Ask Mr. Smarty Plants

On searching for butterflies that come to specific plants, you must think of them as pollinators. While the face of the primrose is fairly flat and looks inviting to butterflies, the butterflies that are around may have mouths adapted to totally different flowers. We suggest you begin by reading our How-To Article on Butterfly Gardening.

To be honest, I am afraid that the material you are reading says "many butterflies,"  because they don't know either. We found an article from Illinois Wildflowers on Oenothera speciosa (Pink evening primrose) that has some pollination information, including:

"Faunal Associations: Small bees collect pollen from the flowers, but they are unlikely to pollinate them. The size and length of the stamens and style suggest that hummingbirds, large butterflies or day-flying Sphinx moths are more likely to pollinate the flowers while seeking nectar, although this is somewhat speculative. Some insects feed destructively on the foliage and other parts of Showy Evening Primrose and other Oenethera spp."

Another (slightly tedious) scientific paper from the American Journal of Botany suggested the night-flying hawk or sphinx moth might be the primary pollinator of the Evening Primrose, which sometimes blooms only one day while other times it blooms two days, and pollination would occur at night, which explains why you're not seeing them crowded around the flowers during the day.

From the US Forestry Service article Celebrating Wildflowers, here is an article on the Hawk or Sphinx Moth.

Even after your nice comment (thank you) we feel we have let you down, but at least you know that we couldn't find out either. However, if it is any comfort, many of the articles on the flower itself branded them as very invasive. They make a lovely roadside flower, possibly crowding out invasive non-natives, but you might want to read the 18 negative comments from the Dave's Garden Forum website.