Several types of caterpillar live only on Brassicas and can strip a young plant to the midribs in a short time. They are different species, but have the same feeding habits and can be dealt with together. Entomologists call them the Cabbageworm complex. They include:
Imported Cabbageworms - The larvae of the white butterflies often seen flying around the garden.
Cabbage Loopers - These arch their bodies as they move along and may sometimes also feed on tomato, cucumber, and potato. The adult is a nocturnal brown moth.
Cabbage Webworm- These are easy to spot with their tan body and four strips running along the length of the body. These webworms are found nestled on the margins of leaves.
Diamondback Moth- This cabbageworm feeds off the mustard and cole family, including cabbage and broccoli. The moth is brown with hints of yellow.
The caterpillars of all of these species may be found chewing their way across Brassica leaves. If you have only a few plants, hand picking is the best way to go.
If you have a whole field then a spray of B. T. is most often recommended.
Parasitic wasps and other predators will kill a lot of caterpillars if given the chance, but not if you start spraying poisons. Encourage them with the usual pollen and nectar bearing flowers. Red Cabbage is not as attractive to caterpillars as the green, though it is more attractive to aphids.
Image: R.J. Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company, Bugwood.org