Instead of sterilizing the substrate, is there any merit to the idea of inoculating the soil with leaf litter from outside, in an attempt to create a stable mini-ecosystem without any single dominant mold or mycelium? That seems to be what the beetles encounter in the wild, and it might prevent a single mold species from taking over quickly. Maybe that's a terrible idea, and it might be why I've never been able to raise a wild-caught larva to adulthood. (That, or the months of neglect. Or a mixture of both).