Onion problem

[ken]

I've just lifted my onions (Sturon, grown from sets). Most were fine, but two were rotten and one partly so. The roots on the two worst affected had disappeared. There was no white fluffy stuff or black pinhead marks which I understand are the classic signs of white onion rot. I cut them open: in one I saw a couple of tiny black insects but no sign of insect life in the others. Any clues?