Cabbage Maggot

Larvae of cabbage maggot are legless, white, and 0.4 inch long or less. Their head is pointed and the rear end is blunt with two brown, buttonlike spiracles encircled by short, fleshy outgrowths.

Adults are dark gray flies about one-half the size of the common house fly. Females are attracted to moist soils high in organic matter and lay their eggs in cracks in the soil. Hatching maggots burrow in the soil to feed beneath the soil surface. After 3 to 5 weeks, they form oblong, brown pupae in roots or soil. Cabbage maggots pupate for 2 to 4 weeks before emerging as adult flies.

There are at least two to three generations per year in coastal areas. In the central coast region, it is possible for cabbage maggot to have more than three generations per year. Cabbage maggots are not a pest in the southern desert. Their numbers increase during cool, wet weather.

Microscopic examination by a taxonomist is needed to distinguish cabbage maggot from other Delia spp., such as seedcorn maggot. Cabbage maggots typically infest direct-seeded cole crops about one month after planting, and infest transplanted cole crops within two weeks after planting. In contrast, seedcorn maggot infests only 1- to 2-week-old seedlings.  

Cabbage maggot feeding causes yellowing, stunting, slowed growth, and in some cases death of the plant. Cabbage maggots can destroy roots of any cole crop. When larvae are numerous, they riddle the roots with tunnels, providing entryways for bacterial soft rot and the pathogen that causes black leg. Though damage is usually restricted to the roots, heavy infestations can attack the flowering heads of cole crops.

Young plants, between seedling emergence until about one month after thinning or transplanting, are the most susceptible to damage. Plants that are attacked after they are well established can usually tolerate moderate infestations if they are otherwise healthy. Older plants may outgrow damage from moderate numbers of cabbage maggots if irrigation is carefully scheduled.

Brussels sprouts and cauliflower may be more susceptible than hybrid cultivars of broccoli, and crops planted in summer and fall suffer more damage than crops planted in other seasons.

Manage cabbage maggot with a combination of cultural control and insecticide application.

Use the following cultural methods to suppress cabbage maggot:

• Avoid hardening transplants near infested fields.
• Schedule irrigation carefully to allow older plants to outgrow maggot damage. Use the online decision support tool CropManage to keep track of irrigation.
• Disc under crop residues immediately after harvest; otherwise, some maggots can survive in residue and develop into adults.
• Avoid successive planting of Brassica crops, especially those planted within one month of a previous Brassica crop.
• Allow crop residue to dry and decompose completely.

Pull up affected plants and check roots and soil to confirm the presence of maggots. If several rows of seedlings are infested, plants may be removed and rows replanted. If the roots are tunneled but no maggots are present, maggots have left roots to pupate in the soil. In this case, insecticide application will not be effective.

For spring plantings where cabbage maggot causes economic injury, use a delayed insecticide application to prevent intolerable damage from this pest. The timing of this application is critical; an insecticide applied two to three weeks (two weeks for cole crops that grow more quickly) after direct seeding more effectively controls these maggots than does a traditional at-plant insecticide application. Insecticide applied later than the seedling stage may not control cabbage maggot.

For direct-seeded crops in the Central Coast region, seed treatment with clothianidin can effectively control cabbage maggot.