onion fly
 

My first garlic crop is looking like it's going to be totally
destroyed by onion fly. Leaves have turned yellow on some, and after
pulling up, 'bulb' (what's left of it) is infested with small white
maggots. Has anyone got any organic methods of prevention? I have some
onions about 4 feet away that don't seem to be affected yet (no
yellowing of leaves). Will these maggots move through the soil to
infect them, and if so, would it do any good putting a barrier (e.g.
piece of wood)into the soil?
thanks
Ross