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How to identify it
- The first signs are stunted, yellowing or wilting onion plants; pull one up and look for cream-colored, legless, carrot-shaped maggots, about 1/4 inch long when grown, in the rotting base or in holes in the bulb.[2][3][1]
- Adults are slender, grayish, bristly flies that look like small houseflies, with their wings overlapping over the body at rest; Wisconsin puts them at about 1/4 inch long and UMaine at under 1/3 inch.[1][2]
- The eggs are white and elongated and are laid at the base of the plants.[1]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Cabbage maggot | A close relative on cabbage-family crops; UMaine notes that the onion maggot fly lacks the three dark stripes on the thorax that the cabbage maggot fly has.[2] |
| Seedcorn maggot | UMaine notes that its flies and maggots are very similar to the onion maggot's.[2] |
Where it lives
Covered as a common garden pest by Wisconsin, Maine and Minnesota Extension; Wisconsin notes it can be the most serious problem in onion production.[1][2][3]
Biology and life cycle
It overwinters as pupae in the soil, in or near onion debris and cull piles; adults emerge in mid to late May (around the third week of May in Maine) and lay eggs at the base of the plants.[1][2]
The eggs hatch in less than a week; the maggots crawl under the leaf sheath into the bulb, feed for two to three weeks and then pupate in the soil, and the next generation of flies appears a few weeks later.[1][2]
There are usually three generations a year, and a full life cycle takes 45 to 65 days; cool, wet weather favors the maggots, while hot, dry weather hurts their survival.[1][2]
Damage and signs
- The maggots kill seedlings, so a patchy, thin stand can point to them; in larger plants they tunnel into the bulb and leave the plant limp and yellow.[1][2]
- Feeding wounds let in the bacteria that cause soft rot, so damaged bulbs rot in storage and can spread rot to other onions; white onions are more susceptible than other varieties.[1][2]
- Besides onions, it attacks leeks, shallots, garlic and chives.[1]
Health risk
It feeds only on onions and their relatives; none of the sources reports any harm to people or pets.[1][2]
Control
Prevention
Move onions away from last year's planting where you can (Wisconsin suggests half a mile for farms, which isn't practical in a home garden), and plant onion sets about a week before the flies are expected to emerge.[1][2]
Keep them out
Floating row covers may keep the egg-laying flies off young onions.[2]
Sanitation
Remove volunteer onions, wild onions and onion debris or culls, which carry the pest through the winter, instead of working them into the soil.[2][1]
Biological control
Ground beetles, birds, parasitic wasps, nematodes and a fungus that thrives in cool, wet weather all attack onion maggots.[2]
Pesticides
Once damage shows, it is too late to control that generation; protecting the spring planting matters most, and any soil-applied product must list onions and onion maggot on its label.[1][2]
Common questions
Sources
- University of Wisconsin-Madison Extension (Wisconsin Horticulture), “Onion Maggot (Wisconsin Horticulture).” hort.extension.wisc.edu/articles/onion-maggot/ (accessed 2026-10-17)
- University of Maine Cooperative Extension, “Onion Maggot (Pest Management Fact Sheet #5031, J. F. Dill; reviewed 2023).” extension.umaine.edu/ipm/ipddl/publications/5031e/ (accessed 2026-10-17)
- University of Minnesota Extension (What's wrong with my plant?), “Onion: holes or cracks in bulb (What's wrong with my plant?).” apps.extension.umn.edu/garden/diagnose/plant/vegetable/onion/bulbholes.html (accessed 2026-10-17)
- iNaturalist, “Delia antiqua taxon page (order and family).” www.inaturalist.org/taxa/347587 (accessed 2026-10-06)