Anyone who has grown red cabbage or its relatives is more than likely familiar with a very hungry visitor: the cabbage worm. The larvae of the common white cabbage moth, this velvety green worm readily eats its way through leaves and into the heads of cabbages, depositing large quantities of BB-sized waste pellets in its wake. In smaller home gardens, best control practices consist of hand-picking worms as they appear, though larger plots may require the use of pesticides.
About Cabbage Worms
There are actually three types of "cabbage worms." The most common is the larvae of the imported cabbage worm, identifiable by the soft green worms which eventually grow to more than 1 inch. Adults are the familiar white moth with a black spot on the hindwing. The cabbage looper, known to children as inchworms, are light green with a white line running down each side and mature into dark brown moths. The larvae of the diamondback moth, the least seen of the three, grows to only 1/3 inch long and has a small brown head on a light green body. All three cause significant damage to cabbages.
Other Host Plants
Cruciferous vegetables, such as red and green cabbage, broccoli, kohlrabi, cauliflower and Brussels sprouts, are the most common cabbage worm targets, but other plants can fall victim to the caterpillar's appetite as well. When any of the three cabbage worm species are present in the garden, they may also feed on leafy green crops like kale, chard, collards and mustard greens, as well as root crops including turnips, rutabaga and radish.
While small, cabbage worms chew holes into the foliage of developing cabbage plants, and eat larger quantities of leaf material as they grow. Damage first appears as small holes which then grow into ragged, perforated leaf edges, and eventually the entire cabbage plant will take on a lacy appearance. Left uncontrolled, larvae burrow their way into developing cabbage heads and destroy the heads from inside.
Handpicking is the most effective method of controlling these insects in small gardens. For larger crops, the bacterial pesticide Bacillus thurunguensis, better known as Bt, should be applied to the underside of leaves once the plant's true leaves emerge after seed germination. Spinosad and pyrethrins are two other natural pesticides which kill caterpillars but not their natural enemies. Several other types of pesticides are approved for use on cabbage to kill these larvae, but preparations of rotenone and other inorganic insecticides will kill cabbage worms as well as their natural predators.
One benefit of avoiding broad-spectrum insecticides is that natural enemies of the cabbage worm will be present in the area to prey upon the larvae. All three caterpillar species are parasitized by separate wasp species, which deposit eggs into the cabbage larvae. After hatching, wasp larvae kill the caterpillars by devouring them from within. Other predators of cabbage worms include ladybugs and their larvae, lacewings, soldier bugs, hover fly larvae, spiders and ground beetles.