Species

Cabbage maggot (Delia radicum).

Related species

Adult flies resemble the common housefly, but are about half the size.

Host/site

The cabbage maggot was introduced from Europe and is most destructive in Canada and the northern U.S. Cole crops, such as cabbage, cauliflower, broccoli, Brussels sprouts, and radish, are most affected by the maggot, and it is the youngest plants that are most susceptible to damage. The tunnels created by the maggots can also be an entryway for pathogens.

Identification

Appearance

• The maggots are small legless and white, and usually about 0.33 inch when full grown.
• The head of the maggot is pointed and the rear is blunt.
• Adult flies are gray and resemble the common housefly, but are about half the size.
• Eggs are small and white and are laid in cracks in the soil or on plant stems right on the soil line.
• Pupae are brown and egg shaped and can be found in roots or surrounding soil.

  Indicators

  Maggots dig through the root systems of plants in the cabbage family, creating tunnels. Affected plants fail to develop normally. Cabbage takes a blue-gray color and other cole crops can appear stunted or pale in color.

  Life cycle

  In early Spring, females lay small white eggs in cracks in the soil or in plant stems right at the soil line. Eggs hatch after a few days and the small, white legless maggots emerge. They feed for one to three weeks and then pupate in roots or in the soil surrounding the roots. Adults emerge after 2 or 3 weeks.

Natural Enemies

Some soil inhabiting fungi may attack cabbage maggot larval populations. Ground beetles also eat cabbage maggots in their egg, larva, and pupa stages. Parasitic wasps may also attack cabbage maggots.

Monitoring

Fly populations can be monitored by placing yellow plastic dishpans filled with soapy water around the garden. Monitor the number of flies found in the dishpan to keep track of populations. This will help determine when to plant seeds or transplants (it is best to plant them after the peak of adult emergence and egg laying).

Action Thresholds

Prevention is the most important part of cabbage maggot management, so preventative measures (listed under cultural controls) should be taken before fly populations become large.

Cultural and Physical Controls

*Overfertilization with manure should be avoided, as excess manure can provide a desirable environment for cabbage maggot egg laying.

• Weeds should be removed from the garden two weeks before planting.
• Older plants are more resistant to cabbage maggot damage, so if possible, transplants and pre-germinated seeds should be planted instead of seeds. If seeds are planted, use a row cover to protect them.
• Also try to plant in late May or early June, when there are fewer flies around to deposit eggs.
• If infested plants are found, they should be removed to prevent contamination of other plants.

Biological Controls

• The aforementioned natural predators of the cabbage maggot, such as parasitic wasps and ground beetles, can lower maggot populations.
• Beneficial parasitic nematodes also work as a good control of cabbage maggots.

Chemical Controls

Cabbage maggots cannot be reliably controlled with pesticides once there is an outbreak, so cultural controls are the best solution for an infestation. Chemical controls can be used as preventative measure, but these soil insecticides are likely to harm natural enemies of maggots such as beetles, so they are not recommended.

References

"Managing Pests in Gardens: Vegetables: Invertebrates—Cabbage maggot." University of California IPM. August 2009. <http://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/PESTS/cabbagemaggot.html>
"Managing Pests in Gardens: Vegetables: Invertebrates: Lifecycle—Cabbage maggot life cycle." University of California IPM. August 2009. <http://www.ipm.ucdavis.edu/PMG/GARDEN/VEGES/PESTS/LIFECYCLE/lccabbagemaggot.html>
Sorensen, Kenneth. "Cabbage Maggot." North Carolina State University Center for Integrated Pest Management." July 2010. <http://ipm.ncsu.edu/ag295/html/cabbage_maggot.htm>
Van Wychen, K. "Cabbage Maggot" University of Minnesota VegEdge. March 2010. <http://www.vegedge.umn.edu/vegpest/colecrop/cabmag.htm>