Onion maggot larvae attack germinating seedlings, feeding on the developing roots and epicotyl, and can continue to feed on the expanding bulbs during later stages of growth. This results in increased rot in bulbs held in storage.

The seed corn maggot adult is a slender, light gray fly, about ³⁄16 inch long. It looks much like a small housefly. The whitish eggs have slightly raised ridges running the length and width of the eggs forming tiny rectangles. Larvae are about ¹⁄4 inch in length, white to whitish yellow, cylindrical, and tapered, with the smaller end in front. Pupae are small brown capsules.

The seed corn maggot is abundant during or following a wet cycle, primarily in spring, and is most common in fields containing a high amount of residue from a previous crop or where manure has been spread.

Seed corn maggots’ damage usually is restricted to the very early seedling stage. Maggots are primarily a pest of onions and do not generally cause economic damage to garlic. They may spread bacterial soft rot.

Biology and life history Three overlapping generations of onion maggot occur each year. Onion maggot overwinters as a pupa in the soil, although some adults may survive in protected areas. Emergence occurs in the spring and typically extends from April to June. Second or summer generation flies are active from late June to early August. A third flight can take place from early August to October. The adults spend little time within the fields during the day, preferring to rest at field margins in taller and denser vegetation. The flies are most active in onion fields during dusk and dawn or on overcast days. Onion maggot damage is greatest following consecutive years of wet springs.

The seed corn maggot overwinters as a larvae or pupae in the soil. Adults emerge in early May and lay eggs singly or in clusters in the soil near plant stems. They prefer soils with high levels of organic matter. Eggs hatch in 7 to 10 days. Larvae feed for 1 to 3 weeks on seeds and germinating seedlings and then burrow into the soil to pupate. They emerge as adults in late June and early July. Adults lay eggs as before, and larvae feed to maturity and pupate in August to form the overwintering stage. There may be as many as five generations each year. It can be a significant pest of onions in some years in the Columbia basin.

Scouting and thresholds Slow emergence and poor stand establishment are signs of maggot activity. Where slow, spotty emergence is observed, dig up seed and inspect it for maggot feeding. Cool soil temperature and periods of excessive moisture favoring slow seed germination and seedling emergence increase susceptibility to maggot infestation.

In the Pacific Northwest, no specific monitoring methods have been developed. In other parts of the country, estimates of adult fly activity are obtained from the use of yellow or white sticky traps, water pan traps, and pyramid traps to assist in determining the necessity and timing of treatments. Treatments for onion and seed corn maggot are preventive and should be considered for fields that are high in organic matter or undecomposed organic material or that have had previous maggot problems.

Avoid planting successive rotations of onion crops. Early spring-planted crops are more likely to be damaged when the soil is too cool for rapid germination and emergence.

If serious infestations are expected, wait until the soil warms up in spring, or, if feasible, plant in fall while the soil is still warm. When planting, use a chain drag or similar implement behind the drill to cover the seed row. Consider planting into a “stale” seedbed. Use an appropriate insecticide in the seed box at planting if problems are expected.

1. chlorpyrifos (Lorsban 4E) at 0.03 lb ai/1,000 ft in furrow or at 1 lb ai/a as a directed spray to the base of the onion plants. REI 24 hrs. Only one application per season. Toxic to fish. 24c SLN ID-010001, OR-000029, WA-000010.

2. cypermethrin (Ammo) at 0.04 to 0.1 lb ai/a. PHI 14 days. REI 12 hrs. An oil or surfactant adjuvant improves control. Do not graze or feed crop residues to livestock. Spray at dusk along the edges of onion fields as flies migrate back into the fields to lay eggs. Wait 30 days after harvest before planting a subsequent crop. Do not apply within 25 ft of an aquatic habitat, or within 150 ft of one when applied from the air. Toxic to fish.

3. diazinon at 2 to 4 lb ai/a. REI 3 days. One treatment per season. Diazinon label in transition. See label for current details. Broadcast and incorporate prior to planting.

4. gamma cyhalothrin (Proaxis) at 0.0075 to 0.0125 lb ai/a. PHI 14 days. REI 24 hrs. Do not exceed 0.12 lb ai/a per season. An oil or nonionic surfactant improves control. Do not apply within 25 ft of an aquatic habitat, or within 150 ft of one when applied from the air.

5. lambda-cyhalothrin (Warrior) at 0.02 to 0.03 lb ai/a. PHI 14 days. REI 24 hrs. Adults of onion and seed corn maggot. Bulb onions only. Do not exceed 0.24 lb ai/a per season. Do not apply within 25 ft of an aquatic habitat, or within 150 ft of one when applied from the air. Toxic to fish.

6. malathion at 1.5 to 2 lb ai/a. PHI 3 days. REI 12 hrs. Use for adult control. Use on direct-seeded dry bulb onions only. Apply as an in-furrow drench at seeding. Use at least 40 gal/a of spray. Only one application per season.

7. permethrin (Ambush, Pounce) at 0.1 to 0.3 lb ai/a. PHI 1 day. REI 12 hrs. Use for adult fly control. Do not exceed 2 lb ai/a per season. Toxic to fish.

8. zeta-cypermethrin (Mustang ) at 0.03 to 0.05 lb ai/a. PHI 7 days. REI 12 hrs. Adult fly control. Spray near dusk when flies re-enter field. Do not exceed 0.25 lb ai/a per season. Do not apply within 25 ft of an aquatic habitat, or within 150 ft of one when applied from the air. Toxic to fish.