Allium leafminer, Phytomyza gymnostoma (Diptera: Agromyzidae), is an invasive species first recorded in the Western Hemisphere in 2015 and has expanded its range into northeastern and MidAtlantic states. Its host range encompasses Allium species grown for food and ornamentals, weedy species, species used for pollinator provisioning, and species of conservation concern. Using field and laboratory studies, we advanced methods for rearing, developed a phenology model for spring emergence, describe pupal development, and report on parasitism. Spring emergence was best detected by scouting wild alliums as opposed to emergence cages, and modeled using 350 degree-days above a lower threshold of 1.0°C. Spring adult flight occurred for about 5 wk. Larval development required 22 and 20 d at 17.5 and 25°C, respectively. Pupal development progressed along a color gradient, and an initial presence of fat cell clusters and an air bubble, followed by an exarate pupa. Pupal developed at 3–5% per day at 3°C and reached 25% per day at 21.5°C, but development was not successful at 30°C. Although parasitism rates were low, we documented two Chalcidoidea parasitoids, Halticoptera circulus (Walker) (Hymenoptera: Pteromalidae) and Chrysocharis oscinidis Ashmead (Hymenoptera: Pteromalidae). Together, these data provide baseline information to advance IPM for this invasive species both in crops and noncrop areas.