I lost most of last years overwintering onions and some of my summer onions due to the allium leaf miner, which I had never heard of until this year. My overwintering onions this year are now covered in environmesh.
I been researching the allium leaf miner and I have put some information on the link below if anyone is interested:
http://notjustgreenfingers.wordpress.com/2012/09/19/the-allium-leaf-miner-and-homemade-pizza/
I hope it helps someone.