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Milk weed
Using a different source, still looks like an orange sulfur.
Orange Sulfur:
http://bugguide.net/node/view/138493
Clouded Sulfur:
http://bugguide.net/node/view/145979
It could be the lighting, but it looks like there is sufficient orange showing through the wing to suggest Orange Sulphur.
Southern dogface (zerene cesonia) has a much more southern range in North America.
Dark Clouded Yellow (Colias croceus) is not found on the North American continent but is a similar looking species.
Clouded Sulphur (Colias philodice)
http://en.wikipedia.org/wiki/Colias_phil...
and Orange Sulphur (Colias eurytheme)
http://en.wikipedia.org/wiki/Orange_Sulp...
are both likely candidates but it is hard to tell without looking at the upperside of the wing color.
Clouded Yellow (while a good visual match) should not be in the Americas. While the ranges of the other 2 could overlap, the wing shape identifies this as the orange orange sulfur.