Onion maggots feed on roots, killing seedlings and older plants. They also burrow into the bulbs, making them unfit for use. Adults are small gray flies that lay eggs early in the spring. You need to destroy infested plants. The maggots tunnel and feed in the bulbs, causing serious damage.
Most injury occurs when the spring seasons are wet.
A third generation may attack just prior to harvest and cause them to rot in storage. Effective control has also been provided by applications of Ethan or triton to the soil prior to planting. Marathon dust or sprays or diazinon sprays are satisfactory for use on growing plants. Remember, all onion or garlic, which are a source of infestation should be destroyed or removed immediately after harvest.