Castings should be harvested about twice a year. When most of the bedding has been eaten, it is time to harvest the castings and add new bedding.
“Harvesting” the castings means removing finished castings from the bins while leaving the worms in the bins to make more castings. To do this, the worms must be separated from the castings they currently inhabit without excessive handling.
There are two common ways to harvest the castings. The first method is recommended. Move everything (worms, castings, partially decomposed paper and food) to one side of the bin. Pick out the partially decomposed materials and put them on the empty side. Place food on top. Cover the food with fresh paper strips. Replace the lid and leave the bin. Within a few weeks, the worms will have finished whatever edible remained in the finished castings, and will move over to the new bedding and food. At that point, remove the castings from the bin (wear gloves). Carefully review or sift to make sure you leave all worms in the bin.
The second method utilizes the fact that worms hate light. This method is less desirable because worms hate being handled and because it requires more effort. Take finished castings and worms out of the bin. Make a pile of the earth castings (worms will be mixed in there) and shine a light on it. The worms will move to the middle of the pile in order to get away from the light. Brush off the outside layer of worm-free castings with a very soft brush. The worms will move closer to the middle to avoid the light. Brush off the outside worm-free layer again. Repeat this until you are left with a ball of worms. Return the worms to the bins. Wash your hands thoroughly after handling castings and/or worms.
Add new bedding to the bin until it is 1/3 to 1/2 full.