are you useing hen manure, wireworms are attracted to it, do you put lime around your cabbage ect' when you rotate, if not I would start doing it , it sounds like wire worm, so now you know they like bird dropings but hate the lime, hope this helps nothing worse when you have a good crop to find these little bliters have tasted every thing befor you,

How large are 'tiny'? The other cause of small holes in potatoes, and more frequently occurring, is keel slugs:
"A burrowing specialist, notorious for its destruction of potatoes, it will in fact attack most root crops and is difficult to control as it spends most of its time underground." to quote a Cardiff University website:

http://www.cardiff.ac.uk/biosi/staffinfo/wocs2.html