Mealwroms can best be kept in an open plastic tray or closed with air holes in the top. Use wheat bran found in feed stores as the bedding. They will eat the wheat bran. Use slices of potatoes or carrots for moisture.
For longer life, place the ones you will not use right away in your refrigerator set at 45F. When you are ready to use them, remove the desired amount and leave at room temperature until they become active. You must take them out of your refrigerator once a week, allow them to warm up and then put some slices of potato or carrot in so they can drink. Dont allow them to be in temps below 45 or higher then 90 or they will die. If not kept in the refrigerator then keep them in a cool dry place.
Oat bran, wheat bran, and corn meal are the most used substrates for keeping mealworms. They will eat their substrate but this does not provide hydration. It is also necessary to provide them with slices of fresh carrot or potato for hydration every 2 to 3 days, or when the current one dries up. .