Your finger and thumb, picking them off, dropping on a firm surface plus application of boot = the most organic remedy.

Prevention = inspection of underside of leaves and squishing of eggs at an earlier stage.

Oh bad luck. It depends what sort of a hold they already have. If it's just a few you can pick them off and look for the tiny yellow or white eggs on the undersides of the leaves. Nets are really the only way to keep the butterflies from laying their eggs.

Looks like we are on the same wavelength at the same time Beattie.

:-)

Grow some Malva Moschata Alba plants. They self seed and will grow again next year. The hoverflies come on summer evenings to the white flowers. Then they fly over and feed on the aphids - that grow into Cabbage Butterflies. You can watch them do this. I dont know if the coloured varieties have the same effect. I have been using them in this way for 20 years. Come the second week in September it turns cold at night, and you will see a sudden crop of Cabbage Butterfly aphids on the Brussels. Then just spray once. Next week it will be too cold for them and they will be gone.
from Dianebulley.

Umm, Dianebulley, cabbage butterflies come from little green caterpillars, not from aphids. I'm afraid that hoverflies wouldn't help much with them.

The best way is to net the plants at the start to prevent the butterflies getting access to them to lay their eggs. If its too late for that keep an eye on the plants and remove any little eggs you find before they have a chance to hatch. Failing that, watch out for the caterpillars before they can do any damage.

Sorry Tugbrethil, that comes of replying when tired.
Of course, the Hoverflies (bless 'em) feed on the
White Cabbage Butterfly Larvae.
I dont have any trouble with caterpillars at all. Cheaper than paying for netting.
Saw a new idea for keeping Pigeons off the Brussels last week. Place 5 ' canes among the plants and around the edges of the area. Then weave fishing line in and out to form a network about 6" above the plants. Seal this fishing line to the canes with electrical tape. There was
no bird damage on these plants at all. I will be doing this when mine are higher than the hoops because the Pigeons settle on the hoops in the winter, and peck through at the top of the plants.

Thanks, Diane. I will have to try this myself, since all of the literature that I have been able to find talks of hoverflies eating aphids, but not caterpillars. Wouldn't have been the first time that the "experts" were wrong.