Onion fly

Identification

An inconspicuous, ash-grey fly 6-7 mm long, resembling a small housefly; the damage is done by its larvae - legless, white, glossy maggots up to 8 mm long. The larvae bore into the bulbs and the base of the stem and tunnel through them, causing the plants to yellow and wilt, while infested bulbs rot and become secondarily infected with bacteria. It feeds on onion, garlic, leek and shallot; two to three generations usually develop over the season, and the first, in spring, attacks the youngest seedlings and is the most damaging. The fly overwinters in the soil as a puparium (a barrel-shaped, brown pupa) a few centimetres deep, most often where onions were grown the previous year. The flight of the first generation usually begins at the turn of April and May and is traditionally signalled by the flowering of dandelions and early fruit trees; the females then lay small, white, elongated eggs singly or in small groups at the base of the plants and in soil cracks right next to the bulb neck. The hatched larvae travel downwards and bore into the basal plate and the fleshy part of the bulb. In the bed, an infestation is recognised by the progressive yellowing and drying of the oldest onion leaves, the wilting of whole plants, and the fact that diseased plants can be pulled out of the ground very easily - tunnels and white larvae are visible in a cut-open bulb and at the base of the stem. The damaged tissue quickly turns into a wet, foul-smelling bacterial rot, so that a single larva can destroy an entire bulb, and the focus spreads to neighbouring plants in the row. The later generations (summer and late-summer) damage bulbs that have already formed, reducing their storage life.

Treatment

The foundation is a multi-year crop rotation and spatial separation of the new planting from last year's onion beds, together with covering the beds with fleece from sowing onwards to prevent the flies from laying eggs. Early sowing or planting from sets helps (the plants are then stronger), as does regularly removing and destroying infested plants along with the larvae, and deep autumn cultivation that brings the puparia to the surface. Growing next to carrots is recommended as a mutual deterrent of the flies. The key to acting accurately is monitoring the flight of the flies with yellow sticky traps set low over the bed - this catches the moment of arrival and egg-laying, the only point at which any intervention makes sense. The larvae hidden inside the bulb are practically beyond the reach of plant-protection products, and most of the former soil insecticides once used against these flies have been withdrawn in the EU, so control rests above all on prevention, while chemistry - if used at all - targets the adult flies at peak flight, before they lay eggs. If a spray is used, bear in mind that pyrethroids are highly toxic to bees and beneficial insects, so they are applied only outside the pollinators' foraging time, under real pressure and in accordance with the label of an approved product. It also pays not to leave onion debris and heads in the field after harvest, because they attract females and favour the pest's overwintering. The greatest importance lies in protecting the first, spring generation on the seedlings - it determines the scale of losses across the whole season.

How to tell it apart from look-alikes

Feature	Onion fly	Cabbage root fly
Host plants	Onion, garlic, leek, shallot - allium crops	Cabbage and other brassicas/crucifers (cauliflower, broccoli, radish, oilseed rape)
Where the larvae feed	The basal plate and fleshy bulb and the base of the stem - tunnels inside the bulb	The root and root collar - the larvae gnaw at and tunnel through the root system
Symptom on the plant	Yellowing and drying of the onion leaves, rotting of the bulb	Wilting and bluish discolouration of the leaves, stunted growth, the rosette pulls out of the soil easily
Appearance of the adult fly	An ash-grey fly 6-7 mm, like a small housefly	A very similar ash-grey fly ~5-6 mm - practically indistinguishable by the naked eye
What decides it	The species is determined by the host plant and the site of damage, not by the appearance of the insect	As above - the key is the host (brassicas) and the tunnelling of the root

Control — organic and chemical methods

Organic and biological methods

Method	How it works	When to use
Crop rotation and spatial isolation	A 3-4-year break from growing allium crops on a given field, and moving the new planting away from last year's, reduces the arrival of flies emerging from puparia that overwinter in the soil.	At the crop-planning stage, before sowing
Fleece cover	Tightly covering the bed with a light fleece creates a physical barrier that denies the flies access to the base of the plants and prevents egg-laying - the most effective mechanical method in a small planting.	From sowing/planting throughout the flight period of the spring generation
Early sowing and growing from sets	More strongly developed, older plants and planting from sets instead of sowing shorten the vulnerable seedling stage, on which feeding by the first generation is the most damaging.	Early spring
Sanitation and deep autumn cultivation	Removing and destroying infested plants together with the larvae, clearing away onion debris, and deep digging/ploughing in autumn bring the puparia to the surface, where they die from frost and birds. A supporting method - it reduces but does not eliminate the population.	Throughout the season as needed and after harvest
Companion planting with carrots	Growing onions and carrots side by side is recommended as a mutual scent masking that discourages the onion fly and the carrot fly. Its effect is only supporting, based on gardening observation rather than hard research.	When planning the layout of vegetables

Chemical methods

Method	How it works	When to use
Contact insecticide on adult flies (pyrethroid)	A spray with a contact product from the pyrethroid group onto adult flies at peak flight (indicated by yellow sticky traps) limits egg-laying; it does not reach the larvae in the bulb, so it only supplements prevention. Pyrethroids are toxic to bees - apply outside the pollinators' foraging time, according to the label.	At the peak flight of each generation, according to monitoring

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.