Watch out for the eggs of the Large White or Cabbage White (Latin Name - Pieris brassicae ) butterfly. The eggs are laid on the underside of brassica leaves. So check your cabbage, sprouts, broccoli etc. regularly. The female butterfly lays batches of 20 to 100 yellow eggs underneath the leaves of the plants.
If seen the eggs should be removed. Soapy water will help unglue the cabbage white eggs from the leaf.