To keep the black soldier fly frass dry, you can mix in rice bran, maize bran or coco peat. These act as moisture absorbers to keep the growing bin not too wet, and not turn too anaerobic or mucky.
Alternatively, you can use chicken feed. Like rice and maize bran, the larvae can eat this and it will keep the area dry and clean. Larvae then won't escape because of excess moisture and it will be easier to harvest the larvae and clean the containers later.
What I've noticed, if you mix in coco peat, is that the larvae don't eat this stuff. It will incorporate with their frass, contributing a part in the final product. However, some coco peat may leave a slightly sour/acidic smell. So take into account this when using.
For some more watery fruits, people let them dry or dewater them a bit before feeding the larvae. If you know some foods may turn runny after a few days in (like jackfruit or soybean waste), add in some foods that may turn dry or may cake (like bread or chicken feed) to balance the mix.
Sometimes, the ambient temperature can help keep things dry. If it's hotter on a day, the frass will dry up more and faster. On rainy days or nights, the frass may turn a bit more moist/wet. Possibly because of the high humidity in the air, and more so for the upper containers.
This is only my guess, but the body heat from the larvae when eating (around 40C/104F) may also warm things up. With some air flow, the moisture could evaporate more, keeping your bin dry, and frass less sticky.
My other guess is when I noticed wet frass (which contains the bsf poop + undigested foods + their exoskeleton), it led me to thinking: Are my larvae having diarrhea? You know that the frass is wet when you use a trowel and scoop through it, it sticks to the trowel. Many times, you'll know just by the look of it. And you will hear larvae slurping 'churp churp' into the foods.
I was afraid I was giving them too much bananas (twin bananas even) and they may turn bored of it. So from here, it rings true of something an experienced grower advises us, give larvae fruits/veggies and starch for energy for a balanced mix.
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