The imported cabbageworm (Pieris rapae) and the cabbage looper are the primary pests responsible for chewing large, ragged holes in garden brassicas like cabbage, broccoli, and kale. The imported cabbageworm is the velvety green larva of the common Cabbage White butterfly. The cabbage looper is a different species that moves with a characteristic “looping” motion. These pests can rapidly defoliate plants. Understanding their natural enemies is key to achieving biological pest control.
Insect Predators and Parasitoids
The most effective biological control agents are specialized insects categorized as predators or parasitoids. Predators consume the larvae outright, while parasitoids use the larvae as a host for their offspring. Generalist predators like ground beetles and green lacewing larvae readily consume young, soft-bodied caterpillars. Lacewing larvae inject digestive enzymes into their prey, including cabbage loopers, leaving behind a shriveled husk.
Parasitoids are more specific in their attack mechanism. The tiny, dark-colored Cotesia glomerata wasp specializes in the imported cabbageworm (Pieris rapae). The female wasp deposits 20 to 60 eggs into a single caterpillar’s body cavity. The developing wasp larvae feed internally, and the host dies when the larvae chew their way out to spin distinctive clusters of yellow, silken cocoons on the leaf.
Avian and Other Vertebrate Eaters
A variety of larger animals contribute to cabbage worm control as they forage through the garden. Many common insectivorous birds, such as sparrows, robins, and warblers, actively search for caterpillars to feed themselves and their nestlings. Tree Swallows and Eastern Bluebirds also hunt the adult Cabbage White butterflies, which prevents egg-laying and breaks the pest’s life cycle.
Ground-dwelling amphibians also play a significant role in reducing garden pest populations. Toads are nocturnal hunters that consume a wide range of invertebrates, including caterpillars and slugs. A single toad can eliminate thousands of insect pests over the course of a growing season. Providing a damp, sheltered habitat encourages these beneficial vertebrates to patrol the garden beds.
Utilizing Microbial Controls
Microbial agents offer a highly targeted method for controlling cabbage worms without harming beneficial insects or vertebrates. The most common microbial control is Bacillus thuringiensis var. kurstaki, widely known as Bt. Bt is a naturally occurring bacterium that produces protein crystals toxic only to the larvae of moths and butterflies.
The caterpillar must ingest the bacterium by eating treated foliage. The toxin then paralyzes the insect’s digestive system. Caterpillars cease feeding almost immediately after exposure and die within a few days, making Bt an effective and specific biological spray. Beyond bacteria, naturally occurring pathogens like the granulosis virus (GV) can also infect and kill imported cabbageworms.
Encouraging Natural Enemies in the Garden
Gardeners can increase the presence of these beneficial species through careful management practices. Broad-spectrum chemical pesticides should be avoided entirely, as they eliminate the insect predators and parasitoids that naturally regulate pest numbers. The adult forms of parasitic wasps and lacewings feed on nectar and pollen, so planting small-flowered herbs and flowers provides them with a food source.
Plants that provide nectar include:
- Dill
- Fennel
- Yarrow
- Sweet alyssum
For vertebrate control, providing shallow water dishes and small shelters encourages toads and frogs to take up permanent residence. These simple changes create a balanced ecosystem where natural enemies can thrive and provide long-term pest control.