I don't have much experience with seedcorn maggot, so I extracted some biology and management recommendations from our former extension entomologist, Marlin Rice. Female seedcorn maggots (true flies) deposit eggs in the soil and larvae feed on organic residue. Decaying organic matter, like manure, is especially attractive to females. Larvae overwinter and will complete development in the spring. First generation larvae will feed on germinating seeds and seedlings, and can possibly kill the plant. Several generations are possible in Iowa, but generally are not an economic concern after stand establishment.
If you suspect seedcorn maggot, carefully dig in the seed furrow and look for evidence. You may not find any maggots, but could see a few feeding scars, tunneling in the seed or stem, or the entire seed destroyed. But the same type of damage could be caused by wireworms, grubs or cutworms.
Rescue treatments for seedcorn maggot do not exist. Areas with persistent populations should consider using an insecticidal seed treatment to protect corn and soybean fields the following year. Seed treatments are encouraged when planting into cool, wet soils with manure applications.