What is the best way to keep cabbage white butterflies off greens?

Apply food grade diatomaceous earth to the soil and spray them down with a horticultural oil.

I am growing cabbage in a raised bed made from a twenty gallon plastic storage bin. The raised bed has height that makes it easier to inspect the cabbage. when I see holes on the leaves or tiny worms, which are from the white butterfly, I take a bowl of soapy water and a chair and sit and wash the worms off the cabbages. now that it's so easy I'll probably wash the cabbage plants once a week

I spray mine with a mixture of Dawn dish soap and water weekly. It has helped just by spraying. But I may take Patty's advice and bathe them as well.