Brassica Pest - Root Maggots
Cabbage maggot damage on a young cabbage plant; note the chlorotic (yellowing) plant parts.
Adult: Dark gray fly about half the size of the common house fly.
Egg: Small white, oval-shaped; typically laid on the soil near the stem of the host plant.
Larva: Small, white, legless maggot with a blunt tailend and pointed head.
Pupa: About ¼-inch long and brown in color.
Cabbage maggot pupae overwinter in crop debris and soil. Adults emerge in early May, and mated females lay eggs in the soil at the base of host plants. Small maggots hatch in 4-10 days and immediately burrow into the stem of the host plant. After about 3 weeks, mature larvae leave the stems and pupate in the soil close to the soil surface. About 2 weeks later, adult flies emerge and lay eggs for another generation. Larvae from this generation feed on roots or stems and develop into the overwintering pupae.
Cabbage maggot larvae feed on the roots of cole crops, and can tunnel through tap roots. Tunnels provide an entry for decay, fungi, and bacteria. Damaged plants show wilting, reduced growth, and lighter green plant parts. Cabbage maggot prefers cauliflower, Brussels sprouts, radish, cabbage, broccoli, collards, kohlrabi, and turnip. Cress, beet, and celery can also be infected. Cauliflower and Brussels sprouts can be more susceptible than hybrid cultivars of broccoli. Seedlings and young plants are most vulnerable while healthy plants can tolerate moderate infestations.
After susceptible crops emerge, watch for wilting, reduced growth, and signs of chlorosis (yellowing). If cabbage maggots are suspected, pull up affected plants and check the roots and soil to confirm maggot presence. If tunnels are found in roots, but no maggots are present, then maggots have already exited roots to pupate in the soil. This timing is too late for an insecticide treatment to be effective.
- Rotate crops. Plant susceptible hosts as far away as possible from where they were planted the previous year.
- Use a set of drag chains when direct-seeding susceptible crops. Drag chains can help eliminate moisture where seeds have just been planted. Adult flies may be more attracted to moist areas for egg-laying.
- Be aware of cabbage maggots in cool, wet spring weather as these conditions are more favorable for cabbage maggot development.
- Plant seeds into raised soil beds to promote soil drying and warming, and discourage egg-laying by cabbage maggots.
- Immediately after harvest, destroy or disc under crop residues. Maggots are able to survive for an extended time in crop residues.
- When several rows of seedlings are infested, remove them and replant.
Carabid beetles, rove beetles, and parasitic wasps may help suppress cabbage maggots. However, biological controls alone generally do not keep cabbage maggot populations below economically damaging levels, especially once a population has established.
Fumigate or treat infested soil before planting. In areas where cabbage maggot causes economic injury, treat with a band of insecticide at the base of the plant at the time of planting or transplanting.
Image 1, Mary Ann Hansen, Virginia Polytechnic , Institute and State University, bugwood.org