Genus Rheumaptera

Should be safe to separate these 2 species where they are allopatric. Ferguson (1955) gives a bit more detail than Covell:
undulata: throughout the Canadian and Hudsonian zones from Newfoundland to Vancouver Island and Alaska, southward in the Rocky Mountains to San Francisco Peaks and White Mountains, Arizona and the Jemez Mountains, New Mexico, and in the coast ranges to Mariposa Co., California.
prunivorata: Nova Scotia south through New England and the middle Atlantic States to North Carolina; westward through southern Quebec and Ontario, New York, Pennsylvania, Ohio to Illinois and Wisconsin.
Perhaps include these comments on the species Info pages.
It would be nice to include a distribution map on the genus Info page showing the distribution of each species and emphasizing the area where they are sympatric and thus cannot be easily identified to species.

and check out the comments. We seem to remember several stating that things were not as easy as they seemed. Maybe Tony Thomas will comment on this topic since our recollection is that he had significant insight into this problem.

Also, All-Leps is our resource on Lep taxonomy and they list four North American species for what that's worth.
Geometridae
Larentiinae
Rheumaptera hastata (Linnaeus, 1758)
Rheumaptera prunivorata (Ferguson, 1955)
Rheumaptera subhastata (Nolcken, 1870)
Rheumaptera undulata (Linnaeus, 1758)