The "casings" are pupae of a moth.

The stages in a moth's life cycle include the following:

1. Egg
2. Caterpillar (the active feeding stage)
3. Pupa (a non-feeding stage while the caterpillar transforms into the adult)
4. Adult moth.

At this time of year, a pupa in the soil is likely to be the winter cutworm, Noctua pronuba.

You can verify this ID by rearing the pupae. To do so, put them in a clear container with a porous lid, perhaps a piece of toweling held in place with a rubber band. Put it somewhere you'll see it daily, but not in in the sun. When the adult moth(s) emerges, check to see if it matches the pictures at the above link.