I use a handful or two of flake soil that finished cycling to inoculate the next batch. I make several batches in the summer. Also, I believe the spent flake soil from the larvae could be used as egg laying medium for the adult beetle breeding boxes. However, since adults don't really need the premium flake soil and the fact organic raised bed soil from the local garden center has shown efficacy in rearing several beetle species, I might just use this "cheap" store bought soil for my own breeding boxes, avoid the nasty frass, and then transfer the eggs/larvae to flake soil once collected.