This is a genus of several species that feed on Betulaceae (Betula and Alnus). Like most tree aphids, I don’t study them particularly, just try to put decent identifications on my samples. Others have brought to bear better expertise on Euceraphis than I could, especially a number of papers by Roger Blackman and co-authors, including: “A year in the life of a birch aphid.”
Euceraphis betulae Koch
This species is common on birches in western North America, apparently introduced from Europe. As noted by others, it is notoriously variable in appearance and is a taxonomically challenging species or species-complex. I am intrigued in variability as simple but dramatic as the degree of wax secretions as demonstrated by photos below.
Euceraphis gillettei Davidson
In western North America this aphid is common in natural systems wherever Alnus (alder) grows. As noted by Blackman and Eastop, this name might be used for more than one species.