With the collected BSF frass, you don't have to throw it away. Instead, you could put the frass to good use around your farms and garden.
But first, for some whom may ask:
What is BSF frass?
For some that may ask, simply, frass is the leftover residue after you harvest out the BSF larvae themselves. Frass usually includes undigested food pieces, BSF poop, their exoskeletons and sometimes the liquid (or leachate).
With that out of the way, let's see some ideas for using frass now.
#1. Feed For Worms
If you raise red worms or have a vermiculture system, you can use the BSF frass to feed the worms. Frass is a good source of nutrients for the worms. This is because of the contrasting eating behavior of black soldier flies and the worms, meaning, what the BSF usually don't eat (and excrete out), the worms will eat.
#2. Compost the Frass
Secondly, you can compost the frass. It is recommended to age or mature the frass for 1-2 months before using.
As the frass we collect is only 12-15 days old or younger (the time during which we raise the larvae), the microbial activity in it is still high (the microbes are still eating digesting the leftover frass).
If we use the immature frass at this point, it may compete for oxygen and nitrogen in the soil, and thus may damage or kill near-by plants. Composting thus helps age the frass, lessen the microbial activity and therefore leave nice broken down nutrient pieces ready for use for our plants. For good effects, you could add in some molasses or trichoderma when composting.
With the liquid part, you can use it to make something like compost tea. With the aged compost, you can use it to fertilize plants or sell it to other people as a stable mature compost.
#3. Use As Fuel Or Biogas
For residue that is a bit more wet, it can be used to feed to biogas reactor for fuel production. Some early results have shown that the biogas amounts generated from the BSF frass is comparable to that from cow manure.
Through anaerobic digestion (meaning microbes producing biogas from frass in an oxygen-lacking condition), this could be used to produce fuel and energy.
Similar to what farmers have done, that is putting pig or cow manure in closed bags to get gas to fuel their stoves, this energy source may come in handy on rainy days when woods or straws are overly moist or wet (hard to catch fire). My dad taught me this (thanks & memories to dad).
The other thing you can do to utilize bsf frass is:
#4. Attract BSF Flies
The liquid part of the frass, also known as leachate, can be used to attract flies (potentially pregnant female BSF) from the wild environment. Sometimes, the smell from the larvae themselves can attract BSF nearby.
I have actually seen this myself on one occasion, where a BSF finds its way from miles and kilometers afar to land on one of my local larvae boxes. I didn't know there were BSF around my natural environment so that was certainly a pretty sweet surprise.
This leachate, the smell of it can also deter other insects like houseflies or wasps from coming near your BSF colony as you're getting it established.
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