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Pieris marginalis
Not a cabbage white, the lack of black marking other than veins, make this an arresting visitor to my garden. It is highly reflective and the black body stands out noticibly
I was just going to comment on the range. marginalis works though. Thank you!
Oops, I was not paying attention to your location. Although It looks like it the most to me, I don't think the Mustard White occurs in your area. Try, the Pieris marginalis - Margined White instead. Look through the images some are plain white like yours and very similar to Mustard W.
http://bugguide.net/node/view/12866