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Pieris napi
Pieris napi is a European species, although the is a Pieris napi group with some US species, but these are not really called Pieris napi. I can't tell you which exactly yours is, I would need a clearer shot and other views. Candidates are Pieris virginiensis and Pieris oleracea. Please, check these in bugguide and compare with yours and check also the range!
http://bugguide.net/node/view/179680/bgp...
http://bugguide.net/node/view/389712/bgp...
http://bugguide.net/node/view/3258/bgpag...