The onion maggot (OM) continues to threaten commercial onion production in New York (ca. 12,000 acres), Michigan (ca. 8,000 acres), and Wisconsin (ca. 2,000 acres). In these states, onions are intensively grown on high organic (muck) soils. Because onion production is specialized and costly, many growers concentrate primarily on the one crop, resulting in minimal rotations to other plant species. This practice invariably increases onion maggot pressure, since this insect usually completes three generations per growing season; and has only one major commercial host in the U.S. Immature stages of the OM dwell within or just outside of underground portions of the onion plant. A system where onions are grown either continuously or in close proximity to last year's plantings must rely heavily upon effective soil chemicals applied at seeding to control the larval stages. In addition, sprays are often applied in an attempt to control the adult fly stage. However, sprays directed at flies cannot be relied upon. This is because flies move in and out of onion fields almost continuously. It has been estimated that a single application of short-lived insecticide will contact only a small percentage of the total onion fly population.