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Cabbage and onion maggots

Quick facts

  • Two important root maggot species found in Minnesota are onion maggots (Delia antiqua) and cabbage maggots (D. radicum).
  • Root maggots can occur in any year but are more common during cool, wet springs.
  • Infested plants can appear discolored, wilted or stunted.
  • Damage can be severe enough to kill these vegetables.
  • Physical barriers, like row covers, are the best way to manage them.

How to identify root maggots

Adults

  • 1/4 inch long
  • Dark gray with dark colored stripes 
  • They resemble small house flies

Larvae

  • Legless maggots 
  • 1/4 inch long 
  • Yellowish-white  
  • Shaped like cylinders, tapering towards the head
    Dark gray adult onion maggot fly
    Onion maggot adult
    A cylindrical, yellowish maggot in a yellowing onion root
    An onion maggot

    Biology

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    Damage caused by root maggots

    • Root maggots have several generations in a year, but most of their damage is limited to early spring plantings.
    • Seedlings and transplants suffer more damage from root maggots during a wet, cold spring.
    • The maggots feed on roots and bulbs, creating tunnels.
    • Plants first begin to wilt and can become stunted and yellowed.
    • Heavily damaged plants can ultimately die.
    Green onion with a tunnel near its roots
    Onion maggot damage
    Brownish dried cabbage leaves
    Wilting cabbage caused by cabbage maggots

    How to protect your plants from root maggots

    Once you notice damage from root maggots it's too late to treat them. Protect your vegetables by preventing or removing conditions that favor root maggots.

    If you regularly have experienced root maggot problems then you probably will see them again. 

    Keep your garden clean

    • Do not use animal manure or green manure in your garden in spring. Rotting and decaying organic matter attracts root maggots and can lead to plant damage.
    • When possible, wait until June 1st to plant varieties that can be attacked by root maggots.
    • Remove target plants in the fall, including their roots, and destroy them. This will kill any pupae that might be left.
    White row cover covering the young plants

    Use a physical barrier

    Row covers are an effective option to prevent adult flies from getting near the plants to lay eggs. 

    • Choose a barrier that allows both sunlight and rain to get to the plants.
    • Make sure to set up the barrier in your garden by the time adult flies are laying eggs, usually early to mid-May.
    • Keep the barrier in place until the end of the month when the flies are finished laying eggs.
    • Floating row covers may not be practical in large gardens.
    • Row covers can be purchased at lawn and garden supply stores and online.

    Do not place row covers if onions or other root vegetables were planted in the same area the previous year. Root maggots live through the winter as pupae in the soil near their target plants. Placing a row cover will trap adults that hatch from the pupae and it will no longer protect the plants from the flies.

    Practice crop rotation to minimize this issue: plant susceptible crops in different areas of your garden or alternate seasons when you grow them.

    Using pesticides

    There is no pesticide available as a pre-plant treatment for cabbage and onion maggots.

    Jeffrey Hahn, Extension entomologist and Suzanne Wold-Burkness, College of Food, Agriculture and Natural Resource Sciences

    Reviewed in 2018

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