What I use for superworms and/or mealworms.
Superworm and Mealworm Bedding (Diet)
I provide approximately 3-4 inches of high quality and nutritious substrate for the worms to eat and burrow in. Main ingredients are rolled oats, bran, baby rice cereal, wheat germ and poultry laying mash. Other ingredients included in small to very small amounts are dried veggies, tropical fish flakes, alfalfa powder, bee pollen and acidophilus. The exact amounts and ingredients are not written in stone. You can play with the recipe and see what works for you.
This bedding provides the worms with a well balanced diet. To keep them hydrated, I use fresh greens, vegetables and fruits. I replace the source of "water" usually every other day or sooner if it appears rotten or moldy.