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If you consider yourself a homesteader, an urban farmer, or just love gardening, black soldier flies are your best friend. If you missed the memo, black soldier fly larvae can upgrade your compost bin to a whole new level. Black soldier fly larvae (let’s stick with BSFL from here on) have such an efficient gut biome that they can compost just about anything. And yes, we mean anything- dairy, citrus, meat, even animal droppings. When the larvae get done composting your kitchen scraps, they can be used for awesome animal feed.
But we’re interested in another thing they produce today: frass. Insect frass (or poop) is a normal byproduct of the composting process, and it can be used in half a dozen ways to improve your garden and animal feed. In this article, we’ll talk about harvesting and using black soldier fly larvae frass to improve your plants, animals, and soil.
Black Soldier Fly Biology
We don’t need to cover every detail about black soldier fly larvae in this article, but there are a few details that matter for collecting frass. The black soldier fly (Hermetia illucens) is a black fly species, similar to common houseflies. It starts life as an egg, and then turns into a sightless grub does one thing and one thing only: eat.
BSFL start out tiny. So small, you can’t even see them. By five days, they will reach about ¼-⅓ their total size as larvae. These “baby larvae” require some special treatment, which we’ll cover in a minute. By ten days, your larvae will be their full size, about half an inch long.
All the while, they eat like crazy and produce lots of frass. And while black soldier flies are perfectly capable of eating other animal waste, they avoid their own. In order to keep them eating and growing, you will need to separate out their frass as it accumulates.
After a few weeks, your larvae will darken in color and quit eating. At this point, you will harvest them to use for feed, or let them turn into flies to start the cycle over again.
What is Frass?
BSFL are simple. Food goes in, frass comes out, the larvae get bigger and turn into feed for your chickens. If you didn’t know what you were looking at, you might not even recognize BSFL frass in your compost bin.
When your larvae are “babies,” their frass is very fine and has a dark brown color, almost like espresso. As they grow to their full size, their frass does too. Eventually, it will look almost like soil.
Benefits of Black Soldier Fly Frass
BSFL frass is rich in nutrients, especially potassium, nitrogen, and phosphorus, all things that plants need. If used as a liquid fertilizer, it can improve plant health and produce better, bigger crops. It’s a totally organic fertilizer, which means you don’t have to dump harsh artificial chemicals into the soil, which then drain into our waterways.
It’s also full of chitin, which can improve the structure of soil and make it more porous, allowing it to store water better. This, again, improves the quality and quantity of your garden’s produce.
But it has another benefit, too. Chitin is what insect exoskeletons are made of, and plants have evolved to detect it. When plants notice higher than normal levels of chitin in the soil, which they sense through their roots, it causes a kind of immune reaction.
From the plant’s perspective, the soil is full of insects, which means it’s about to be overrun and eaten. So its defensive adaptations go into overdrive to repel the insect invasion. But in reality, the plant isn’t at risk at all, and now it has its defenses up, which will make it impervious to any actual pests that show up.
BSFL frass has also been shown to be an effective ingredient in fish meal, particularly for omnivorous species like tilapia. Some research has suggested that it can improve chicken feed as well.
How to Collect Black Soldier Fly Frass
Right off the bat, remember that collecting frass is not optional. High concentrations of frass in your compost bin will stop your BSFL from eating, which is bad. We recommend sifting frass out regularly to keep your larvae happy.
To separate frass from your larvae and the rest of the stuff in your compost bin, you need two sieves: a “wide” 5mm sieve and a “narrow” 3mm sieve.
First, dump your compost bin onto the 5mm sieve over a plastic bin. Uneaten food will stay on top, while your grubs and their frass will fall through. The food scraps go back in your compost, the grubs and frass will get sieved again.
Dump them onto the 3mm sieve over another plastic bin and shake it continuously to keep your BSFL from getting stuck in it. Their frass will fall through, while your larvae stay on top. If your larvae are still babies, you will need to use an even smaller 1mm sieve to get their frass. Five-day larvae will fall right through a 3mm sieve.
How to Incorporate Frass into Your Homestead
Once you have your frass separated from your black soldier fly larvae, you have a couple options on how to use it. The simplest is to use it as fertilizer in your garden. Just spread it over the soil. This is especially effective before you till. The frass will improve the water holding capacity, organic content, and nutrients of your soil and keep harsh chemicals out of our collective water system.
You can also use frass to make “tea,” a powerful liquid fertilizer that you can spray straight onto your plants. We normally mix about a half a cup of frass into a gallon of water and let it steep for a few hours. You can let it keep steeping overnight, even. Then simply transfer it into a spray bottle and spray it on your plants to give them a boost.