The surest way to do this is to rotate your cabbage crops and cover them with floating row cover. The rotation is essential to protect the plants from eggs already in the ground, which would hatch out under the tents. Fall plantings of broccoli, brussels sprouts, and kale should be less susceptible than spring plantings. Wilting cabbages may also indicate club root, a fungal disease. Rotating cabbage plants to different parts of the garden will reduce club root.
Cabbage loopers are light green caterpillars which loop as they walk.
They eat large ragged holes in the leaves and bore into developing heads. Hand picking is the most effective non-toxic control, and it can make a sizeable dent in the infestation quickly. If these methods fail, B.t. is an effective biological control, but remember that it kills all caterpillars, so use it with care. Leafminers tunnel between leaf layers to produce a telltale series of white or brown tracks and blotches. Spinach, beets, turnips, and cabbage are especially vulnerable.