The presence of the cabbage worm in a garden often signals a problem for anyone cultivating plants in the mustard family. These caterpillars create ragged holes and burrow into the heads of crops like cabbage, broccoli, and kale, leading to significant crop loss. Gardeners frequently wonder how these pests appear so quickly and consistently throughout the growing season. This article traces the origin of this common garden intruder, following its life cycle back to the adult insect that introduces it to the garden environment.
Identifying the True Cabbage Worm
The most common culprit, known formally as the Imported Cabbage Worm (Pieris rapae), is a velvety, matte green larva that offers excellent camouflage against the leaves of its host plants. When fully grown, the caterpillar reaches approximately one inch in length and is subtly marked with a faint, thin yellow or broken yellow stripe running down its back and along its sides.
It is important to differentiate this true cabbage worm from two other major brassica pests. The Cabbage Looper (Trichoplusia ni) is typically a paler green and moves with a distinctive “looping” motion, arching its back because it lacks a pair of mid-body legs. The Diamondback Moth larva (Plutella xylostella) is much smaller, rarely exceeding one-third of an inch, and reacts to being disturbed by wiggling violently and often dropping from the leaf on a silken thread.
The Adult Source: The White Cabbage Butterfly
The immediate origin of the cabbage worm is the egg laid by its adult form, the Imported Cabbage Butterfly, often called the Cabbage White. This is a small, pale white butterfly commonly seen fluttering around gardens during the day. The adult female is responsible for the infestation, as she actively seeks out host plants to lay her eggs upon.
The butterfly is identifiable by the black markings on its wings; males usually have one black spot on each forewing, while females display two. The female uses chemical cues to locate host plants, ensuring the emerging larvae have an immediate food source. She deposits tiny, yellow or pale white, oblong-shaped eggs that are characterized by distinct longitudinal ridges. These eggs are laid singularly, often on the underside of the host plant leaves.
The Life Cycle: From Egg to Garden Pest
The continuous presence of the cabbage worm in the garden is due to its rapid life cycle, which allows for multiple overlapping generations each season. The larva hatches in approximately three to eight days, depending on the temperature. The caterpillar begins its feeding stage, passing through five growth stages, or instars, over the course of about two weeks.
Once the larva reaches its full size, it leaves the plant to find a sheltered spot for the pupal stage, transforming into a chrysalis. This pupa is typically a pale green or brownish color and is secured to a plant stem, fence, or garden debris by a silken thread. The pupal stage lasts about one to two weeks before the adult butterfly emerges.
The entire development from egg to adult can take as little as three to six weeks, allowing for three to five or more generations to occur throughout a single growing season. The pest overwinters successfully as a chrysalis, usually hidden within garden debris or in a protected location, ensuring that new adult butterflies emerge early in the spring.