Yes, you can use mushroom plugs. I make a substrate composed of wood pellets and wheat bran, water it with boiled water until it's at an adequate level of moisture. The moisture level should be just wet enough that water doesn't drip in a downpour when you squeeze a handful. After getting it to an adequate moisture level, I fill individual containers with the substrate and then set it in a pot of water that I set to boil for about an hour. After that, I let the containers of substrate cool and then carefully inoculate the substrate by dropping a number of plugs onto the surface of the substrate. I try to do this where I've previously cleaned an area that has low foot traffic to minimize the chance of contamination. I also work as quickly as possible when inoculating and try to open the lids of the containers only just enough to slip the plugs in before closing them.
Pearl oysters are recommended since they are pretty quick colonizers and reduce the chances of contamination resulting from spawn runs taking too long to finish. Dorcus species aren't supposed to be very picky about the type of fungus used, but you do want to make sure the fungus is a white rot fungus.
You cannot place the plugs into the substrate or breeding logs because if the substrate and logs are suitable for breeding and rearing, they've already been attacked by fungus and are no longer have sufficient nutrition for the fungus to grow. The spawn must complete its run before you can use the substrate. You can make your own breeding logs by inoculating fresh logs with plugs, but it'll be years before they're suitable for use.
