Cabbage Maggot (Delia radicum)
Cabbage maggot is the immature, damaging stage of an anthomyiid fly (Delia radicum L.). Although a minor pest in other regions, Cabbage Root Fly thrives in the PNW climate and is a major concern for brassica growers. Susceptible crops include broccoli, cauliflower, cabbage, turnip, oilseed rape, and many others.
Delia radicum female adult (Photo by Ken Gray)
Cabbage maggot (Photo by Ken Gray)
How to ID Pest
Adults are dark gray, about 5mm, and the body is covered with fine black hairs. Adults are often confused with the common housefly or Delia platura, the seedcorn maggot. Cabbage Fly maggots (immatures) are very small, yellowish-white, and legless, with pointed heads. The posterior end of the maggot is blunt and has 2 brown spiracles present. Pupae are dark reddish-brown.
This pest overwinters in the pupal stage in soil or crop residue. In the PNW, adult flies emerge in early spring, approx. 200DD (growing degree days) after January 1st. Female flies lay eggs directly at the base of host plants or in nearby cracked soil. Maggots hatch in 4 to 10 days and migrate below ground where they feed continuously for approximately 3 weeks. They then leave the roots and pupate on or just below the soil surface. Depending on the time of year, pupae will either a.) emerge as adult flies 2 weeks later or b.) remain as pupa and overwinter. There are multiple generations per year.
Crops Affected & Damage
This insect pest feeds on all cole crops, but prefer cauliflower, brussel sprouts, and winter or spring planted brassicas. Young plants are most susceptible. Hybrid cultivars of broccoli are usually less damaged but are still considered a host plant. Damage is caused by immature flies (maggots) as they tunnel into developing feeder roots and the established taproot. This tunneling directly reduces uptake of water and nutrients, and more importantly, can increase pathogens such as blackleg and soft rot.
Scouting & Monitoring
Monitoring for adults is done by predicting adult flights based on degree-day models, and setting pan traps (a.k.a water traps) at field edges in early spring. Yellow sticky traps also can be effective. Adult flies are attracted to yellow because it simulates a flowering brassica crop.
If adults are found, scout the field for wilted plants or areas of reduced growth that may indicate damage by root-feeding maggots. Pull plants that appear to have symptoms and look for maggots on the root and in the surrounding soil. If the root has empty holes, but no maggots are found, they have left the roots to pupate and an insecticide treatment would be ineffective.
Cabbage maggot may be suppressed by natural enemies such as carabid beetles, staphylinid (rove) beetles, and parasitic wasps. Providing habitat and other conservation techniques to preserve natural enemies are recommended. However, biological controls alone are not an effective means to control this pest, especially once a population has established.
Crop rotation is integral to controlling cabbage maggot. If replanting a susceptible crop in the same field is unavoidable, direct-seeded crops should be followed with a drag chain to reduce moisture-gradients within the seed row because female flies use moisture as an oviposition cue. A better option is to grow seedlings in a greenhouse and fumigate the soil before transplanting. Residue should be disked immediately after harvest, and repeated at least once thereafter. Ask your seed provider if resistant brassica varieties are available.
Consult the PNW Insect Management Handbook for currently labeled crops and rates.
References and Citations
Dreves, A. 2006. Phenology and monitoring of the cabbage maggot (Delia radicum L.) in brassica root crops. PhD dissertation, Oregon State University.
Hollingsworth, Craig S. (Ed.). 2011. Pacific Northwest Insect Management Handbook. Corvallis: Oregon State University.
Jessica Green, Oregon State University
Danny McGrath, Oregon State University