Onion maggots overwinter as pupae in the soil. As temperatures rise in early spring pupae mature into adults and emerge from the soil.
Females begin to lay eggs in less than two weeks.
Females may lay about 200 eggs in less than two weeks in three separate groups. When plants are young eggs are deposited around the base of the onion, but as plants mature and precipitation drops, females deposit eggs directly on the leaves and, bulbs or roots, in cases where mature onions are left to dry in the field. Larvae hatch from the eggs in a couple days and bore into plant tissues. Larvae feed on the onion plant for about 2-3 weeks before pupating in the soil. Three generations occur annually in Virginia.