Seedcorn Maggot

Seedcorn maggot larvae are legless, white, and up to 0.4 inch long. They cannot be distinguished from other Delia spp., such as cabbage maggot, without microscopic examination by a taxonomist. However, unlike cabbage maggot, seedcorn maggot does not attack plants after the seedling stage and is rarely found tunneling in larger roots. Their damage is typically restricted to cole crops that are 1 to 2 weeks old.

Adults are dark gray flies about one-half the size of the common house fly. Females are attracted to lay eggs in moist soil that is high in organic matter. Larvae burrow beneath the surface to feed for 1 to 3 weeks on seeds and germinating seedlings. Mature larvae form oblong, brown pupae in soil.

Seedcorn maggot can be a problem in coastal areas on broccoli and cauliflower if the weather is cool and wet. There are several generations per year. Seedcorn maggots are most prevalent during cool spring conditions, especially after wet winters. Their numbers may decline in summer.

The larvae damage and kill germinating seeds and very small seedlings. Affected seedlings may wilt, be abnormally light green, and stunted. Once the stand is established and seedlings have developed a few leaves, seedcorn maggot is unlikely to cause economic damage.

The key to reducing damage from seedcorn maggot is allowing plant debris and other organic matter to completely decompose before planting. Once crop seedlings emerge or right after transplanting, watch for wilting, atypically light-green plants, or reduced growth, which may indicate a seedcorn maggot infestation.

Use primarily cultural controls to manage seedcorn maggot:

• Disc under cover crops at least 2 weeks before planting.
• Allow crop residue to completely decompose before planting cole crops.
• When direct seeding, attach drag chains behind the planter to eliminate the moisture gradient from overturned soil that may attract egg-laying females to the seed row.
• Transplant instead of direct seeding.
• Insecticides are generally not necessary in desert areas where seedcorn maggot is not a problem. Cole crops typically grow out of seedcorn maggot damage in this region.

In areas where seedcorn maggot damage is a problem, seed treatment or a soil-applied insecticide are effective forms of control.

In the San Joaquin Valley, spring-planted fields with soil high in organic matter may require insecticide application. However, to avoid damage from seedcorn maggot, it is best to plant only after organic matter has decomposed.