The Pieris rapae butterfly, known as the Cabbage White or Small White, is a familiar sight in gardens and agricultural fields across the globe. While it flits between flowers, it is also a significant pest of brassica crops. This dual identity makes it a subject of both natural observation and garden management.
Identifying the Cabbage White Butterfly
The adult Cabbage White is a small- to medium-sized butterfly with a wingspan around 50 millimeters. Its wings are a plain white to creamy yellow, with distinct black markings. The tips of the forewings are marked with a patch of charcoal-grey or black. Below this tip, small black spots on the forewings can be used to differentiate between the sexes; males possess a single spot, while females have two.
The Large White (Pieris brassicae), for instance, is bigger and has a more pronounced black band at the wingtips. In North America, it might be confused with native checkered whites, but the Cabbage White’s simple markings are a reliable identifier. The underside of the hindwings is a plain, dull yellow, sometimes lightly dusted with black scales.
Life Cycle and Host Plants
The butterfly undergoes four distinct life stages, beginning with the egg. Females lay small, yellowish, cone-shaped eggs individually on the undersides of host plant leaves. These eggs feature approximately 12 longitudinal ridges and are often deposited in open, sunny areas. From these eggs hatch the larvae, which are the pest stage of the butterfly.
The caterpillar is velvety green with a faint yellow stripe running down its back and subtle yellow patches along its sides. It feeds on the foliage of plants in the Brassicaceae family, which includes:
- Cabbage
- Kale
- Broccoli
- Cauliflower
- Collard greens
After growing for two to three weeks, the larva finds a secure location to pupate. The chrysalis is attached by a silken girdle and can be green, grey, or pink, camouflaging with its background. This stage lasts for about one to two weeks before the adult butterfly emerges.
Origin and Global Spread
Pieris rapae’s native range is across Europe, North Africa, and Asia, but it is now abundant in many regions where it is not indigenous. The butterfly’s expansion into new territories was driven by human activity. It was accidentally introduced to Quebec, Canada, around 1860, marking the beginning of its North American colonization.
From this initial introduction, the species spread rapidly across the continent. Its success as an invasive species is due to its high adaptability and the widespread cultivation of its host plants. Within a few decades, it became established throughout North America, and similar introductions led to its presence in Australia and New Zealand in the 1930s.
Managing Infestations in Gardens
For gardeners, managing Cabbage White caterpillars is focused on protecting brassica crops. Preventative measures are the first line of defense. Using floating row covers can physically block adult butterflies from laying eggs on the plants. Regular inspection of leaves allows for the manual removal of eggs and caterpillars.
Encouraging natural predators can provide a form of biological control. Birds and predatory insects, such as social wasps, prey on the caterpillars. Several species of parasitic wasps and tachinid flies also target various life stages of the butterfly.
For a more targeted approach, products containing Bacillus thuringiensis (Bt) are effective. Bt is a naturally occurring soil bacterium that produces proteins toxic only to specific insect larvae, including caterpillars. When ingested, the proteins disrupt the caterpillar’s digestive system, leading to its death. This organic control does not harm other beneficial insects, pets, or humans.