in the Journal of Cleaner Production published in late April assessed market perceptions of BSF products among Australian farmers, who agreed that BSF-derived fertilizers would improve the environmental sustainability of the agricultural sector. They also said they were interested in using the product.
Barragán-Fonseca said insect farming can take place either in small structures or large industrialized facilities, meaning that both big and small players have a role. She said she’s now exploring options beyond the BSF, looking into the effects of other insect residual streams from the production of the yellow mealworm (Tenebrio molitor) and the house cricket (Acheta domesticus).
She said she’s obtained positive results from these.
Banner image: The black soldier fly is gaining momentum in food production systems. Image by Oktavianus Mulyadi via Unsplash (Public domain).
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Citations:
Barragán-Fonseca, K. Y., Greenberg, L. O., Gort, G., Dicke, M., Van Loon, J. J. A. (2022). Amending soil with insect exuviae improves herbivore tolerance, pollinator attraction and seed yield of Brassica nigra plants. Agriculture, Ecosystems & Environment, 342. doi:10.1016/j.agee.2022.108219
Elissen, H., van der Weide, R., Gollenbeek, L. (2023). Effects of black soldier fly frass on plant and soil characteristics: A literature overview. Wageningen Plant Research, WPR-996. doi:10.18174/587213
Kragt, M. E., Dempster, F., Subroy, V. (2023). Black soldier fly fertilisers by bioconversion of livestock waste: Farmers’ perceptions and willingness-to-pay. Journal of Cleaner Production, 411. doi:10.1016/j.jclepro.2023.137271
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