If you’re wondering what’s eating your cabbage or other Brassica then it’s likely to be the caterpillar of the Cabbage white Butterfly / Large White Butterfly Pieris brassicae
Pieris brassicae Cabbage White Butterfly – Large White Butterfly
The butterfly lays a large number of her eggs on the food plant, namely your Brassica.
The caterpillars are tiny when they first emerge but will grow quickly as to their appetites.
Brassica crops can be totally destroyed by these allotment / vegetable pests
Photography by Neil Bromhall. Complete Garden plant finder, planting advice and pruning guide CD-ROM with 3,500 plants plus 9,000 photographs