I had to give up the 3 drawer system- the beetles could get OUT of the drawer they were in because the top of the drawers do not touch the frame, sometimes they fly and they would land outside and I hate having bugs loose in my house. The screening on the bottom did not work very well either because the screening would catch when trying to monitor/tend to the beetles and it would end up dumping wheat bran everywhere thru the screening when the drawer was opened. I went to having the beetles in covered bins with holes in the lid that have screening over them, and am very pleased with that.
I now have several drawer sets with various stages, some are just pupa drawers and some are just babys hatching from paper/frass that was in with the beetles and then theres the big fat mealies that supply the pupa drawer and the freezer for chicken bugfests for winter. I have to spend about 2 hours every 3 days to put everyone where they belong and wet the sponges and add a dry piece of bread and stick in new carrot sticks. (Potato slices mold for me so I dont use them) I have found that the closed bins that hold the beetles do need the moisture but if you stack the bins, the sponges stay moister longer in the ones under the top one even with separating boards between them. I am careful to switch them around every so often because with the sponges not drying faster in the under bins I think theres more likelihood of a mold starting that will kill that bin off. I label the beetle bin with the month I started it so I know about what stage it is in. I lay the sponges on a piece of hard plastic (plastic wrap or folded plastic bags make a mess) I cut from something or other to keep it off the wheat bran- it can cause mold if laid directly on the substrate. I save bread and dry it and lay a slice in the mealies and beetles-they can eat a whole slice in just a few days.
I do NOT put anything in the pupa drawer except wheat bran as a cushion and for food for the new beetles till they get put into a beetle bin. A small percentage of the pupa do seem to "die"-shrivel up-dehydrate whatever. I just toss them.
I have to say, that frass is sure something for plants. I am in a state that has approved Medical marijuana so I "could" sell mine for that purpose, I dont approve so I'll have to find a way to sell it for another purpose. (long story-divorced ex because he went crazy when state went ahead for Med. Marijuana, his plants never lived (amazing what a few drops of vinegar does to a seedling or cutting) then started buying it-800 to 1200$ a month. Trouble was, there wasnt anything wrong with him to warrant being high 24/7-he just liked being high. I said over my dead body, so here I am alone. again)