**Imported Cabbage Worms** (*Pieris rapae*) are the larval stage (caterpillar) of the small, white **Cabbage White Butterfly**. They are the most common and damaging pest of plants in the brassica family (the “cabbage family”) worldwide, including **cabbage, broccoli, cauliflower, Brussels sprouts, kale, and collards**. Originally from Europe, they were introduced to North America and are now a perennial garden pest, capable of chewing large, ragged holes in leaves and contaminating heads with frass.
Taxonomy and Classification
Imported Cabbage Worms belong to the order Lepidoptera (moths and butterflies), family Pieridae (white and sulfur butterflies). They undergo complete metamorphosis. The adult butterfly is a common sight in gardens, often fluttering low over brassica plants to lay its eggs. They typically have multiple generations per year, ensuring continuous pressure on the crops throughout the growing season.
Physical Description
The adult is a white butterfly, and the larva is a velvety green caterpillar.
- **Adult Butterfly:** Creamy white wings. Males have one black spot on each forewing; females have two.
- **Larvae (The Worm):** Velvety green caterpillars, $1$ to $1.25$ inches long when mature. They have a faint, yellow-orange stripe running down the center of the back. Their green color provides excellent **camouflage** among the leaves.
- **Eggs:** Tiny, cone-shaped, yellowish eggs laid individually and upright on the undersides of leaves.
- **Damage Sign:** Large, irregular, ragged holes chewed in the leaves. In cauliflower and cabbage, they bore into the developing head. The presence of green or dark brown **frass** (fecal pellets) near the feeding sites is a key indicator.
Distribution and Habitat
Imported Cabbage Worms are found globally in all temperate climates where brassica vegetables are cultivated. Their habitat is restricted to plants in the mustard family. They are prevalent in home gardens and commercial fields, overwintering as pupae (chrysalides) attached to plants, fences, or garden debris.
Behavior and Conflict
The caterpillars are active feeders, and the adult butterfly is highly mobile, allowing infestations to establish quickly.
- **Feeding:** Young larvae prefer the undersides of outer leaves; older larvae move inward, causing the most significant damage by chewing into the center head or flowering parts of the plant.
- **Contamination:** Besides consuming the foliage, their frass pellets left in the developing heads or leaves render the crop unmarketable or unpalatable.
- **Successive Generations:** Because new butterflies emerge throughout the growing season, control measures must be maintained from spring through fall harvest.
Management and Prevention
Control is integrated pest management (IPM), with an emphasis on exclusion and biological control.
- Cover susceptible crops with fine-mesh **floating row covers** immediately after planting. This prevents the adult butterfly from landing and laying eggs on the plants.
- Apply **_Bacillus thuringiensis_ var. *kurstaki* (Btk)**, an organic biological insecticide, to the leaves. Btk is highly effective against young caterpillars but must be reapplied frequently. It is completely harmless to humans, pets, and beneficial insects.
- **Hand Picking:** Regularly inspect plants and hand-pick the caterpillars and destroy them, especially in small gardens.
Conservation and Research
Imported Cabbage Worms are managed as a continuous, high-impact vegetable pest. Research focuses on maximizing the effectiveness of biological control agents and identifying natural plant defenses within brassica varieties that deter the butterfly from laying eggs.