Caterpillars are the larval stage of butterflies and moths. There are several species that feed on cabbages, other brassicas and other plants including turnip, swede, horseradish and nasturtiums.

The three species that are often found are:

• Large white butterfly (Pieris brassicae)
• Small white butterfly (Pieris rapae)
• Cabbage moth (Mamestra brassicae)

Holes are eaten in the outer leaves of all brassicas and damage may also be seen on the inner leaves of cabbages when the heart is cut through. Caterpillars and their excrement are often found on the plants.

• Large white butterfly caterpillars are yellow and black with obvious hairs on their bodies (see picture above)
• Small white butterfly are pale green and covered in short, velvet-like hairs. Green veined white caterpillars are similar in appearance
• Cabbage moth caterpillars are yellowish green or brownish green, with no obvious hairs on their bodies
• Diamond back moth caterpillars can feed extensively on leaves, causing a characteristic windowpane effect. They are green and reach about 1cm long when fully grown and can be very numerous. They will often drop from the plant when disturbed. This species may become more of a problem in the UK as the climate changes

Check susceptible plants frequently from spring onwards so action can be taken before a damaging population has developed. When choosing control options you can minimise harm to non-target animals by using the methods in the non-pesticide section below. Pesticide treatments are likely to kill natural enemies and are only likely to be successful if the entire plant can be reached.

Non-pesticide

• Where possible tolerate populations as cabbage caterpillars are food for many other creatures including parasitoid and social wasps and birds. The butterfly caterpillars are often infected with the parasitoid wasp Cotesia glomerata, the larvae of this wasp burst out of the caterpillar and spin yellow cocoons
• Growing brassicas under fine netting or horticultural fleece can exclude adult butterflies and moths from laying eggs on the crop. Care must be taken to ensure the netting does not touch the plants or the adults can lay eggs through it
• Inspect plants regularly and pick off the pale yellow butterfly eggs, white spherical moth eggs and caterpillars when seen
• A biological control is available for caterpillars, this is supplied as a mixture of pathogenic nematodes, suppliers of which can be found in a list of Biological control suppliers. The longer the treated caterpillars and foliage stay wet, the greater chance of the treatment being effective, so apply during cool dull/ damp weather. Whilst the nematode controls are usually either insect or mollusc specific, they therefore have the potential to infect non-target animals within those groups. They should therefore be used with care and only when there is a specific problem to treat. 
• One product is sold as a repellent against cabbage caterpillars, Grazers G3
   

Pesticides

• If numbers of larvae are too high for hand picking, control may be achieved by spraying with pesticides. Spraying at dusk is likely to give the best results
• Organic contact insecticides containing natural pyrethrins (e.g. Bug Clear Ultra 2, Neudorff Bug Free Bug and Larvae Killer). Several applications of these short persistence products may be necessary to give good control
• Further information about the use of pesticides available for management of aphids is available on the pesticides for gardeners leaflet.

Downloads

Pesticides for gardeners (link downloads pdf document outlining pesticides available to gardeners)

Biological control suppliers (downloads pdf document)

• The adult butterflies and moths lay eggs on brassica leaves. Large white butterflies lay clusters of yellow, skittle-shaped eggs on the top or bottom surface of leaves. Small white butterflies lay yellow eggs singly on the underside of leaves. Green veined white lays single white eggs. Cabbage moth lays white spherical eggs in clusters on either surface of leaves. Diamond back moth lays flattened pale yellow-green eggs singly or in groups of up to eight.
• The butterflies have two generations during the summer; cabbage moth has two or three overlapping generations. Diamond back moth can potentially have several generations in the summer
• White butterfly caterpillars are likely to be seen in June-July and August-September; cabbage moth caterpillars are active in July-September. Diamond back caterpillars may occur from mid summer onwards
• With the exception of diamond-back moth, when fully fed, the caterpillars leave the plants to pupate. Cabbage moth pupates and overwinters in the soil and cabbage white butterfly larvae pupate on suitable vertical surfaces above ground level. Diamond back pupae can be found on the plant in a loose silk cocoon.