Eggs hatch in 1-9 days, depending on temperature of the soil. In 1-3 weeks the maggots become full grown and pupate. Adults emerge from their puparia in approximately 9-26 days. The larva of the seedcorn maggot attacks germinating seeds or transplants, but is only a pest early in the season before the soil warms up. Seedcorn maggot damage is generally most severe in cool wet periods of the year because seed germination is the slowest at this time.
Little damage is likely to occur once favorable growing conditions set in.
Seedcorn maggots are particularly damaging when crops are planted into soils infested with large populations of maggots, which have built up on residues of the previous crop, which have not thoroughly decayed before planting. Injury to plants results mainly from maggots feeding on the sprouting seed or on seedlings. This pest attacks many spring planted vegetable crops including beans, peas, corn, curcurbits, and onions. The maggots burrow into the seed, feed on the endosperm, and often leave only a hollow shell. Secondarily, by breaking through seed coats, they allow disease organisms to enter, causing seeds to rot.