Hi Mike, yes, keep them in the fridge, in an airtight bag. Use them within 3 or 4 days. Put them in water while you fish. Leave a few out to darken and turn to floaters, and you can try these as hookbait as an alternative to the sinkers.
Btw – the sinking/floating business: Casters are the stage between maggot and fly. When they first turn, they are full of liquid and sink. As they develop, the fly embryo takes shape and they float as the air around the fly in the shell changes the balance.. You're aiming to arrest their development and keep them at the sinking stage, hence the airtight bags and keeping them under water, but you can't hold them back indefinitely and they'll die and begin to decompose if you store them too long. Thus the relatively short shelf life.
A few random caster points. Tackle shop casters can vary in quality. The best casters come from white maggots. You can tell a) by the goo inside when you crush one b) by the colour of the paler, newly turned ones. Dying maggots tends to shrink them, giving you smaller casters. Understandably, some shops caster all their left-overs, but the best ones use only whites. You can keep them in the plastic bag you buy them in, but often the casters touching the bag suffer “fridge burn” and get darkened patches. To keep your casters in the kind of luxury you yourself would enjoy, decant them so they almost fill an airtight container, like a baitbox with no holes in the lid.
On top of them place some dampened newspaper, cut to fit the box. When it's full to the brim, or a bit over, put the lid on tightly. These casters will thank you for it by outshining and outlasting their cousins, degrading in a poly bag.
When to use them? Well, they're very good, in groundbait or loose fed, for attracting and holding bream, but since your local bream hurl themselves on your pellet and corn feed, you would be catching the same fish with a dearer and more temperamental bait. They're a great river bait for pretty well all species, used in various ways, but I know you're mainly on stillwaters just now. So I'd suggest you choose a stillwater with plenty of roach and target them.
A good feed for roach is a mix of hemp and casters, 50/50 is as good a ratio as any to start with, in a bait tub, just covered in water, and once you've got your rig set up – a 3 or 4 bb waggler, set just off bottom, with the shot mainly round the float and just a couple of small shot down the line, fished a couple of rodlengths out – you can feed a dozen bits of this mix every couple of minutes, or every cast when you start to get bites. On the hook you can use a caster, two casters or even a maggot or two. And be prepared for the fish to start intercepting the falling feed – the sign is that you get bites as your bait drops – whereupon you can shallow up to meet them halfway.
This post has gone on a bit, but I wasn't sure how much you already know. Casters are a bit dearer and more trouble to care for than maggots, but they will catch a better class of fish – the immature fish that dive on maggots don't seem so interested - and it's well worth getting to know the in's and out's. Some of my most enjoyable fishing has involved nothing more than a cupful of casters and a winter canal.