Onion maggot, Delia antiqua Meigen (Diptera: Anthomyiidae), is the most important early-season insect pest of commercial onion in the northeastern United States and Canada. Larvae of the pest feed on belowground tissue of onions and can cause substantial damage to seedlings. Delia antiqua is managed with seed treatments, and damage from the pest is highly variable across onion growing regions. The goals of the research presented here was to investigate factors associated with D. antiqua damage to determine what may be influencing the disparities in damage observed across the region and to evaluate the mechanism of control of the principal seed treatment, spinosad (Regard SC), used in its management. Relationships between plant damage and surrounding landscape, planting date, soil temperature, and soil organic matter were identified. Spinosad uptake in plant tissue, susceptibility of larvae through contact and ingestion, and behavior of larvae exposed to spinosad-treated plants revealed insights into the seed treatment’s mechanism of control and highlighted areas in need of further research. Collectively, the results from these studies fill key gaps in our understanding of D. antiqua activity and control and will be used to inform areas of future research and improve management of this economically important pest.