Root maggots can be extremely destructive garden pests. As they feed they can destroy the root systems of plants causing slow and irregular growth and even death. They are particularly troublesome for early season plantings.
Unseen, underground, they feed on the root systems of many different vegetable crops including cabbage, radishes, turnips, carrots, and onions. Plants suffering from root maggot attacks may appear yellowish, stunted, and will sometimes wilt during the day when sunlight is at its peak. Tunneling that occurs as the maggots feed also leaves plants open to rot diseases such as black rot.
How to Identify Root Maggots

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Adults are dark grey flies that look like a common housefly, only smaller. They are about ⅕ of an inch in length. They will lay their eggs in the soil at the base of their chosen host plants. The maggots which hatch.