Onion and Garlic Maggots: Identification & Prevention

August 18, 2025 Bill Hageman

Red and white onions representing crops vulnerable to onion maggots

Onion and seedcorn maggots are soil-feeding fly larvae that can damage garlic roots, developing stems, and bulbs. Management is primarily preventive because there is little a home gardener can do once larvae are feeding inside a plant.

How to Recognize Maggot Damage

Garlic plants yellow, wilt, or collapse, sometimes rapidly.
Roots, stems, or bulbs have feeding tunnels or holes.
Damaged tissue may develop secondary decay or an unpleasant odor.
Larvae are creamy white, legless, tapered, and found in or near injured tissue.

Do not diagnose from yellow leaves alone. Excess water, root disease, nutrient problems, stem and bulb nematode, and normal maturity can look similar. Lift an affected plant and inspect the entire root and bulb area.

Onion Maggot vs. Seedcorn Maggot

Both pests can attack roots and developing stems. Onion maggot can continue feeding in expanding bulbs. Adult flies are small and gray, so damage and larvae are often easier for gardeners to recognize than the adults. A local extension office can help confirm uncertain specimens.

Remove Active Sources

Remove severely infested plants and nearby decaying allium material. Do not leave cull onions, garlic bulbs, or piles of damaged plants exposed near the bed, because decomposing material can support fly activity. Dispose of suspect material according to local extension recommendations, particularly when disease may also be present.

Rotate the Planting Area

Plant garlic and related alliums away from the previous season’s infested area when possible. Pupae can overwinter in soil near host plants. Rotation also matters when using row cover: covering garlic planted over an infested bed can trap emerging adult flies beneath the barrier.

Use Row Cover at the Right Time

A securely anchored insect-exclusion fabric can prevent adult flies from reaching a clean planting area during egg-laying periods. Install it before adult activity begins, allow enough room for crop growth, and seal the edges. Do not depend on row cover where susceptible alliums grew recently unless local guidance confirms the site is appropriate.

Fly timing varies by region and weather. Use local extension alerts rather than a fixed national calendar.

Manage Soil and Organic Residues

Adult flies may be attracted to recently disturbed soil and decaying organic matter. Incorporate green residues or manure well before planting when local recommendations advise it, and avoid placing decomposing material directly against garlic stems. Maintain drainage and avoid damaging plants during cultivation.

What About Insecticides?

There are no practical rescue treatments for larvae already feeding inside plants in the home garden. Do not apply a general soil insecticide or beneficial organism based only on suspected damage. If considering any pesticide, confirm that the current label permits use on garlic or the applicable crop group and lists the target pest and application timing. Follow every label direction and local restriction.