Answer: You've experienced on of the dilemmas of butterfly gardening! Many gardeners try to find a happy medium, allowing a few caterpillars to survive while not totally devastating their desirable plants.
The lowest toxicity, organic insecticide for caterpillars is Bacillus thuringiensis, otherwise known as B.T. It is sold under many brand names. Your local garden center manager can help you find the one they carry. Check the label for instructions.
B.T. only lasts a few days and therefore must be applied whenever caterpillars reappear. It is most effective on young caterpillars, and is not a contact poison like most synthetic products. When the caterpillar ingests leaves with B.T. on them they soon stop feeding and die.
Maybe you have room for some extra bushes. One for you and one for the butterflies!
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