This is a cabbage white butterfly, although it seems to have a bit of yellow on its underside. As far as I know, it neither eats cabbage nor looks like cabbage, so I don't quite know where it got its nickname.
UPDATE: Apparently the name derives from the fact that the worm of the species is an absolute scourge to cabbage plants.
Cabbage worms are voracious consumers of the leaves of many of the brassicas.
If you grew cruciform vegetables in your garden, you'd know why they're called cabbage whites.
Because they're a plot to salt the Earth's atmosphere with methane so the planet will spontaneously combust before humanity becomes a threat to the Galactic Authority, has been my impression.
It doesn't but it's caterpillars sure do.
All of the above.
Amen. Or bingo or something.
Yep I gave up trying to grow brussels sprouts and broccoli in my garden due to them.
I tent those things with white garden cloth pinned to the soil on all sides. Pain in the neck because you have to keep rearranging it as the plants grow, but brocc and sprouts are so damn wonderful straight out of the garden, I do it every few years anyway.
Thanks a lot, whoever managed to accidentally introduce them. As if we didn't already have enough indigenous pests in the vege patch.