🧅 Onion How hard it is to control: ★★★★☆ 4/5

Onion fly

Larvae of the fly Delia antiqua eat out the inside of young bulbs and the base of the leaves. The first generation in May can wipe out a whole direct-sown crop, because young plants have no chance to recover.

Symptom photos

Onion fly — Onion base — white onion fly maggots in blackened, rotting tissue — Onion base — white onion fly maggots in blackened, rotting tissue Photo: Rasbak at Dutch Wikipedia, licence CC BY-SA 3.0

Symptoms

Leaves of young plants yellowing and wilting
White, legless larvae in the base of the bulb
The inside of the bulb eaten out and foul-smelling
Plants dying and toppling over
Gaps in the row where seedlings have died

What it does to the crop

Young plants lost and a thin stand
Secondary soft rots in the damaged bulbs
Unmarketable bulbs that will not store
Losses of up to tens of percent in a direct-sown crop

What encourages it

First fly flight in May, second in July
Light soils that have dried out
Onions grown after onions or right beside them
Weak, slow-growing plants

Prevention

Cover the crop with fleece from sowing for the first few weeks
Intercrop with carrots to mask the onion smell
Keep distance from last year's allium plots
Deep autumn cultivation to destroy pupae in the soil
Remove and destroy infested plants along with the larvae

⚠️ Before you reach for a spray

This atlas is informational and still growing — different diseases can look almost identical. Confirm the diagnosis and read the product label before applying any plant protection product. If you are unsure, ask your local extension or advisory service.

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📅 When to Sow Onion? →