Onion fly: how to recognise and treat it

Pest

Symptoms

Seedlings collapse and die; one maggot can kill several neighbouring seedlings.
On older plants the outer leaves turn yellow and wilt.
White, legless maggots up to about 10 mm long feed in the roots and bulb; the bulb rots and becomes inedible, and damaged bulbs rot in store.

Cover seedbeds and new plantings with insect-proof mesh or fleece right after sowing, edges sealed with soil; do not cover a bed where onions grew last year, as flies can emerge underneath.
Water a mix of insect-killing nematodes into the soil to control young maggots; they also affect other soil insects, so use them only where needed.
Lift badly damaged plants carefully and destroy them before the maggots finish developing.
Encourage ground and rove beetles, birds and hedgehogs, which eat the maggots and pupae.

Sprays reach only the adult flies, not maggots inside the bulb; where registered for home use on onions, pyrethrins or a pyrethroid such as lambda-cyhalothrin can be aimed at the flies — follow the label dose and pre-harvest interval.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Grow onions and shallots from sets rather than seed — they survive the first generation of maggots better.
Remove cull bulbs and self-sown onions; do not leave damaged bulbs near the bed after harvest.
Do not grow onions in the same place in successive years, and avoid soil with fresh manure or undecomposed organic matter — let manure break down before planting.
In problem spots sow later, once the soil has warmed; yellow sticky traps show when the flies are active.

Figures checked against these sources.