neem oil works wonders in organic gardens. Its a natural herbicide that keeps insects at bay.
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Flamethrower. Just very lightly braze over the plants, and it should kill the pesky butterflies.
the butterflies are migrating they are eating just the leaves and that is it
If they are just eating the leaves and not the harvest and it's not too bad, I'd just pick off the ones I could find and ignore it. A little leaf damage won't hurt much. I don't go completely organic so I dust once or twice with Sevin early.
Since there are already holes appearing it means that you've already got lots of caterpillars crawling around on them. This website has a list of many of the natural predators of caterpillars, encouraging these beasties to flourish in your garden will reduce the amount of caterpillars in the long term. Additionally, fine mesh over the plants will prevent the butterflies from landing on your plants in the future and laying their eggs. You could have it either as a low level cage that you can remove to harvest and weed or as a high level cage covering the whole area, that you would walk beneath. If you want to keep it organic and can't spray anything chemical on them the website above recommends garlic spray to deter and kill the caterpillars. I hope this helps. Good luck!