Like all insects meal worms need a moisture to thrive and reproduce. There is a ton of info out there on the web on this subject. In our years of breeding insects we have tried all the methods of providing moisture. Adding water directly to the substrate will cause mold and kill your colony. We have worked out several methods to successfully provide moisture to our meal worm colonies.
First you can use a simple paper towel folded in half or quarters, add some water and squeeze out the excess. Place the paper towel on the top of the substrate. This can still cause mold. So you need to leave the top off the meal worm tray. Check the meal worm colony frequently and make sure there is no mold. Now the best way to provide moisture is from sliced vegetables. We have been using potato slices or whole carrots.
The meal worms get their moisture directly from the vegetables and receive nutritional value as well. In a few days all you need to do is dig through the substrate and remove the dried vegetables. In the last few years we have discovered what we believe is the best moisture source there is.
The nopal cactus (opunita streptacantha). We slice the nopal cactus pads into strips and place these on top of the substrate. When the worms are finished the only thing left is the thin skin of the cactus. This species of cactus has few thorns and the few there are can be removed easily with a knife. We have had absolutely no problems with mold and have found that the meal worms consume the cactus before any other moisture source. If you have access to nopal cactus we strongly recommend trying it as a moisture source for your meal worms.