Seedcorn maggots ( Delia platura Meigen) are occasional pests of corn and soybean, especially in the Midwest region of the USA. A 2-year field study demonstrated that the presence and emergence of adult seedcorn maggots were governed by timing of tillage operations. Two distinct seedcorn maggot cohorts were recorded each year, one following an early plowing and another after a late plowing. Peak emergence of adult flies occurred following the accumulation of ca 408 thermal units. Similar trends in adult emergence were observed from plots with and without soybeans. A large population of seedcorn maggots during the second year reduced plant stand, but no seed yield differences were obtained.