Could this be the Balkan Green-veined White, A. balcana?
Mts, Bulgaria, July 2004
Upperside of butterfly above, taking salts from sweat on my finger.
Belgium, June 2001
This is an extremely widespread species, found
across Europe and the mountains of north Africa, in a huge variety of
habitats flying throughout the season in a number of broods. It is also
extremely variable in intensity of dark markings on both surfaces and
also the extent of the markings. Spring butterflies are, on average,
darker than subsequent broods.
The variability between populations has led to a large number of
named forms and subspecies and even species. The Balkan Green Veined
White (Artogeia balcana) is still up for debate as a separate
species although physical differences in the genitalia are reported to
make cross paring with napi very difficult/ unlikely in the wild.
The two photos above of a butterfly found in SW Bulgaria show greatly
reduced underside veining which is associated with balcana.