onion fly

Leeks and onions are occasionally attacked by the Onion Fly, which is particularly troublesome in the larval stage, when the maggots tunnel into the plant tissue.  The Onion Fly, which looks like a small grey house fly, lays its eggs in the spring and early summer, near the base of the leek or onion, on the leaves and in the soil nearby. The maggots are white and about half an inch (1.25 cm) long when fully grown.

Like onions, young leeks seem particularly vulnerable. Lift and destroy infested plants.  When lifting the infested plants, make sure there are no maggots left lying on the ground, and because the flies are attracted to the smell of fresh manure, make sure the leeks and onions are not planted on freshly manured soil.

There are chemicals to treat the soil where onion fly maggots are a big problem. These are readily available at most garden centres and nurseries.