Onion Maggot

• Seedling onions yellowing and wilting at the base despite adequate moisture — larvae are feeding inside the stem at the soil line
• Transplanted onion sets with softened, rotting bases when pulled — maggots tunneled into the bulb
• White maggots, up to 1/3 inch long, found inside the base of affected plants or at the soil surface nearby
• Adult flies (small gray-brown, resembling a housefly) hovering near the soil at the base of onion rows in spring
• Bulbs harvested early showing brown tunneling and secondary soft rot where bacteria colonized maggot entry wounds
• Entire short rows or clusters of onions dying together — females lay eggs at the base of multiple plants in one area

The onion maggot fly overwinters as a pupa in soil and emerges in spring, typically coinciding with the white fly lilac bloom phenological window. Females seek out freshly transplanted or germinating Allium plants by scent and lay eggs at the base, just below the soil surface. First-generation larvae feed inside the stem and bulb base; when large enough, they pupate in the soil. Two to three generations occur through the season, with the second and third generation often causing more damage to maturing bulbs than the first. Damaged bulbs frequently develop secondary bacterial soft rot that makes the entire bulb unusable.

1. 01Rotate onion family crops to a different bed each year — larvae pupate in soil near the previous crop and emerge close to where they developed
2. 02Cover onion rows with floating row cover or fine insect mesh immediately after planting sets or seeding — this is the primary home-garden control
3. 03Remove crop debris promptly after harvest; do not leave onion culls in the garden, as decaying tissue attracts egg-laying females
4. 04Avoid planting near last year's onion bed — a buffer of at least 20 feet reduces but does not eliminate risk
5. 05Apply beneficial nematodes (Steinernema feltiae) to the soil around onion plants in early spring to target larvae in the soil — effectiveness varies by soil temperature and moisture
6. 06Delay spring planting by 3–4 weeks past the first fly emergence window to miss peak egg-laying, if your season permits

If maggot damage is occurring despite row cover, an extension specialist can help determine whether the cover is being applied and sealed properly and whether your regional fly flight timing requires adjusting your approach.

• Onion Maggot— University of Minnesota Extension
• Onion Maggot Management— Cornell Cooperative Extension