Black soldier fly value as an alternative source of protein and fat is increasingly widely appreciated, which is a good thing. However, it should not overshadow another important product of fly farming, namely frass. This is an excellent soil amendment that is a good fit in sustainable agricultural systems, including organic farming.
The original meaning of the word “frass” is insect feces. It was commonly used to monitor the presence of concealed pests, such as termites. Black soldier fly frass, however, also includes the unconsumed leftovers of larval substrate, as well as skins cast by growing and molting larvae. It is essentially an equivalent of manure produced by other farmed animals, which also consists of a mixture of feces, urine, animal bedding such as sawdust or straw, feed waste, etc.
Just like traditional manure, frass is a valuable soil amendment that improves its physical and chemical properties and serves as a source of nutrients both for cultivated plants, as well as for beneficial soil microbes. However, it also has an additional benefit of containing chitin, a biological polymer that is an important structural component in insects, nematodes, crustaceans, and fungi. As old skins are shed by molting larvae, they disintegrate and become incorporated into frass, thus enriching it with chitin. This chitin provides ample food for certain species of bacteria and fungi that can digest it. As a result, populations of these microorganisms become more abundant and make life difficult for pest nematodes, insects, and fungi that also live in the soil and need chitin to maintain integrity of their bodies. As their chitin is being destroyed, mortality of these pests skyrockets.
Similar to other types of manure and compost, black soldier fly frass may have a wide range of properties depending on larval substrate, rearing conditions, time of incubation, etc. In some cases, it may be ready to use right after coming out of a larval bin. In other cases, it may be advisable to wait for it to mature or to mix it with other composts. Therefore, frass properties need to be tested using well-established compost testing techniques. This will maximize the benefits of subsequent field applications.