The caterpillars are a known 'pest 'of vegetables including potatoes, beets, and yes, turnips. See here; http://lepidoptera.butterflyhouse.com.au/acro/nigerrima.html
Glad you put pest in inverted commas. Some beasties that are seen as pests really are not. Even the cherry slugs that have decimated my cherry trees this year are food for cuckoos. And no the cuckoos have not been particularly in evidence this year. Might be more food elsewhere.
Actually I also find it interesting the way that butterflies tend to be known by the adult form while moths are known by the larvae. It just fits with the butterfly image of something beautiful (even if it eats your cabbages) while the moths are either disregarded or regarded as a pest. Moths tend to be much more beautiful than butterflies (aside from skippers which of course are something else again).