They do look like "white grubs" which are the larvae of Scarab beetles. That could include several different species, like Japanese beetles, June beetles, European chafers and so on. They can be identified by the pattern of spines on their bottoms (raster). For the non-squeamish look on Google for raster pattern white grub and you could try and identify them yourself. It's not always easy to do, so your extension office should be able to ID them as Rick suggested.
Did you use garden soil in the pots? Just wondering how they got in there.