Onion maggots are small, white larvae that burrow into onion bulbs and seedling roots underground, tunneling through the tissue and often killing young plants outright. They’re the larval stage of a small fly called Delia antiqua, and they’re one of the most damaging pests for anyone growing onions, garlic, leeks, or other allium crops. If your onion seedlings are wilting and yellowing for no obvious reason, or your stored bulbs are rotting from the inside, onion maggots are a likely culprit.
What Onion Maggots Look Like
The adult is a small gray fly, roughly the size of a housefly but slimmer, that lays its eggs at the base of onion plants right at the soil line. The eggs are tiny, white, and easy to miss. Once they hatch, the larvae are legless white maggots, tapered at the head end, that feed entirely underground. You’ll rarely see the maggots themselves unless you pull up a damaged plant and inspect the bulb or roots.
How They Damage Your Crop
The first generation of onion maggots in spring primarily attacks seedlings, feeding on the developing shoot and young roots up through the 3 to 5 leaf stage. This is when plants are most vulnerable. Damage shows up as wilting, yellowed foliage that eventually collapses. Leaves may turn rotten, and badly infested seedlings simply die. If you notice patchy, poor stands in your onion bed, onion maggots are a strong possibility.
Later in the season, the second and third generations shift their feeding to the expanding bulbs of maturing plants. They tunnel through bulb tissue, creating entry points for bacteria and fungi. This feeding may not kill the plant during the growing season, but it sets up bulbs for storage rot. You can harvest what looks like a healthy onion only to find it breaking down weeks later in storage.
Life Cycle and Timing
Onion maggots overwinter as pupae in the soil near where allium crops were grown the previous year. Adult flies emerge in spring, typically in early to mid-May, and immediately begin seeking out onion plants to lay eggs on. The full cycle from egg to adult repeats two to three times per growing season, meaning pressure can build as summer progresses.
The spring generation is the most damaging because it coincides with the vulnerable seedling stage. Understanding this timing is the key to effective prevention: if you can protect seedlings during that first wave of egg-laying, you avoid the heaviest losses.
Cultural Controls That Work
Crop rotation is the single most effective non-chemical strategy, and the distance matters. If possible, plant this year’s onions more than a half mile from where you grew them last year. For home gardeners without that kind of space, simply moving your allium planting to a different bed each year helps reduce the number of overwintering pupae near your new crop.
Floating row covers physically block adult flies from reaching your plants to lay eggs. Set them up by early to mid-May, before the first egg-laying flight, and keep them in place through the end of the month. Choose a cover that lets sunlight and rain through. One critical rule: do not use row covers over soil where onions or other alliums grew last year. Pupae from the previous season will emerge under the cover, trapping the flies exactly where you don’t want them.
Planting timing can also help. Delaying your planting toward the later end of the onion planting window lets seedlings avoid the peak first-generation egg-laying period. For crops susceptible to root maggots generally, waiting until June 1st to plant is a useful guideline, though with onions you’ll need to balance this against the bulb sizing that depends on an earlier start.
Avoid applying fresh animal manure or green manure to your garden in spring. Rotting, decaying organic matter attracts the adult flies and increases the chance of plant damage. If you use cover crops or organic amendments, terminate and incorporate them at least 3 to 4 weeks before planting.
Biological Control Options
Several natural enemies target onion maggots, though none offer complete control on their own. Ground beetles and rove beetles are generalist predators that eat maggot eggs and larvae. Around 100 species in these two beetle families will feed on them, but because they aren’t specialists, their impact is inconsistent and you can’t rely on them as a standalone solution. Encouraging their habitat by maintaining ground cover and minimizing tillage helps.
Parasitic nematodes are commercially available and can infect maggot larvae in the soil. Two species are the main options. Soil conditions like moisture and temperature affect how well they perform, and some nematode species may not be compatible with soil-applied insecticides, so check before combining treatments.
Three groups of insect-killing fungi naturally infect and kill onion maggots. These fungi tend to cause higher mortality in adult flies than in larvae, which limits their effectiveness against the underground feeding stage. All three are available as commercial products, but there’s an important catch: fungicides applied to manage onion diseases like downy mildew or white rot can interfere with these beneficial fungi. If you’re using both approaches, you may be undermining one with the other.
Chemical Control
For commercial growers and gardeners facing heavy infestations, insecticide seed treatments and soil-applied insecticides at planting are the primary chemical tools. Seed treatments coat the seed before planting and protect the emerging seedling during its most vulnerable period. In-furrow applications placed in the planting row at seeding time offer similar early-season protection.
Timing matters more than product choice. Chemical controls are most effective when they target the first generation, protecting seedlings through the 3 to 5 leaf stage when feeding damage is most likely to kill plants. Later-season chemical interventions are less practical because the maggots are already inside the bulb tissue where sprays can’t reach them.
Preventing Storage Rot
Even moderate maggot damage during the growing season can lead to significant losses in storage. The tunnels maggots create in bulbs become highways for bacteria and decay organisms. Inspect bulbs carefully at harvest and cull any with soft spots, visible tunneling, or signs of rot at the base. Proper curing (drying bulbs in a warm, well-ventilated area for two to three weeks before storage) helps, but it won’t save a bulb that’s already been compromised by maggot feeding. The best defense against storage rot is preventing maggot damage in the field through the cultural, biological, and chemical strategies above.