I see Bath whites every year but a bit at random - not in colonies as such. As a rule the further you go south the commoner they are, though they are replaced in their Eastern range (and in Brittany, bizarrely) by the Eastern Bath white, Pontia edusa. Ironically, although the butterfly takes its name from the town of Bath in England, all the British specimens turn out to be edusa too. There is no way of separating these species externally - they are said to be sufficiently distinct biochemically to preclude interbreeding.
