Try diatomaceous earth

nostr:npub1pncg62q25h704u6qcf56hnmxx46jdlwfpw2t860ld5685s0sjzms9rqlgr those are butterflies, they'll find your brassicas anywhere. It's you or them. One year I picked them all off and killed them; I did have brassica but have regretted killing them ever since 🦋🦋🦋 https://en.m.wikipedia.org/wiki/Pieris_brassicae

as far as I know brassica likes high pH soils and cooler weathers.

The collards will regrow if the stems were mature enough, at least in the Caribbean they do. The larvae looks similar to monarch butterfly larvae but I know it isn't. The larval stage is easiest to control, either pick off the worms or spray using some type of organic pesticide.

Utilize floating row covers to prevent adult butterflies from laying eggs on the plants. This method is most effective if applied when plants are young. Regularly inspect the plants for eggs, larvae, or worms, and remove them manually. Crush the eggs and dispose of the worms away from the garden. Interplanting with aromatic herbs like thyme, sage, or rosemary may deter the adult butterflies from laying eggs. A bacteria that specifically targets caterpillars, Bt can be applied as a spray to the affected plants. It's considered an acceptable option in organic gardening as it is non-toxic to humans and other animals. Planting a sacrificial crop that the cabbage worms prefer may divert them from your main crop. Mustard is often used for this purpose. If it fits within your permaculture system, free-ranging chickens or ducks can help by foraging for cabbage worms. After harvest, turning the soil can expose pupae, leading to their desiccation or making them accessible to predators.

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They are in the wrong place and certainly not endangered. It is quite legitimate to pick them off. You might like to leave a few. Sharing is good.

Netting would have kept them out.