The three species of caterpillars that attack cabbage (Brassica oleracea var. capitata) are the larval forms of moths. Called cabbage worms, the caterpillars are easier to kill while they are young, so be ready to take action as soon as you see the white, pale yellow or gray speckled moths hovering around your crop. Although older plants can tolerate the caterpillars without serious damage, you’ll want to get rid of them before they attack the cabbage heads.
Types of Caterpillars on Cabbage
As a group, the types of caterpillars that attack cabbage are called cabbage worms. The type most often seen in home gardens is the imported cabbage worm. Mature caterpillars are light green in color and sometimes have a pale yellow stripe down the length of their bodies. Cabbage loopers are green or yellow-green in color. You can distinguish them from the other caterpillars because they move by raising their back like an inchworm. Diamondback moth caterpillars are the tiniest of the bunch at less than one-half inch in length. These little green caterpillars are wide in the middle and tapered at the ends. They have two legs protruding from their last segment that form a “V” at the end of their bodies.
Identifying Caterpillar Damage
Caterpillars chew ragged holes in the leaf tissue between the veins, and leave deposits of dark fecal matter (frass) where they feed. Very young diamondback moth caterpillars feed inside the leaves for a few days before breaking through to the surface of the leaves. Young cabbage plants are the most susceptible to serious damage from caterpillars. Older plants can tolerate some feeding, but it’s best to get the pests under control before they begin chewing on the heads.
An Organic Insecticide: Bacillus Thuringiensis
Bacillus thuringiensis (Bt) is a safe bacterium that kills caterpillars without harming humans and other animals. When a caterpillar eats the Bt spores, toxins paralyze his digestive tract and he can no longer feed. It may take as long as 48 hours for the caterpillar to die, but he can’t do any further damage after ingesting the toxin. Bt is the insecticide of choice, because it is specific for caterpillars and doesn’t harm their natural enemies or the environment. Of the three strains of Bt, the “Kurstaki” strain is most effective against caterpillars that attack cole crops.
Using Bacillus Thuringiensis
The easiest form of Bt for home garden use is a ready-to-use dust, because you don’t have to mix the product or use any special equipment for application. Read the label completely before using the product. Wear long sleeves when applying the dust. A mask isn’t required, but it will prevent breathing in the dust, which can irritate your lungs. Use about one-half to 1 ounce per 50 square feet. The product is only effective if the caterpillar ingests it, so cover the leaf surfaces completely. It’s best to apply the dust when you aren’t expecting rain, which can rinse away the insecticide.
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