Abstract The phenology and control of the onion maggot, Delia antiqua (Meigen) (Diptera: Anthomyiidae), were investigated in commercial onion fields in Michigan from 1978 to 1980. First-generation emergence of adults occurred at 399 Dδ (degree-days) (4.4°C base). Second- and third-generation emergence occurred at 1142 and 1840 D°. Adult flight activity was correlated with time of damage to onion plants by 1st and 2nd generation larvae. Third-generation larvae were only observed on unharvested mature onions. Phenologies of the various life-stages are compared to development rates determined in the laboratory. Application of foliar insecticide sprays for adult control did not provide additional protection from root maggot damage where granular insecticides had been applied at planting. Recommendations for onion maggot control are discussed.