The onion fly is a common pest of onion but may also attack garlic, leeks and shallots. Young plants wilt and die and on older plants the white maggots bore into the bulb tissue around the root area which then softens and rots. Most damage occurs during June and July although plants can be attacked at other times of the year.
Infected plants should be destroyed to prevent the larvae going into the soil to pupate. To help reduce the number of over wintering pupae dig the area in autumn or early winter. Onions grown from set are less susceptible to damage from the first generation of maggots than seedlings. Covering the crop with horticultural fleece will exclude the female flies thus reducing the risk of attack.
There are currently no chemicals available which control onion fly however we recommend Nemasys 'Grow Your Own' Multiple Pest Killer which is a unique mix of nematodes to target a broad range of pests including onion fly, carrot root fly, cabbage root fly, leatherjackets, cut worms, ants, sciarid fly, caterpillars, gooseberry sawfly, thrips and codling moth.