Onion Maggot (Delia antiqua) overwinters as pupae in the soil. Adult flies emerge in spring and lay eggs near the base of onion plants. The small white maggots hatch and tunnel into roots and bulbs. Young seedlings may wilt and die, while older onions develop feeding tunnels that allow soft-rot organisms to enter. Several generations can occur, and damage is often worse where onions are grown repeatedly in the same location.

How to Fix It
Crop rotation: Don't plant onions or other susceptible alliums where onions grew the previous year.
Insect netting: Cover onions early, before adult flies can lay eggs, and seal the edges securely.
Remove infested bulbs: Pull badly affected plants and dispose of the entire bulb rather than leaving larvae in the garden.
Clean up after harvest: Remove onions, culls and crop debris that can provide food for developing maggots.
Avoid damaged onions: Injured or decaying bulbs are especially attractive to later generations of onion flies.
Beneficial nematodes: Appropriate insect-parasitic nematodes can suppress maggots in the soil when conditions are suitable.