Cabbage maggots overwinter as pupae in the soil. Small grey flies emerge in spring and lay eggs in moist soil near the base of brassicas. The white maggots hatch and burrow into the roots, where their feeding interferes with water and nutrient uptake. Young plants may wilt, yellow, become stunted or die. On turnips and radishes, the maggots tunnel directly into the edible root, leaving scars and channels and creating openings for rot. Damage tends to be worse during cool, wet spring weather.

How to Fix It
Insect netting: Cover plants as soon as they are seeded or transplanted to prevent adult flies from laying eggs. Seal the edges well.
Crop rotation: Move brassicas and susceptible root crops away from areas where they grew the previous year, since pupae overwinter in the soil.
Remove infested plants: Pull badly affected plants, including the entire root, and dispose of them rather than composting them.
Clean up after harvest: Remove old brassica roots and crop debris that can support developing maggots.
Time planting: Where cabbage maggots are a recurring problem, later plantings can sometimes avoid the heaviest spring egg-laying period.
Beneficial nematodes: Soil applications of appropriate insect-parasitic nematodes, particularly Steinernema feltiae, can suppress cabbage maggot larvae when soil moisture and temperature are suitable.