Clouded Yellow (Colias croceus)
Of the three species of Clouded Yellow generally regarded as being found in the British Isles, the Clouded Yellow (Colias croceus), Berger's Clouded Yellow (C. alfacariensis) and Pale Clouded Yellow (C. hyale), C. croceus is by far the commonest and the easiest to identify; alfacariensis and hyale are both similar in appearance and extremely rare immigrants.
When observed in flight, the orange appearance of croceus is unlike any other British butterfly. When settled, the lemon-coloured underside of croceus allows us to distinguish this species from both alfacariensis and hyale which both have much paler undersides. This diagnostic generally holds true even in the helice form of female croceus where the orange colouring is replaced by a delicate creamy white. Distinguishing alfacariensis from hyale in the field is a completely different matter and unless one is accustomed to regularly seeing either species it would be very difficult to make an accurate identification of the adult insect.
C. croceus has a distribution befitting a strong-flying, highly-migratory species and can be found almost anywhere in the British Isles including coastal cliffs, open downland, and fields containing its larval foodplants. Many immigrants remain near the coast where they feed, mate and lay eggs. Others disperse inland and this species can be found in both Scotland and Ireland during good seasons. Numbers vary greatly from year to year but in 1947, one of the infrequent mass immigration years, an estimated 36,000 adult butterflies were recorded in the British Isles.
The above images show (i) a female nectaring from Devil’s-bit scabious and (ii) a female at rest during the early evening sun.