Delia antiqua, Delia platura and Delia florilega are three root maggot species commonly considered pests in Eastern Canadian onions. The onion maggot, D. antiqua, is considered the primary root maggot pest in onion but it remains unclear whether the other two species are also causing damage. In order to develop updated management strategies for root maggot, we tested adult oviposition and feeding preference by Delia larvae on four growth stages of onion using bioassays and we determined the Delia species composition across the four major onion growing regions in eastern Canada. Delia species oviposit readily on onion at the 5–7 true leaf growth stage but damage on onions is not statistically different between Delia species in our zero-inflated models. The four eastern Canadian onion growing regions have different proportions of Delia species. Southern Ontario and Quebec were the only two regions where Delia antiqua was collected. The highest average numbers of Delia spp. were caught in Quebec and Nova Scotia. Our study shows that timing is important in implementation of management strategies for root maggot in Eastern Canadian onions.