I just came across a posting by Kelly Slocum on Sept 8, 05. She is a worm expert that used to post here and she said if the castings are kept in the dark, they can be stored for up to 3 years w/o loss of potency. Hope this helps. You can search her name at the bottom of the page for some of the older postings. I find them very informative.

Does anyone know why they have to be kept in the dark once the worms are gone? I put mine in an old plastic container that food came in while waiting for the babies to come out of their cocoons. It's plenty dark in he middle for the newbies. Does the sun affect the bacteria in the compost??

I'm just guessing but i would think that the ultraviolet spectrum of sunlight sterilizes the castings.