The signs to look for are; holes in the foliage, caterpillars and excrement pellets on the leaves.
The caterpillars of the large cabbage white butterfly are up to 45mm (1¾”) long and are hairy with yellow and black markings, and those of the small cabbage white butterfly are up to 30mm (1¼”) long and have a velvety green appearance.
Cabbage moth caterpillars are up to 38mm (1½”) long and are greenish-brown in colour with relatively hairless bodies.
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Cabbage moths have one generation with caterpillars feeding from July to September.
Both cabbage white butterflies have two generations a year with caterpillars active between May and October.
The life cycle; of butterflies and moths is divided into 4 phases.
The egg stage: the caterpillar eats its way through the shell to provide energy.
The caterpillar stage:
This is the stage most detrimental to the gardener i.e. when the caterpillar/s feed on the host plant.
When fully fed, the caterpillars crawl away and pupate above ground level in sheltered places.
Small cabbage whites and the cabbage moth caterpillar bore into cabbage heads, whereas the larvae of the large cabbage white remain on the outer leaves.
As the Caterpillar grows it expands, necessitating it to moult.
This is normally between 4-6 moults for most caterpillars.
The pupa stage:
The body is extensively rearranged to form the adult moth or butterfly.
Moths usually construct a protective cocoon in which to pupate.
Some spin leaves to form a simple cocoon, whilst others incorporate bark and wood to make an elaborate chamber.
Butterflies are mostly suspended from leaves and stems.
The caterpillar spins a silk thread to hang from.
The adult stage:
As the pupa skin splits open, the adult begins to emerge and swallows air to expand the body.
When fully emerged, the adult rest whilst pumping blood into the wings to expand and dry them.
Control;
Prevention is often easier than cure!
To avoid the situation, cover the young plants with agro-fleece after planting out, this will prevent the moths & butterflies depositing there eggs on the plants.
It is easier to control younger caterpillars than older ones, so regular inspections for signs of these pests are worthwhile.
Hand picking will eliminate small infestations.
Otherwise spray with a suitable insecticide.
Biological controls are also available by mail order.