Seedcorn Maggot

Seedcorn maggot (Delia platura) larvae feed on germinating seeds or seedlings of corn and soybean (Photo 1). Feeding can delay development or kill the plant. Plant injury is especially prevalent during cool and wet springs when plants are growing slowly. Infestations tend to be field-wide instead of in patches like for many other pests. To confirm seedcorn maggot injury, check field areas with stand loss and look for maggots, pupae, and damaged seeds (hollowed out seeds or poorly developing seedlings).

Identification

Adult: The adult fly is grey to brown in color with red eyes. Adult seedcorn maggots are 1/5 inches long and look like a small house fly (Photo 1).

Egg: Eggs are usually not observed in the field, but they are elongated and white in color.

Larva: Seedcorn maggots are white or pale, legless, and 1/4 inches long with a tapered body. The maggots have a black mouth with hook-like mouthparts to feed (Photo 2).

Pupa: The pupa is brown and looks like a “wheat seed" (Photo 2).

Biology

Seedcorn maggots overwinter in Iowa as pupae in the soil. Adult flies emerge and mate in April and May, then females lay eggs in the soil. Eggs typically hatch within 2-4 days, and maggots feed for 2 to 3 weeks before pupating in the soil, where they remain for 7 to 14 days before emerging as adults. The entire life cycle can take as little as 21 days to complete, resulting in 4-5 generations per year in Iowa.

Plant Injury

Corn and soybean seeds injured by seedcorn maggot larvae can show a range of symptoms (Photo 3). The most obvious is reduced stand or gaps in the row. This happens when maggots burrow into the seed and consume the embryo, preventing germination. Even if plants germinate, seedlings are typically weak and may die. Feeding by seedcorn maggot may lead to secondary infections and cause damping off. 

Risk Factors

Fields with a history of seedcorn maggot infestation are at higher risk. Fields that have been recently tilled, regardless of residue type, are highly attractive to egg-laying females. Additionally, females seek fields with high organic matter to lay eggs, including fields with manure, cover crops, or weeds that were recently incorporated. In general, any condition that delays germination or causes slow plant growth may also increase damage from seedcorn maggot.

Scouting

Scouting for seedcorn maggot should be prioritized in fields that are at higher risk for injury. Scouting could be combined with other early-season scouting activities, especially stand counts since the most common and obvious indicator of seedcorn maggot infestation is stand loss. To confirm seedcorn maggot injury, check areas with stand loss and look for maggots, pupae, and damaged seeds (hollowed out seeds or poorly developing seedlings). Infestations tend to be field-wide instead of in patches like for many other pests.

Management

There are no rescue treatments for seedcorn maggot. The focus should be on minimizing the risk of infestation (see the previous section) and avoiding planting during peak fly emergence.

Tracking development: This fly species has a lower developmental threshold of 39°F and upper threshold of 84°F. Peak adult emergence for the first, second, and third generations occurs at 360, 1,116, and 1,872 accumulated degree days since January 1, respectively. We track the development of seedcorn maggot each year and post scouting reminders on ICM News. You can track seedcorn maggot develop on the Pest Maps and Forecasting page.

Table 1. Seedcorn maggot development and associated activity.

*GDD accumulated from January 1, base 39°F. 

**These pupae will either start the fourth generation or overwinter. 

Cultural: No-till fields are less attractive to egg-laying females. Target planting when soil and moisture conditions are conducive to quick germination and vigorous growth to reduce seed and seedling pest problems. Farmers with persistent seedcorn maggot infestations should consider a later planting date, shallower planting, higher seeding rates, and earlier termination of cover crops (Bessin 2004). Waiting two weeks (or 450 growing degree days) after tillage or manure applications to plant corn or soybean should provide enough time for the seedcorn maggots to complete development and move to another host (Gessell and Calvin 2000). Target planting during the "fly-free" period to minimize risk.

Chemical: There are no rescue treatments available. However, insecticidal seed treatments can adequately manage seedcorn maggots unless there are high densities of maggots. If an insecticidal seed treatment is not used, tracking GDD and modifying planting date is the best option. In-furrow insecticides may be an option for fields with a history of infestation by seedcorn maggot or fields with a high risk of economic infestation (see Risk Factors).

If significant stand loss occurs, replanting the field is an option. A replant decision should be based on percent stand loss and cost of additional seed. Resources for replant decisions are available:

Replant resources for Corn
Replant resources for Soybean

References

Bessin, R. 2003. Seedcorn maggots. ENTFACT 309, University of Kentucky Cooperative Extension Service.

Gesell, S., and D. Calvin. 2000. Seed corn maggot as a pest of field corn. Entomological notes, Department of Entomology, Penn State University.

Holm, K. and E. Cullen. 2012. Insect IPM in organic field crops: seedcorn maggot. A3972-01. University of Wisconsin Extension.