**Onion Maggots** (*Delia antiqua*) are the destructive larvae of a small fly, and they are a severe pest of **allium crops** (onions, garlic, leeks, shallots, and chives). The conflict is complete crop loss: the fly lays eggs at the base of the plant, and the tiny maggots bore into the bulb or stem, feeding on the tissue. This feeding destroys the growing point, causes the plant to wilt and turn yellow (known as “flagging”), and often leads to the death of the entire plant or, in storage, secondary bacterial soft rot.
Taxonomy and Classification
Onion Maggots belong to the order Diptera (flies). They undergo complete metamorphosis. The flies are small and nondescript, but they are highly attracted to the scent of alliums, particularly damaged or newly transplanted plants. They have multiple generations per year, with the first generation being the most destructive.
Physical Description
The destructive larvae are 1/3 inch long.
- **Adult Fly:** Small, grayish-brown fly, similar in appearance to a small housefly, often seen crawling near the soil line of onion plants.
- **Larva (Maggot – Key ID):** Soft, white, legless, tapered maggot with no distinct head capsule, found feeding inside the onion bulb or stem.
- **Damage Sign (Key ID):**
- **Flagging:** Sudden wilting and yellowing of the central leaf of the plant.
- **Rot:** Soft, brown decay inside the bulb, often with multiple maggots present.
- **Seedling Loss:** Severe mortality in spring seedlings and transplants.
- **Conflict:** Severe commercial and garden crop destruction.
Distribution and Habitat
Onion Maggots are found in temperate regions worldwide where alliums are grown, particularly in areas with rich, moist soils. Their habitat is the soil surface near the host plant and the interior of the onion bulb.
Behavior and Conflict
The conflict is total loss of the crop and storage failure.
- **Chemical Attraction:** The female fly detects the characteristic odor released by the onion plant, especially during thinning, transplanting, or damage, leading to massive egg laying.
- **Direct Destruction:** A single maggot can kill a small seedling. A few maggots can ruin a mature bulb, making it susceptible to bacteria.
- **Overwintering:** They overwinter as pupae in the soil, emerging in early spring to attack the earliest plantings.
Management and Prevention
Control is integrated pest management (IPM), with a heavy focus on cultural control and exclusion.
- **Timing:** Plant seeds early or delay planting until after the first major fly flight (which is the most damaging).
- **Rotation:** Practice strict crop rotation; never plant alliums in the same area two years in a row.
- **Sanitation:** Promptly remove and destroy (do not compost) any damaged or infested bulbs and all crop residue after harvest.
- **Row Covers:** Cover small plantings with fine-mesh insect exclusion netting immediately after planting to prevent the flies from accessing the plants to lay eggs.
- Use insecticide-treated seed or granular soil insecticides at planting time, often a necessary practice in commercial production.
Conservation and Research
Onion Maggots are managed as major agricultural pests. Research focuses on developing fly-specific pheromone traps for monitoring, understanding insect resistance to current seed treatments, and exploring biological controls like parasitic wasps.