Q What are caterpillars?

A They are the larvae of moths or butterflies that feed on the leaves of many plants. They strip foliage, tunnel into the hearts of cabbages and spoil produce with their droppings or frass.

Caption: There are several different types of caterpillars that will attack brassicas

Q Which caterpillars will attack cabbages?

A There are two cabbage-white butterflies that do serious damage: the large white (Pieris brassicae), and the small white (Pieris rapae).

Damage is also caused by moth caterpillars. The cabbage moth (Mamestra brassicae) is the most common, especially in gardens. The diamond-back moth and the garden pebble moth are locally important. The silver-Y moth (Autographa gamma) is only periodically important, as in 1996 when many flew in from the Continent. Occasionally, other moth caterpillars are seen, such as the tomato moth, the flax tortrix moth and dot moth caterpillars.

Q Can cabbage white caterpillars be told apart?

A The large cabbage white butterfly likes to lay its eggs in sheltered spots, so gardens are ideal. Its caterpillars are pale green, often with a yellow hue, and heavily dark-speckled. Look out for a strong, yellow line down the centre of their backs and light hairs. Caterpillars of the small cabbage white butterfly are velvety green, again with a yellow line down the middle of their backs. They are both about 3-4cm long when fully mature.

Both butterflies are white with black wing markings and are very common. There’s also a green-veined white, but it doesn’t often attack cultivated plants.

Cabbage moths are plain grey-brown with some white markings. Flying at night, they are seldom noticed by gardeners. Their light-green/brown caterpillars are 4cm long when fully grown. Unlike the cabbage-white caterpillars, they don’t have many hairs.

Diamond-back moths are elongated and narrow in shape, about 6mm long, with long antennae and white wing edges. When closed, the white marks form a rough diamond pattern. Its light-green caterpillars live on the leaf undersides in a web, and can reach 14mm long.

Night-flying garden pebble moths are small (25mm), brown and yellow. Caterpillars are yellow to green, with a darker green stripe on their backs and a yellowish stripe down their sides.

Caption: Cabbage white butterflies are the most common caterpillars on brassicas

Q Are only cabbages attacked by cabbage-white caterpillars?

A Cabbage-white caterpillars feed on any cruciferous or cabbage-family plants. Vegetables are the usual hosts, but ornamental plants are also eaten – stocks and nasturtiums being favourites. Diamond-back moths and garden pebble moths will feed only on cruciferous or cabbage-family plants, but cabbage moths will also be drawn to lettuce and some ornamental plants, too.

Q Can the symptoms be mistaken for anything else?

A Slugs can spoil the hearts of cabbages, but instead of leaving frass, they leave slime trails. The eggs of slugs can be confused with ladybird eggs, so, if you’re not sure, wait until they hatch then spray any caterpillars. Pigeons also strip foliage, which may be mistaken for caterpillar activity.

Q When should I expect to see caterpillar attacks?

A Large whites emerge in the spring and seek out cabbage plants, laying as many as 200 eggs in clumps beneath the leaves. Two weeks later, the eggs hatch and caterpillars emerge. They feed in a colony at first then disperse, but not far, so if you find one, you will find others. Once fully grown, they pupate away from plants, soon hatching into butterflies. The second generation in July and August are numerous and very damaging.

The small white is similar, emerging a little earlier in the spring, laying eggs and producing caterpillars that feed singly. The chrysalids form on the plants. There is also a second generation of small whites, which cause damage into the autumn. The second generation of both types will not hatch until spring.

Cabbage moths are on the wing at night during May and June, laying batches of round eggs on the leaf undersides that will hatch after two weeks. The caterpillars feed for at least a month before they mature. They pupate in the soil and generally hatch the following spring. But a few will hatch in late summer and into the autumn. They can be found in early winter if it’s mild. The chrysalids of the second generation hatch in the spring.

Diamond-back moths overwinter as pupae on plant leaves and stems. They emerge the following year in late spring to early summer, flying both by day and night. Eggs are laid beneath the leaves. When they hatch, the caterpillars feed beneath a web under the leaves for about three weeks, until fully grown. They then pupate on the stems and leaves of their host plant, spinning cocoons. There are several generations per summer, with a two-week pupation period.

Garden pebble moths overwinter as cocoons containing caterpillars or as pupae in the soil. They emerge in late spring and early summer to lay clusters of eggs on leaves. The caterpillars feed for three weeks before pupating. There is a second emergence in late summer. The caterpillars from these remain throughout winter, pupating the following spring.

Q Are some years worse for caterpillars?

A The severity of the attacks is different each year, due to the effects of weather, immigration from across the Channel and from diseases, which sometimes break out in the moth and caterpillar population.

Q What should I do about caterpillars?

A Serious outbreaks are best sprayed as soon as possible. Light attacks on a few plants can be dealt with by rubbing off any eggs, and picking off and destroying as many caterpillars as is practical. To find the cabbage-moth caterpillars, you will have to search in the centre of plants.

Q What sprays can be used to kill caterpillars?

A Contact insecticides are very effective. There are many available, such as Westland Resolva. Always follow the instructions.

Q What should organic gardeners use to kill caterpillars?

A Try a spray based on nematodes, such as Nemasys Caterpillar Killer.

Q Can brassicas with caterpillars in the hearts be saved?

A Once the caterpillar is inside the heart or head, it is almost impossible to kill. Treat early, before the pest has a chance to get inside. Once caterpillars are inside, salvage whatever leaves you can by cutting out the damaged parts.

Q What should I do with plants that have been attacked by caterpillars?

A Chop up the plants and compost them. If the caterpillars are deeply buried in a warm compost heap, they are unlikely to pupate successfully.

Q Do the caterpillars have any natural enemies?

A Parasites, birds and diseases kill many caterpillars. With luck, these will destroy any remaining ones for you.

Q How can I avoid the caterpillars in the future?

A Cover plants with insect-proof mesh. Use hoops to support the mesh off the plants and make sure there are no gaps. Bear in mind that winter sowings of cabbage.

Suppliers of nematodes and insect-proof mesh

Green Gardener 01493 750061 greengardener.co.uk

Harrod Horticultural 0845 402 5300 harrodhorticultural.com

The Organic Gardening Catalogue 01932 253666 organiccatalogue.com