Pest
Onion fly
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Watch for | Watch for in May | Watch for in June | Watch for in July | Watch for in August | Watch for in September |
What it is
Onion fly looks like a smaller, greyer house fly. It lays its eggs at the base of onion plants, and its maggots feed inside the young bulbs. It is not a problem every year in every garden, but where it is established it can wipe out a row of seedlings in a fortnight.
Symptoms
The first sign is usually a seedling whose leaves go yellow and limp, then collapse. Pull it up and the base comes away easily, often soft and smelly, with several white, legless maggots up to about 8 mm long inside. Later attacks on older plants cause bulbs to rot from the bottom, sometimes followed by other rots. Onion white rot also causes yellowing and collapse, but it leaves white fungal growth on the base instead of maggots.
Cause
The adults emerge in late spring from pupae that spent the winter in the soil. Females are drawn by the smell of onions, particularly the strong scent released when foliage is damaged by thinning or weeding, and lay eggs in the soil beside the plants. The maggots feed for around three weeks, pupate in the soil, and a second or third generation can follow in July and August.
Control
Lift and destroy affected plants as soon as you see them, taking the soil immediately around the roots so you remove maggots that have already left the bulb. Do not compost them. Firm the soil back around neighbouring plants. There are no garden insecticides worth using against this pest.
Prevention
Grow from sets where you can, as they are much less attractive than seedlings. Cover seed-sown rows with fine insect mesh or fleece from sowing, sealing the edges. Sow thinly to cut down on thinning, and when you do thin, water first and remove the thinnings. Rotate alliums around the plot and dig the old bed over in winter so birds can eat the pupae.
Plants often affected
Could it be something else?
These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.
Months to keep an eye out: May, June, July, August and September.
Sources
Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.