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Betula is governed by the botanical code, which I do not understand.

Varieties are no longer allowed by the code of zoological nomenclature. Varieties proposed before 1961 may be treated as subspecies.

Subspecies are rarely used for Diptera. Mammalogists, on the other hand, love subspecies. Slovenia is near the intersection of 3 of the 22 subspecies of Sciurus vulgaris -- the central European fuscoater ("a bright red form, with a white venter"), the western Balkan lilaeus ("a generally brownish form"), and Italian alpinus ("an intense russet brown over the dorsum").

I only note subspecies when I consider the difference interesting. For example, 2 of the 21 subspecies of Tamias minimus are colored tan instead of brown to match the color of the ground where they live in the "badlands" of western North America.