If the shiitake log has been completely spent by the mycellium it is by definition already ready for most species of stag beetle once ground up. This is essentially white rot wood. This is how the breeding logs they sell in Japan are sold. These logs are capable for rearing most stag beetless. This is also how the mushroom substrate they sell is created, it is a byproduct of shiitake log production. In Japan they sometimes it without being composted further, and it is suitable substrate for several stag beetle species. For rhino beetles, you will definitely have to compost it further. It would not hurt for you tor ferment it further for your stag beetles either. A word of note though, not all stag beetles can survive on a white rot wood only substrate... but that should only really matter if you happen to keep exotics somehow. Think of it like kinshi that has been accidently fruited. It isn't as good as kinshi for stag beetles, but the lignin has been nearly completely broken down though.
I got some logs a couple months ago and breaking down all of them by hand was a pain in the ass. Not to mention the amount of mold it grow if you keep it wet even for like 2 days. I've tried giving the wet logs directly to both rhino/stag larvae and it seemed like there was little to no nutrition left in it considering the fact that they took forever to grow/molt but to be honest it's probably because it's not composted enough to the point where the larvae can get all the available nutrients. I think the best way to use these is to crush them into fine pieces and then adding it to your flake soil mix for fermentation. In my experience, the flake soil that was fermenting generated a lot more heat and was able to finish faster. If you happen to ferment large amount of logs, let me know how it goes. I'm interested to see if the oak logs are worth the hassle.
Does the nutrition come from the additives added or the fermentation process? I know the larvae of Trypoxylus dichotoma are sometimes found in broken down shiitake logs in Korea and Japan and surely there aren't any additives added.
Yes, in fermentation process. If the compost left outside for a while, fermentation may occur naturally. T. dichotomus and some other scarab larvae are found in such compost simply because females lay eggs here and there wherever she can. Adult beetles can't really tell whether it is actually nutritious or not..
