When eggs hatch, cabbage maggot larvae tunnel into roots. Plants wilt and shrivel when feeding is heavy. When you pull up wilting plants, rice-size white maggots can be seen feeding on the roots.
Floating row covers (fleece) are an effective way to prevent egg laying by adults. However, rather than covering the plants, it is sufficient to have vertical row cover “fences” on either side of the row, because adults locate host plants by flying low, from side to side. When only a few plants need protection, cut a round of cardboard, cloth or heavy paper, slit it to the middle, and install it as a skirt around the plant’s main stem (a 'cabbage collar'). This simple method protects seedlings from egg laying by adult cabbage flies. Ground beetles are important natural predators, and they are encouraged by mulch. Cabbage root maggots are most problematic in late summer, when populations are high enough to damage fall crops of cabbage, cauliflower, and closely related crops.
Remove badly affected plants and swish their roots in warm water to remove maggots. Allow them to drown or feed them to your chickens.
Yellow sticky traps or yellow pans of water can be used to monitor and trap onion flies.