You can pick them off the plants after you got them all net the plants but make sure that it's fully secure so that birds can't get underneath and be trapped.
I pick them off sometimes. I do use a product called BTK though with good success. A neighbour used to run a commercial organic brocolli farm, and used BTK as it was certified organic. He told me about it. It is a natural bacteria that when the caterpillars injest it they stop eating and eventually die. It is used as an aerial spray here for some of the forests when there is an infestation. I spray every week, at the first sign of cabbage moths in my broc, cauli, kale and cabbage. I've also sprayed ornamentals when I see leaf munching. It also gets rid of bad bouts of tent caterpillars etc. If I keep it up I have minimal damage. It's pretty harmless otherwise and doesn't bother other bugs or bees etc. It's not regulated as a pesticide here, and Canada is pretty tight about those things. I like that organic vegi producers can use it - so I feel pretty okay about it. A little goes a long ways, and I keep it in the fridge to keep it fresh.
I guess btk is bacillus thuringensis. I squish the eggs before they hatch and check my brassicas every day or 2 when the cabbage butterflies are about.
another name for Bt is Di-Pel. comes as a powder for most people and a concentrate that can be diluted for a spray if you can fine that. both work well.
