Many control methods target agricultural pests’ reproductive traits and so, knowledge of these traits is crucial. The seedcorn maggot, Delia platura (Diptera: Anthomyiidae), is reported as a cosmopolitan polyphagous pest species which can be found in high numbers in numerous crops. Two morphologically identical genetic lines of D. platura (H- and N- lines) with distinct distribution ranges were recently discovered. While many biological traits have been described for D. platura, no study to date has been conducted on the life history strategies and reproductive behaviors of its two lines. Using laboratory-reared colonies originating from the Montérégie region in Québec, this project investigates the effect of group composition (sex-ratio and density) on the mating success and pre-oviposition period of the two D. platura lines. We found a substantial increase in mating success with increasing proportion of males within mating groups for both lines while group density had negligeable effects. These results corroborate reports of D. platura high-density swarms in which sex ratios are usually male-biased. The pre-oviposition period decreased as the ratio of males to female increased at low density only for the N-line while the opposite trend was observed at high density for both lines. These results suggest differences between the mating systems of these two lines, with the H-line females being choosier towards potential mates than those of the N-line. We also describe reproductive traits of both lines along with their implications for integrated pest management strategies such as the sterile insect technique or the release of individuals carrying pathogens, both of which must take into account the high degree of sexual selection present in such group contexts. Keywords: reproductive biology, mating dynamics, seedcorn maggot, swarms