White cabbage moth control - plant nasturtiums. 

I don't encourage white butterflies in my garden for any reason. A chemical free alternative is using an empty white icecream container, draw on some butterfly shapes and cut them out. Makes 12 if you start on the corners and use the lid. Draw the right wing pattern with a black permanent marker. Attach to thin sticks with twisty ties. Plant the sticks with the butterfly against a leaf. As the plant grows move the butterfly up the stick. These act as a Parking Full notice. I have seen butterflies aim for a brassica plant, see my plastic ones and move off to another garden. These fake butterlies last for years, just needing a touch up with the pen when the markings fade in the sun.

ok