Seedcorn maggot (Delia platura), Diptera

Seedcorn maggot biology & life cycle

The seedcorn maggot is an early-season pest of soybeans, corn and edible beans. It tends to be more prevalent during cool, wet springs and in manured or reduced-till fields with decaying residue. Seeds attacked by seedcorn maggots may never emerge, leaving large gaps in the stand. Seedcorn maggots overwinter as pupae in the soil and adults emerge in early spring. Females often deposit their eggs on fresh manure applications or decomposing crop residue. Larvae hatch in seven to 10 days and remain in the field for one to three weeks feeding on residue, seeds and young seedlings. Larvae may be active at soil temperatures as low as 10°C. Seedcorn maggots complete three to six generations per year; the first generation is typically the most damaging.

Identifying Seedcorn maggot

Seedcorn maggot: larvae

The translucent, yellowish-white maggots are small (< 5 mm), legless and oblong-shaped with a pointed anterior. They lack heads but have small black mouth hooks in front.

These images show seedcorn maggot larvae and larval damage to soybean cotyledons below ground; the plants didn’t emerge.

Seedcorn maggot: Pupae

Maggots usually feed for two to three weeks before changing into the brown pupal stage in the soil. Pupae do not damage plants or seeds. Adults emerge from the pupal case in about seven to 14 days.

Seedcorn maggot: adult

The adult seedcorn maggot resembles a small house fly – greyish-brown and about 5-mm long. It can be seen easily flying over freshly worked soil or where manure has been previously spread. First generation adults emerge in April and May, mate, and lay eggs, beginning a new cycle. The entire life cycle may require no more than 21 days.

Seedcorn maggot: management

• Plant only once the soil has reached an appropriate soil temperature
• Avoid heavy manure applications in the three to four weeks before planting
• Kill or plough down green manure or cover crops at least three to four weeks ahead of planting
• Insecticide seed-treatments will usually control seed corn maggot
• Scout for seedcorn maggot by walking fields at emergence:
  - Look for wilted plants and/or gaps in the plant stand where the seedlings have failed to emerge.
  - Dig around the seed trench to look for the cause of the damage. Seedcorn maggots are often found in the developing corn seed or bean cotyledons. They will also tunnel up the stem of newly emerged seedlings. Bean seedlings may have feeding visible on emerged cotyledons and/or stems. Damaged plants may be stunted or delayed.
  - Do a plant population count and record the percentage of the field impacted by insect damage.
  - Nothing can be done to rescue a damaged field except re-planting if necessary.