Hmm its cabbage root fly maggots/larvae; bad luck! They can be kept at bay by making cardboard ‘collars’ to rest on the surface around and close to young brassica plants. Bit too late for those plants.
In most cases its rare to suffer damage like you have, it depends on weather etc. I lost a lot of calabrese one year, but don’t bother with the collars. Possibly fleece covers help to keep them off new pantings in early spring, but all my covers are off now & damage is just slight, so far.
