Onion Fly Identification Guide
Recognize this gray, bristly fly and its legless larvae that tunnel through onion bulbs.
Read the full Onion Fly encyclopedia entry →
Key Visual Features
The onion fly is a small fly closely resembling a house fly in general build, which can make it tricky to identify without a closer look.
- Adults are about 1/4 to 1/3 inch (6-8 mm) long with a slender, ash-gray body and darker gray stripes along the thorax.
- The body is covered in noticeable bristles, especially along the legs and thorax, giving it a slightly fuzzy appearance under magnification.
- Wings are clear to slightly smoky and lie flat over the abdomen at rest, and the legs and head are gray to black without the bright yellow accents seen in some related root-maggot flies.
- Males and females can differ slightly in shape, with males often more slender than the broader-bodied females.
- The larvae are legless, creamy-white maggots, tapered at the head end, that feed inside onion bulbs and can occur in clusters within a single infested bulb.
Where and When You'd See It
Onion flies are found around onion, leek, garlic, and shallot plantings, where adults lay eggs at the base of seedlings or in the soil near maturing bulbs. Adults are most active in mild spring weather and again in late summer, seeking out onion-family plants by scent. They tend to rest on or near host plant foliage and in surrounding soil, avoiding the hottest, driest parts of the day. Larvae remain hidden inside bulbs or roots underground, so an infestation is usually discovered when a bulb is pulled and found to be hollowed out or soft.
Similar-Looking Bugs
- House flies are larger and stockier with a more checkered thorax pattern, while onion flies are smaller and more uniformly gray.
- Carrot rust flies have a shiny black-green body with a yellow head, clearly different from the dull gray, bristly onion fly.
- Cabbage root flies look very similar in size and gray coloring but are associated with brassica crops rather than onions, so host plant is the best distinguishing clue.
- Fruit flies are smaller, more rounded, and often have colored eyes, not resembling the elongated gray onion fly.
Quick ID Checklist
- Slender, ash-gray, bristly fly about 6-8 mm long
- No bright yellow head or legs, unlike the carrot rust fly
- Adults found near onion, leek, or garlic plantings
- Larvae are pale, legless maggots feeding inside bulbs
- Infested bulbs may appear hollowed, soft, or tunneled
Frequently asked questions
How is the onion fly different from a house fly?
The onion fly is smaller and more slender with a plain ash-gray body, while house flies are larger and have a more distinctly striped, robust thorax.
What do onion fly larvae look like?
They are small, creamy-white, legless maggots that taper at the front end and feed in groups inside onion and related bulbs.
Which plants are onion flies typically found on?
They are associated with onion-family crops such as onions, leeks, garlic, and shallots.
How can I confirm onion fly damage in a bulb?
Cut into or examine a soft, hollowed bulb for tunneling and the presence of small, pale, legless maggots inside.