The cabbage worm, specifically the larva of the Imported Cabbage Butterfly (Pieris rapae), is a highly recognizable pest in gardens and commercial agriculture across temperate regions. These velvety green caterpillars are the progeny of the small, white butterflies often seen fluttering near vegetable patches. While the adult butterflies primarily consume flower nectar, their offspring are voracious leaf-eaters responsible for significant defoliation and crop loss.
Primary Food Source: The Brassica Family
The cabbage worm’s diet consists of plants belonging to the Brassica family. This specialized feeding relationship is dictated by unique chemical compounds within these plants. The caterpillars are drawn to and require glucosinolates, which are characteristic of the Brassica order.
These specialized compounds act as both feeding stimulants and oviposition cues, guiding the female butterflies to lay their eggs exclusively on these host plants. Upon plant damage, glucosinolates break down into defensive compounds, but the cabbage worm has evolved mechanisms to detoxify them, often converting them into less harmful nitriles. This adaptation allows them to thrive on plants that are toxic to many other insect species.
Key members of the Brassica family that serve as the worm’s primary food source include cultivated vegetables:
- Cabbage
- Broccoli
- Cauliflower
- Brussels sprouts
- Kale
- Collard greens
- Kohlrabi
The imported cabbageworm shows a strong preference for these crops. Even within this family, certain varieties, such as Chinese cabbage and turnip, are sometimes less preferred than the standard cabbage and broccoli.
Secondary Host Plants
While the Brassica family is the host, the cabbage worm is not strictly limited to these cultivated crops. They will occasionally feed on other plants that contain similar chemical profiles. This opportunistic feeding expands their potential habitat beyond the vegetable garden.
Examples of these secondary host plants include nasturtiums and various weeds within the mustard family. Wild hosts like field pennycress and shepherd’s purse can sustain the first generations of the pest early in the season before cultivated crops are widely available. Other secondary targets can include horseradish, radishes, and sweet alyssum, which also contain the necessary glucosinolate compounds.
Feeding Behavior and Damage
The newly hatched larvae begin feeding on the outer leaves, often on the underside where they are less visible. As they grow, their chewing mouthparts create progressively larger, irregularly shaped holes in the foliage.
A characteristic sign of their presence is the appearance of “shot-hole” damage, which often progresses to severe defoliation where only the leaf veins may remain.
The most concerning damage occurs as the caterpillars mature, when they begin to burrow deep into the developing heads of plants like cabbage and broccoli. This internal feeding contaminates the edible portion of the crop and can cause the plant to become stunted or fail to form a head. The presence of “frass,” which is wet, greenish-brown fecal material, is another tell-tale sign of active feeding, particularly deep within the leaves or florets.