I have had a test mealworm drawer farm up and running for some time and I think now is a good time to share it. I chose the Sterilite
29 Quart drawer for a few reasons. The drawers need to prevent insects from easily gaining access so it needs to be enclosed; no open sides. I also wanted a deeper drawer so that I could stand egg crates up on their side to experiment with density as well as methods to keep the worms from climbing up into the egg crates; beetles only.
I also wanted the ability to have multiple farms to try to time them at different points so that I always have worms. So I went with two drawers. They stack but not in a captive way, so I attached them together using rivets so that the weight of one drawer can hold the unbalanced weight of an open drawer.
In my past experimenting I found that no matter what I tried, the farm got too moist, which inevitably leads to the "three M's": (Mold, Mildew and Mites). Some folks cut lots of holes to alleviate this but then you can get insects flying in. Some will also glue in small mesh screen but I have found that only keeps out the larger insects and not the really annoying gnats, fruit flies and whatnots. And even with holes, you are still relying on convection to move air and vacate moisture, so you really need inlet holes and a good design for that to even work properly. I did it differently and have found great results so far.
I installed a small 12 volt computer fan pulled from an old power supply to exhaust air from the farm. This thing is installed on the lower part of the top drawer so that it can spread the draw across the back and sides of the drawer to prevent any specific high flow areas that might pick up debris. I also drilled a lot of holes between the two drawers so that the fan could draw air from below so that both drawers get exhausted. If i were doing only a single drawer I would have used a smaller fan, smaller voltage power supply or perhaps soldered in a potentiometer so I can adjust the speed. There is just enough opening above and around the front of each drawer to allow air to seep in while still blocking bugs. Any moisture or scent that might attract bugs is now only coming from the rear of the exhaust fan which makes it impossible for them to enter. An added benefit to this is that the constantly moving air makes the farm not smell. Nothing smells anymore, not even when I pull out a drawer to work on.
Iterations of the project have taught me that there is absolutely nothing that can be done to automatically separate the beetles from the worms. If the beetles can climb, so can the worms. And strangely enough, I realized that both worms and beetles have a propensity for particular actions: the worms like to climb and the beetles like to dig. With this, I came up with something that has been working for me.
First, I keep two egg crates in the worm drawer. The adult worms and fresh beetles tend to congregate on, in and under these things. The crates make it VERY easy to harvest mealworms for feeding, and they also make it easy to collect freshly hatched beetles to put into the nursery. After every harvesting, I will fully tap the crates clean of any debris so that they can collect a new clean batch of adult worms for next time. It has never been this easy.
Second, I keep a container inside the drawer with egg crate in it to allow the beetles to breed and lay eggs safely without adult worms trying to eat them all. The nursery is simply a smaller container that you keep in the drawer itself. I have been using square plastic ice cream buckets. Being in the main drawer allows the beetles to share the respired moisture created by all of the worms, and it makes it easier to service the farm. All you normally have to do is collect beetles one by one and drop into the nursery and then weed the refuse a little bit. I will take a drawer over to my bench to work on it. Every two or three days I usually first harvest a mass of worms for that night of treating, then I dust off the worm crates, collect beetles, weed the drawer and then finally feed.
Dont forget to feed the farm. The beauty of the exhaust fan is that things no longer spoil the farm. It is easier to feed, but I still usually go with carrots and occasional lettuce and cooked potato leavings. Wet stuff just causes problems in a worm farm, so I choose the less moist stuff for feeding. Worms also seem to very much enjoy boring into spent corn cobs.
Harvesting beetles is easy now. You simply pluck them off of the crate, lift the nursery and you'll find a bunch, and you can dig around a little in the substrate to get a solid feeling of completion.
It is a good idea to make notes on the drawers themselves using a dry erase marker. I track when I cleaned out the drawer and nursery.
Six weeks seems to be enough time to hatch some eggs and then dump the entire contents of the nursery into the worm drawer for a refresh. The worm drawer itself may last longer or shorter depending on what you do with it. I keep mine pretty well weeded but there does come a time when you need to clean it out. The good news is that if you clean the worm drawer just before you dump the nursery into it, you dont waste any small worms. All of the worms will be of larger size and they can be screened out easily. This makes the whole idea and process of worm farm drawer cleaning easier.