The damage is worse after mid-summer or in dry seasons. The onion maggot overwinters as a pupa in the soil. The adults emerge in mid-May and feed on yellow flowers at the edge of the field. They then lay eggs at the base of onion plants. The maggots hatch and burrow into the plants.
Mature maggots leave the plant to pupate in the soil.
There are 3 generations per year and maggots may be in harvested bulbs. Use Diazinon granules according to label directions. Visuals associated with this text. This information is for educational purposes only. References to commercial products or trade names does not imply endorsement by MSU Extension or bias against those not mentioned.