I was dismayed last week, when I popped out to check on things in the garden, to find this:
The stem has been chewed through, right at the base, and just under the soil I found a little maggoty grub curled up where it had been. (Sorry, no photo; I was in too much of a hurry to squish the little bugger.)
This is the work of a cutworm - a brutally destructive pest I had not come up against until now. Cutworms are the larvae of several species of moth. They lay their eggs in the soil, and when the grub hatches it wraps itself around the first plant matter it comes across - usually a stem of a young plant - and tucks in. Seems a remarkably inefficient way to feed, killing off the whole plant in one bite, but these things are sent to try us... From the look of the grub I found, and the fact this is a brassica plant, I'm guessing the turnip moth is a pretty likely culprit - it's also one of the most prevalent types of cutworm.
Where cutworms are a problem, young plants can be protected with collars around their stems at soil level and sticking just below the soil. A few layers of newspaper can be used, or paper cups or toilet rolls, foil pie trays, 1" diameter plastic pipe, or short pieces of drinking straw, slit lengthways so that the stem can expand and the collar be removed later. (Once the stems are about the thickness of a pencil, cutworms are no longer a danger.) I've seen cornmeal around the base of the plant recommended too - apparently the larvae gladly eat it but cannot digest it, and die - and another method is to stick toothpicks or cocktail sticks in the soil round the plant, in the hope the grubs will wrap round those instead (though it seems to me you'd have to check them for grubs regularly, or they'll just move on to the next stem...) Winter soil cultivation can also help, by exposing overwintering larvae.