Cabbage Maggot

• Transplanted brassicas wilting and failing to recover within days of planting, even with adequate water
• White maggots, up to 1/3 inch, found tunneling in the roots and stem base when plant is pulled
• Roots of affected plants brown, tunneled through, and often colonized by secondary rot pathogens
• Older plants showing stunted growth and bluish-purple discoloration of outer leaves — a sign of root system stress
• Plants in early spring and early fall plantings most severely affected, corresponding to peak fly emergence
• Soil surface near stem base sometimes shows slime or tunneling in fine soil when plant is uprooted

The cabbage maggot fly resembles a small gray housefly. Adults emerge in spring — often timed with forsythia bloom — and immediately seek brassica plants for egg-laying. Females lay eggs in clusters at the stem base, just below the soil surface. Larvae hatch within a week and tunnel downward into roots. Pupation occurs in the soil; adults emerge and produce two or three more generations through the season, with peaks in spring and fall corresponding to brassica planting times. The fly is closely related to the onion maggot and uses similar environmental cues to locate host plants.

1. 01Use transplant collars: cut 6-inch squares of cardboard or thick paper, make a slit to the center, and slide them flat around the stem base at transplanting — this prevents females from laying eggs at the soil line
2. 02Cover transplants with floating row cover immediately after planting and seal the edges — remove only for brief periods if the plants need inspection
3. 03Delay spring transplanting by 2–3 weeks if your season allows, to move past peak spring fly emergence
4. 04Rotate brassica crops to a different part of the garden each year; larvae pupate in the soil near the previous crop
5. 05Apply beneficial nematodes (Steinernema feltiae) to the planting hole at transplant time, particularly in soil with a history of heavy maggot pressure
6. 06Avoid planting brassicas adjacent to last year's brassica bed — spring flies emerge near where they overwintered as pupae

If root maggots are destroying transplants every spring despite collars and row cover, your extension office can advise on whether the collar method is being applied correctly and whether local fly pressure is unusually high.

• Cabbage Maggot— University of Minnesota Extension
• Cabbage Root Maggot Management— UC ANR Integrated Pest Management