Somewhere I read the grubs didnt like soil in the substrate. My compost *looks* nearly like soil, but is nearly entirely cellulose (a gallon of food waste in over a cubic yard of leaves and grass with another gallon of livestock urine) and the happy maggots are, well, very happy. Is it the mineral content of soil they do not like? Also, do you know if they use the nitrogen waste itself or the bacterial load feeding on it as a food supply? I'm wondering if they will occupy a nearby pile of just leaves and urine as opposed to a more mixed-content pile.
I haven't heard about BSF not liking soil, but I'm pretty sure it has no special appeal to them either. I don't think it hurts them. BSF grubs want to eat, and anything that isn't food to them is probably an obstacle at best. We know that they'll navigate through grass/leaves, but I doubt there is enough available nutrition to make it a viable food source regardless of nitrogen or bacteria.
In all of the commercial BSF bio-conversion setups I've read about I think they all divert urine from the waste before feeding it to the grubs. I don't think the urine would hurt them chemically, but it adds an anaerobic zone due to flooding.
I highly doubt you'll be able to attract BSF to a pile of leaves only, or if you do they won't stay in that pile. You've already seen how mobile the grubs are and as soon as the "pickins" get too thin they'll go elsewhere. That's the reason you need to design BSF units to contain grubs that try to leave. They may have happily lived in the unit for weeks but they'll migrate out at the first sign that the food supply is dwindling. By containing them you can hold them in a unit so that they're still there when you get around to feeding them. They just aren't "patient" enough to wait to be fed.
