The cabbage white butterflies have started flying around in the last couple of days and it's time to keep a look out for caterpillars on the brassicas. One fat green caterpillar caused the holes on this cauliflower plant:
Inspecting the underside of the leaves for eggs or caterpillars regularly means you can deal with them before they spoil your crop. Rub out the eggs or squash small caterpillars whenever you see them. Sometimes it is easier just to cut off an infested leaf and remove the whole lot that way. Caterpillars are qute mobile and will find their way back if merely dislodged from plants.
The Large White lays clumps of yellow eggs which then hatch out into hordes of stripey voracious caterpillars, the Small White tends to lay single eggs which develop into a green caterpillar. Covering the plants with horticultural fleece or mesh will easily prevent the butterflies from laying eggs in the first place.