This large spectacular butterfly is the great spangled fritillary. I usually see this butterfly late in the summer, and almost always on the flowers of tall thistle or field thistle(as seen here).
Around here, all thistles are considered noxious weeds. Tall thistle and field thistle are very good butterfly flowers, and thankfully, they thrive even when people try to eliminate them. In fact, I would recommend that the tall thistle should be named the state flower of Iowa, replacing the wild rose.
Why? I say, why not?
Both have thorns, although those of the tall thistle are easier to avoid. Both have spectacular flowers, although those of the thistle are larger and up at eye level instead of on the ground.
Iowa allows people to buy a car license plate with a painting of the state bird (the goldfinch) on the state flower(the wild rose). I have never seen a goldfinch on a wild rose, but I have seen them on the tall thistle.
The great spangled fritillary is one of the largest butterflies in this area, although it probably is not seen as often as some butterflies. Late in the year some of them can be quite ragged.
The larval food is violets, as is the case for about half of the fritillary species.