Cabbage worm, also known as cabbageworm.
Cabbage worms are the larval form of the Cabbage White butterfly. It is useful to know what both the larva and the butterfly look like, because seeing either near your plants most likely means that you'll start seeing damage to your brassicas (such as cabbage, broccoli, and kale.)
Butterfly: off-white wings, with one or two grayish-black spots per wing. The wingspan is roughly two inches across.
Larva (which is the form that does the actual damage): velvety green, inchworm-type caterpillar that is roughly one inch long.
Eggs: These will be found on the undersides of leaves, and are yellow and oval-shaped.
The larva build a chrysalis in the fall, and hatches as the Cabbage White butterfly in spring. The butterfly feeds on nectar, and lays single eggs on the undersides of plants of the brassicas family. The eggs hatch in five to seven days. The larva (cabbage worm) feeds voraciously on brassicas, building up energy to make its chrysalis and metamorphize over the winter.
Signs of Cabbage Worm Infestation:
Because of their voracious appetite, an "infestation" can be as few as two or three worms per plant. Look for holes being chewed from the centers of leaves in kale and cabbage, as well as entry holes chewed to the interior of heads of cabbage. In particular, look on the undersides of leaves, because this is where the cabbage worms usually hang out. They also produce dark green droppings that are fairly noticeable.
Effect on Garden Plants:
A serious infestation can result in the death of the plant, since the more leaves that the cabbage worm manages to eat means that the plant's ability to photosynthesize is reduced. A minor infestation can make plants look unsightly, but they are still edible. Just wash them carefully and inspect cabbage and broccoli for any cabbage worms that have made their way to the interior of the heads.
Organic Controls for Cabbage Worms:
Check your plants frequently for worms, especially if you have seen the butterflies nearby. Check plants thoroughly, and hand-pick and destroy any worms you find. If you have a serious infestation, purchase bacillus thuringiensis (BT) from your garden center and apply it according to the directions.
To prevent infestation in the first place, protect your plants with floating row covers, especially in spring and early summer, when egg-laying activity is at its highest.
To prevent the worms from burrowing into cabbage heads, insert each head into a nylon stocking, and leave it on until you harvest the head.