Onion Fly

Garden pest · Delia antiqua

Description

Onion fly is a small gray fly whose larvae, often called onion maggots, feed on onions and related alliums, including shallots, garlic, and leeks. Adults lay eggs in soil near the plant base or on the lower leaves. The eggs hatch into white, legless maggots that burrow into the roots and bulb’s basal plate. Their feeding damages young plants directly and opens the way for bacteria and fungi that cause soft rot. Seedlings are especially vulnerable and may die quickly; larger plants can survive but produce damaged, poorly storing bulbs. The insect overwinters as a pupa in the soil, and several generations may occur in a growing season, depending on climate. Most damage is hidden below ground, so a declining onion may look as though it needs water until the bulb is lifted. Onion fly is a pest of alliums, not a general pest of garden vegetables.

Symptoms

• Yellowing leaves
• Wilting
• Stunted growth
• Seedlings collapse at soil level
• Brown, mushy roots
• Soft or rotting stems
• Insects visible on the plant

How to Identify

Look for yellowing, wilting onions that fail to recover after watering, particularly among young plants. Gently lift one: damaged roots, a soft or rotting bulb base, and small, white, legless maggots inside or around it strongly indicate onion fly. The adults are gray and resemble small houseflies, so their presence alone is not proof. Onion basal rot can cause similar yellowing and decay but has no maggots. Allium leaf miner and leek moth cause more conspicuous damage within the leaves; look for tunnels, feeding marks, or larvae there. Inspect several plants, as secondary rot can obscure the original feeding damage.

Prevention

Cover onions with fine insect-proof mesh or horticultural fleece immediately after planting, and keep its edges sealed so egg-laying flies cannot reach the plants. Do not put a cover over a bed where onion flies may already be emerging from the soil. Rotate alliums to a different bed and promptly clear away infested plants and crop debris; rotation reduces local carryover but cannot stop flies arriving from elsewhere. Start with healthy sets or transplants and avoid damaging bulbs when planting. Check young plants regularly, especially near previous allium crops, so an infestation can be removed before larvae finish developing. Keep beds weeded and plants growing steadily, while recognizing that good care cannot prevent attack on its own.

Treatment

Lift suspicious plants and inspect the bulb base. Remove infested plants, bulbs, and nearby visible maggots; seal them in a bag for disposal rather than putting them in a home compost heap. This limits the number that can pupate, although it will not repair damaged plants. On a small planting, check adjacent plants and remove any eggs or maggots you can see. If flies are still laying eggs, protect unaffected onions with tightly secured insect-proof mesh, provided flies are not emerging beneath it. Beneficial nematodes, such as Steinernema feltiae, may help against larvae in the soil where a product is sold for this use; apply according to its label and keep the soil moist. Results vary, and larvae already deep inside bulbs are difficult to reach. Insecticides are generally of limited value once maggots enter bulbs. If considering one, use only a product specifically approved locally for this pest and crop, and follow its label.

Plants Affected

Chives, Garlic, Leek, Onion, Shallot

Not sure this is your problem? Try the problem diagnoser.