Yes indeed white cabbage butterflies are pestering our poor broccoli seedlings as though they are the last ones on the planet. The smallest leaf had 20+ eggs at least and derris dusting is quite difficult as well as unsightly.
I went to Mitre 10 today and bought 2 meters of bird netting. White in colour as the black would reduce the amount of light falling on the plants.
A few sticks to prop it up and and hopefully it will keep the beasts at bay.
Not exactly pretty but effective I hope.