Michgan State University Extension:
During this time of year in many backyard vegetable gardens, members of the cabbage family are growing vigorously, but their leaves are beginning to take on the appearance of lace. Several caterpillar pests find cabbage, Brussels sprouts, kale, collards, broccoli, cauliflower and related cole crops very appealing.
Identifying caterpillar pests
One of the most common caterpillar pests of the cabbage family is the cabbage butterfly. Cabbageworms are the larvae of cabbage butterflies, a species with white wings that have black margins and black spots. They can be seen fluttering around vegetable gardens where they stop to lay eggs. In three to five days, the eggs hatch into velvety, pale green caterpillars. After feeding for two to three weeks, larvae are full grown and pupate. Younger larvae chew holes in the foliage, leaving the veins behind. Older larvae may keep feeding on leaves or tunnel into heads. There can be four to five generations per year. They overwinter as pupae near their host plants.