Sprinkle damp leaves with corn meal, rye flour or a mixture of one part salt to two parts flour. Cabbage worms that eat this coating will bloat and die.
Handpicking in the early morning makes a sizable dent in the worm infestation right away.
Badminton Racquet Technique
The white cabbage moths can be ubiquitous in the home landscape. Kids can be enticed to kill the moth on the fly with a badminton racquet. This technique requires some skill in avoiding damage to garden plants and flowers.
Netting or Fleece Cover
You can make barriers from a material that allows sun, air, and rain to get through but prevents the adult, white butterfly from getting access to your plants. Nylon netting, fine screening, or agricultural fleece are all effective barriers, preventing the butterfly from laying eggs at the plant's stem. Cover transplants or a newly seeded area with the preferred material immediately so the moth can never get to plants and lay her eggs on them. Drape the barrier material directly over the plants and seal all the edges to the ground. Provide lots of extra material so when the plants grow larger they don't strain against the covering.
Use the stocking part of pantyhose or a regular nylon stocking to cover cabbage heads as soon as they start to form. The nylon stretches as the cabbage grows, allowing air, sun, and moisture in but keeping the cabbage butterfly out.
Intercropping or Companions
In experiments done with cabbage, all interplants or companions tested seemed to attract more moths of the imported cabbage worm into the plots for egg laying then if no interplants had been used. Catnip and tansy were most attractive. Catnip companion plantings also increased imported cabbage worm on broccoli. Nasturtiums demonstrated no effect on imported cabbage worms on collards, although gardeners often recommend them. Tansy interplanted with broccoli actually increased imported cabbage worm larvae. Other research is now in progress, but it appears that many of the customary companion plantings designed to reduce imported cabbage worm have no scientific basis.
Attract Beneficial Insects
Parasitic (Non-stinging) Wasp - Braconid and trichogramma wasps parasitize the eggs of imported cabbage worms. They can be attracted to your yard by flowers that look anything like daisies.