When young, onion plants will be seen to wilt for no apparent reason. Frequently the feeding damage will cause the plant to break off as it is pulled from the ground.
Floating row covers (fleece) are an effective way to prevent egg laying by adult root flies. Be sure to get covers installed early, before the newly emerged adults fly in search of host plants. At the end of each season, be sure to pull all onions from the garden. Dig and move perennial onions every fall.
By pulling up a sample plant the crop can be checked for the cream coloured onion root fly larvae feeding on the roots. Remove and destroy badly damaged plants, as the maggots can move from one plant to another. Dig around the area where infested plants have been removed to expose maggots to insectivorous birds and other predators.
Regularly dig and turn the soil during the winter months to expose over-wintering pupae to insectivorous birds and other predators.