Little white grubs-not vine weevil (I think)

They could be very young vine weevil grubs which haven't matured much yet, couldn't they? They don't usually seem to get a brown head until later in their development, before they start growing legs. If they were these, I would expect all the roots of your mizuna and land cress to have been eaten though. They may just be some other kind of beetle grub.
It's difficult to know whether you should destroy them, isn't it? Are the ones in your trough curled round into a U shape? If so, I would kill them. I keep an insect book by me for when I'm in the garden, because so many of these grubs are difficult to identify, but there are several other beetle which have white grubs, according to this book of mine! Rather you than me on this one. Perhaps someone else will be of more help. Annie (Cumbria)
P.S. American land cress will survive through the winter, by the way. You don'r have to pull it up each year.

Thanks for this Annie. They don't curl up and they move much more than vine weevil grubs. They seem to have vanished now that I have pulled the mizuna out - the landcress is still there, perhaps they have headed under it instead.

I'm not sure the cress will survive in the Czech Republic - it can drop to -30C in the winter months. But I will certainly leave it (if the grubs don't get it) and see what happens.....

they might be ant larvae, they are grub like in the early stages.

could you post a pic if you came across them again.

I'll try......