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Overview of the Large White Butterfly (Kålfjäril, Pieris brassicae)

The most likely culprit when your cabbage patch is decimated is the large white butterfly, known in Swedish as kålfjäril (Pieris brassicae), or more precisely, its caterpillar—the green cabbage worm. These caterpillars can rapidly devastate cabbage and related crops, making them a significant pest for both gardeners and farmers.

Physical Description

The adult large white butterfly is an impressive sight, measuring 5–6 cm across. Its wings are predominantly white, with distinctive black tips on the forewings, and the hindwings each display a single black spot on the edge. The female is easily distinguished from the male by having two round black spots and a black streak on the front wings, while the male lacks these round markings but shares the same size and striking pattern.

The caterpillars (larvae) are lime green, just over 3 cm long when fully grown. They have a yellowish line running down the back, are covered in small black dots, and sport short, white bristly hairs along their bodies—a combination that makes them easy to identify when scouring your cabbage for pests.

Image Example

Adult female *Pieris brassicae and larva on a leaf (photo: Wikimedia Commons)*

Life Cycle and Behavior

Kålfjäril lays its yellowish eggs in clusters on the undersides of cabbage and other brassica leaves. Once hatched, the larvae feed voraciously, often consuming entire leaves and sometimes stripping whole plants to bare stalks. As they grow older, they become more destructive, particularly in their later stages, and can quickly cause serious damage if not addressed.

When the caterpillars are ready to pupate, they leave the plant and seek out vertical surfaces such as fences, walls, and house exteriors, where they secure themselves in place with a silk thread around the middle. The pupae that have overwintered emerge as adults in April–May. In a single summer, the large white butterfly can produce two to three new generations, allowing populations to increase rapidly during warm months.

Impact on Gardens and Agriculture

The main host plants for the large white butterfly are cabbage, kale, broccoli, and other members of the brassica family. Occasionally, they also use plants like nasturtium as a decoy. Their larvae are infamous for their ability to devastate even large plantings, since a single group can consume nearly all available foliage in a short time. Their bodies also accumulate mustard oils from their diet, which provides some protection from being eaten by birds and other predators.

Control and Management

Manual Control

One straightforward method for hobby gardeners is to pick the caterpillars off by hand and move them to the forest or a meadow, where they may continue their feeding undisturbed and serve as food for small birds.

Biological Control

There are effective biological solutions available today for controlling butterfly larvae. At the first sign of gnawed leaves, you can use a product such as Turex, which contains bacteria (Bacillus thuringiensis Berliner var. kurstaki/aizawa) that are toxic only to caterpillars. The powder is mixed with water and sprayed onto the plants. This method is not harmful to plants, people, or most beneficial insects. Turex and similar products are widely available online, including in smaller packages for amateur gardeners at Lindesro and other retailers. Major plant nurseries also stock these biological treatments. After application, the bacteria cause larval disease and death within a few days.

Natural Predators

The kålfjäril's larvae are also preyed upon by parasitic wasps such as Apanteles glomeratus, which lay their eggs inside the caterpillars, effectively helping to control populations naturally.

Summary Table

Key Points

• The large white butterfly is both beautiful and a major pest of cabbage and other brassicas.
• It is easy to identify from distinctive markings on the adult and larvae.
• Regular inspection, manual removal, and safe biological methods such as Bacillus thuringiensis provide effective protection without resorting to chemical pesticides.
• Biological controls such as Turex are easy to use and available in convenient sizes for home gardeners—for example, from Lindesro.
• Support natural enemies such as parasitic wasps to help keep cabbageworm populations in check.

By understanding the life cycle and behavior of the large white butterfly and using these targeted control methods, gardeners can significantly reduce damage and protect their cabbage crops using environmentally friendly means.