Four years of field trapping indicated 3 distinct broods of onion maggot adults in a New York growing season. Early- and late-season injuries closely followed the population trends. Laboratory studies using 8 constant temperatures established a threshold of 40°F for adult emergence. The average air thermal unit accumulations for 50% catch of spring, second, and third broods were 712, 1187 and 1257, respectively. Incorporation of temperature accumulations, earliest dates of population buildup, and field-monitoring provides New York growers with an early-warning system.