After weeks of adding food wastes the bedding goes down due to a combination of worm activity and micro organism activities. Decomposition and composting are taking place. The large amounts of castings might become harmful to the worms. So if we see bedding becomes darker in colour, it is the time to consider harvesting the castings and changing the bedding.
If the worm bins are correctly maintained, worm population should be double every 2 to 3 months. To harvest the worms from the old bedding, dump the contents of the worm bin on a sheet of plastic or a table. The worms will go down into the pile if we expose them to strong light. After a short time remove the top layer of the bedding up to the point we encounter worms. Wait a short time, and continue peeling the pile. Repeat the process until no more bedding left. We will end up with lots of worms cling together into a red meat ball. This is stressful for the worms so we need to quickly weigh them and put them back to their new bin.