The butterflies are attracted to a pheromone or a smell in the brassicas. You'd have to somehow block that smell, to put the butterflies off ... they can smell them a mile off.

I've got ruddy PillarCats underneath my netting ... every plant had a dozen of them, munching away. I found the best way to pick them off was to shake the leaves quite vigorously, and the caterpillars drop to the soil - free treats for the birdies

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~ "I would rather do a good hours work weeding than write two pages of my best; nothing is so interesting as weeding" ~ Robert Louis Stevenson

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