If you want your garden to be a sanctuary for butterflies, you have to make sure to feed the very hungry caterpillars.
It’s not enough to grow just the nectar-rich, colorful flowers that attract butterflies; if you want to keep them coming back for more, provide them with a garden rich in food for their larvae.
Larvae are picky about what they eat, so make sure you provide the correct plants for the types of butterflies you want frequenting your garden. For example, monarchs eat milkweed–and only milkweed.
In this helpful and insightful article from Gardening With Confidence’s Helen Yoest, you will learn about sustainable butterfly gardens and, in particular, the parsley-loving eastern black swallowtail butterflies: http://gardeningwithconfidence.com/blog/wildlife-host-plants/parsley/