I have had success in the past when I want to harvest a bottom tray by putting a favorite food in the top try and waiting a couple of days. By then all will have moved out of the bottom tray that are going to. There may still be a few worms in the bottom tray so then you are left with a decision . If you want to save those worms its will need to be done manually. I have so many worms that if I have only a few worms in the bottom tray I usually just screen the castings and those few worms along with the castings go into my storage bucket till I need it.

When I harvest a bottom tray, I place it on the top of the worm factory where I have a shop light mounted about 12 inches over it and turn on the light. After a few minutes I scrape off the top layer of castings, since the worms have moved deeper to escape the light. I continue this process until I have collected all castings in the tray.