There are two different types of plants you can grow for butterflies: nectar food sources and larval food sources. Nectar sources attract the adult butterfly and many different types of flowers will serve as a nectar source.
Providing larval food plants is where butterfly gardening diverts from all other types of gardening. With these plants you are feeding the caterpillars that eventually turn into adult butterflies. It is hard for many people to allow a critter to eat up their garden plants. However, butterfly larvae are picky eaters and most need a specific type of plant in order to survive.
In addition to planting milkweed for monarch butterflies, here are some other examples to try.
Let's start with swallowtails. The pipevine swallowtail young feed on pipevine plant, while the zebra swallowtail larvae feed on pawpaw trees. Black swallowtail females lay eggs on plants in the carrot family for their larvae to eat, including dill, parsley, carrot, parsnip, and other natives in that plant family. If you see giant swallowtails, there must be a source of wafer ash or prickly ash nearby. Tiger swallowtail larval hosts include wild black cherry, tulip poplar tree, as well as apple, maple, ash, and others. Finally spicebush swallowtail caterpillars eat spicebush and sassafras.
Fritillary butterflies are one of my favorite. Most fritillaries feed as larvae on violets, and some also on passion vine. The common violet is our state flower and is very beautiful in bloom. I've started using common violet more in my landscape in hopes of seeing more fritillary butterflies in my neighborhood.
To learn more, I recommend the new Butterflies of Illinois: A Field Guide by Michael Jeffords, Susan Post, and James Wiker. This beautiful book contains descriptions, field photos, and life-size specimen photos of all Illinois' butterfly species.
I also encourage you to visit the Kim St John Butterfly Habitat at Wildlife Prairie Park. The house showcases native Illinois butterflies using a modest structure of metal hoops covered with netting. The house covers 2,600 square feet and is filled with larval and nectar plants.
Finally, reduce pesticide use and use more native plants. Butterflies are insects and are susceptible to most insecticides. Many insects feed on the native plants they evolved with over time. Although adult butterflies feed on many different types of nectar, their larvae need specific (and often) native plants in order to survive.
Plant your own butterfly garden this summer. Then, sit back and enjoy the show. Butterfly gardening is very rewarding.