The butterfly in this photo is seeking nectar at swamp milkweed, Asclepias incarnata, which is also a favored host plant. Monarchs will lay eggs on nearly all of Ohio's 15 species in the milkweed subfamily, though.
Midwest Native Plant Conference, so I briefly borrowed an egg for photo purposes. Honey vine is a native albeit somewhat weedy member of the milkweed family, and monarchs frequently use it as a host plant. This egg is hot off the presses; in about four days a tiny caterpillar will emerge.
NOTE: Barb Huebner was kind enough to give me five monarch chrysali in various stages of development, so I could make photos of the emergence process. This is one of them. Barb raises scores of monarchs, all of which are released back to the wild. She gets the caterpillar livestock from milkweeds growing wild in her Columbus, Ohio yard. By raising caterpillars in a protected environment, survivorship soars as the caterpillars are removed from the risk of being killed by various predators, many of which are other insects. Her work and similar efforts by legions of others is almost certainly bolstering monarch populations.
Things happen very fast at this point, and I was popping off shots every few seconds.