The onion fly passes the winter in the soil in a maggot stage called the pupae. Pupae are brownish, oval, and slightly larger than a grain of wheat. As soon as the weather warms up in spring, the pupae develop into adults. They emerge in mid to late May in the onion-growing areas of B.C. These adults are light brown and look something like the common housefly. Look close and you will see that the onion fly has larger legs and a narrower abdomen.
During late May and June, onion flies will be seen in the onion fields and often on dandelions around the field borders. Adults mate when they are about 6 days old and begin laying eggs after an additional 3 to 4 days.
Each female’s lifespan is about 30 days, and during that time may lay up to 200 eggs. Eggs are laid in 3 to 4 batches, with about 7 to 8 days between each batch. The eggs are white, elongated and small. They are usually laid around the base of the small onion seedlings, but as the plants become larger, and especially during dry weather, they may be laid in the leaf sheaths.
Eggs hatch in a few days and the larvae bore directly into the onion plant. These larvae are tiny, white, legless maggots that taper toward the front end. They grow to a length of about 1 cm when mature, and have conspicuous dark feeding hooks on the tapered end. They may attack the onion at any stage of development and feed within the plant for about 2 to 3 weeks. When the larva becomes full-grown it leaves the onion and pupates in the soil.
In B.C., there are three generations each year. Adults of the first generation are present from mid-May to late June. Second-generation adults appear in early July and continue into early August, while third-generation adults are present from late August to early October.