Answer: It sounds like onion maggot. They appear in cool, wet weather. They eat the root, and the plant may collapse. A fly, similar to a housefly, lays eggs, which hatch into the larvae. Lift infested plants and destroy them. Clean up all plant debris at the end of the season. Rotate where you plant onions next year. Floating row cover can be used on young seedlings to help prevent the flies from laying eggs. Be careful not to damage the plants when weeding, as this seems to attract the flies.
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