Plants grow slowly, turn yellow, wilt, and die. Bulbs may rot in storage. White legless maggots, up to 1/3 inch long, burrow inside the bulb.
Discard and destroy maggot-infested onions. Clean all debris from the garden at the end of the season to reduce the number of overwintering pupae. You may keep adult flies from laying eggs with row covers. Rotate the onions to a new place in the garden, then put the covers in place at the time the onions are set out or seeded. Be sure they are tight to the ground, without spaces that flies can enter.