Onion maggot, Delia antiqua (Meigen), is a serious pest of onion Allium cepa L. in northern temperate regions. Over the last decade, D. antiqua has been managed principally using a pesticide seed treatment package containing the reduced-risk insecticide spinosad. While spinosad protects onion seedlings from D. antiqua, very little is known regarding how protection occurs. The main objectives of this study were to assess susceptibility of 1- and 2-wk-old larvae to spinosad through two different modes of exposure: ingestion and contact, and to evaluate larval feeding behavior in choice and no-choice tests with onion seedlings grown from treated and untreated seeds. Results showed that spinosad was more than twice as lethal to 1-wk than 2-wk-old larvae when it was ingested, but was equally toxic to both larval ages via contact exposure. In choice assays, larvae preferred feeding on untreated plants; however, without a choice, larvae fed and survived equally well on untreated and treated plants, suggesting that spinosad may have a deterrent effect. In a field study, levels of spinosad within young onion plants and in the soil around roots were monitored in addition to the cumulative number of onion seedlings killed by D. antiqua. Spinosad was detected in the soil and in both aboveground and belowground plant tissue, indicating that spinosad translocates into foliage, but declines in plant tissue and soil as plant mortality from D. antiqua feeding increases. Together, these results provide valuable insight into how spinosad protects onion seedlings and reveal key areas in need of further investigation.