Cabbage white butterflies Pieris spp.

Look for

Large holes in the leaves of brassica crops and nasturtiums are usually the work of cabbage white caterpillars. Before an attack you may notice large numbers of white buttlerflies around your plants, as they lay eggs. These hatch into larvae which make small holes in the leaves as they start to feed on them, becoming much larger as the caterpillars start to feed more vigorously. Caterpillars also produce small brown or dark green granular deposits as they feed.

Plants affected

• All types of brassica are affected by cabbage white caterpillars, including cabbages, cauliflower, Brussels sprouts, swede and turnips.

About Cabbage white butterflies

• The caterpillars of a few species of butterflies can cause extensive damage to cabbages and other brassicas, eating holes in leaves and tunnelling into the hearts. Plants begin to rot and become spoiled with excrement.
• Two species are particularly important pests: the large white butterfly, Pieris brassicae, and the Small White butterfly Pieris rapae.
• Both species are commonly seen flying within our gardens and are affectionately called summer snowflakes.
• While the small white is a native species, the large white has usually migrated from southern Europe becasue they don't often survive a north European winter.
• Large white butterflies have often been seen crossing the English channel in swarms of many hundred individuals.
• Cabbage white butterflies overwinter as pupae. Those that survive the cold months will hatch into butterflies during the spring.
• The butterflies mate then each female lays a few hundred eggs, on the leaves of cabbages, other related plants and nasturtiums.
• The large white's eggs are often laid in batches of ten to 20,which hatch into yellow and black slightly hairy caterpillars in about two weeks.
• Large white caterpillars feed for a month or more and can reach 50mm in length before they turn into pupae.
• The small white's eggs are laid singly and hatch into bright velvety-green caterpillars. Unlike the large white, these caterpillars often burrow into the hearts of cabbages to feed.
• Cabbage white caterpillars leave their food plants when ready to pupate and usually attach themselves to a vertical surface such as a fence or wall with a silk girdle before their final skin shed.
• During the summer months, the pupae soon hatch into the next generation of butterflies and the cycle continues.
• The severity of cabbage white problems can vary considerably from year to year due to weather, immigration and diseases.

Treatment

Chemical

Products containing the following chemical ingredients are all effective on Cabbage white butterflies

• Pyrethrum

Note: It is important to read manufacturer's instructions for use and the associated safety data information before applying chemical treatments.

Organic

• Inspect the undersides of leaves and remove any egg clusters.
• Remove caterpillars by hand, particularly before cabbages begin to develop hearts.
• Encourage insectivorous birds within the garden by using bird-feeders in winter and nest boxes in spring.
• Don't remove dead older caterpillars that have become parasitised because these will produce many more parasites that will attack and kill further generations of caterpillars.
• A naturally occurring bacterium, Bacillus thuringiensis, kills only caterpillars and not the predatory insects, and is very effective when sprayed thoroughly above and below leaves.

Prevention

• Inspect susceptible plants on a daily basis during the growing season and remove any eggs or caterpillars that are found. The appearance of white butterflies hovering around susceptible plants almost certainly means that eggs have been laid.
• Cover plants with insect-proof mesh or fleece, although this can lead to overheating during the warmer summer days.
• Transfer any parasitised caterpillars to plants where new caterpillar infestations are expected.
• Avoid planting colourful, high nectar plants near the cabbage patch as these will encourage adult butterflies into the garden.