In conjunction with efforts to identify efficient insect pollinators for seed multiplication of cross-pollinated plant species stored and maintained by USDA–ARS Western Regional Plant Introduction Station (WRPIS), experiments were conducted to assess and compare the efficiency of the house fly, Musca domestica L. (Diptera: Muscidae), and Calliphora vicina Robineau-Desvoidy (Diptera: Calliphoridae), and different densities of each fly species, to pollinate leek, Allium ampeloprasum L., plant inventory (PI) accessions in field cages for seed yield maximization and high germination. Cages with flowering plants were exposed to 0 flies or stocked with 100, 250, and 500 M. domestica or C. vicina pupae per week for 6 (2002) and 7 (2004) wk. Seed yield (weight per cage) increased linearly as fly densities (C. vicina or M. domestica) increased from 0 to 500 pupae per week, with 500 fly cages averaging 340.7 g (C. vicina) and 70.5 g (M. domestica) of seed in 2002 (PI 368343) and 615.3 g (PI 168977) and 357.5 g (PI 368343) in 2004 when only the C. vicina was used. For 0, 100, and 250 fly cages, seed yields averaged between 2.3 and 175.3 g in 2002 and 10.7 and 273.1 g in 2004. Mean 100-seed weights between treatments ranged narrowly between 0.4 and 0.5 g in 2002 and 0.3 and 0.4 g in 2004, and germination rates of seed lots from “fly cages” were mostly ≥80% in both years. The C. vicina is an efficient and cost-effective pollinator ($388.97 for pupae and shipping, compared with $2,400 for honey bee, Apis mellifera L. [Hymenoptera: Apidae], nuclei) for caged leek accessions, with 250 and 500 C. vicina pupae per week required to produce sufficient seed (130 g) to fill an accession storage bag in the WRPIS gene bank.