I don't know what these grubs are actually called but I call them 'ground grubs'. I'm pretty sure they are a moth larvae. They are bad for your garden but great for fishing. They are a bit like a whitchetty grub, just not as big, found in dirt instead of trees, and I don't think you should try to eat one.

Go to stores like WalMart, or hardware stores, and there is a chemical just for that, I believe by Ortho, but there are others. It's a soft, dry, granular product that you lightly sprinkle on the soil. This works great on lawns but I'm not positive it's OK for planter boxes but I would assume so. Check the label.

Are these chafer grubs? http://en.wikipedia.org/wiki/Cockchafer http://www.kendall-bioresearch.co.uk/chafer.htm