By checking your Brassicas regularly, you should be able to detect any caterpillars before they cause too much damage. Pick them off as and when they're seen. The best is to look when adult butterflies fly consistently over as it might be an indication that they are trying to lay eggs.
Cabbages can be covered with netting to prevent eggs from being laid on the plants.
Other barriers include diatomaceous earth, wood shavings or crushed eggshells.
These insects are heavily preyed upon by other garden wildlife, including reptiles, mammals, birds and other insects.
Attract natural predators to your garden by providing refuge spots. These can include anything from strips of native grasses, wildflowers, hanging baskets, climbers, or old bits of log/ deadwood.
They're also heavily parasitized by wasps, which will insert eggs into the egg, caterpillar and pupal stage.
Pteromalus puparum, braconid, ichneumonid, eulophid, encyrtid, pteromalid and chalcidid wasps will all use this butterfly as a host for their offspring.