Onion fly
A small fly whose white maggots eat the roots and bulbs of onions, leeks and related crops, causing yellowing, wilting and rotting plants.
Also known as: Onion maggot, Onion root fly
Symptoms and identification
Leaves turn yellow, flop and then collapse, usually starting with the outer leaves. Bulbs have soft, rotting areas hollowed by small white maggots, and young seedlings die. The damage is most severe in young plants.
Causes
Adult flies lay eggs on the soil near plants, mainly in late spring and again in summer. The maggots feed on the roots and bulb base for about 3 weeks before pupating.
Organic treatment
Remove and destroy affected plants and the maggots, and do not put them in the compost. Firm the soil around plants.
Prevention
Cover crops with fine insect mesh straight after sowing or planting, thin seedlings in the evening to avoid scent that attracts flies, and rotate crops. Grow from sets rather than seed, as larger plants are less vulnerable.
Chemical treatment (use with care)
There are very few insecticides approved for home gardens. Prevention with mesh and rotation is the most effective control.