Dear UK Butterfliers,
Distinguishing Pale and Berger's Clouded Yellows is never easy on the field, sometimes not at all. The best what you can do is what Peter mentioned. You have to net the specimen and carefully examine the upperside.
here are some keys what is essential to remember during the determination:
Please, don't forget that these keys must be used in combination, single keys may result false determination. It is very helpful to have the other species in your other hand but I know it's impossible.
Berger's CY: MALES ONLY The ground colour is very bright yellow. The forewing basal area has black scales only at the inner margin but not the costal margin. The whole basal area of the hindwing is scattered by black scales.
The apex of the forewing is less arrow than the PCY (a bit round shaped). The center spot of the hindwing is bright orange. The black band on the hindwing margin is not prominent.
Pale CY: MALES ONLY The The ground colour is usually paler yellow. The black scales cover nearly the whole basal area of the forewing and hindwing too. The apex of the forewing is visibly arrow. The center spot of the hindwing is paler orange. The black band on the hindwing margin is broad.
The females usually have the same features but not eves as clearly as on the males
![Smile :)](./images/smilies/icon_smile.gif)
From the underside, it is impossible to distinguish the two species, apart from the shape of the forewing apex, but this key is usually not sufficient!!!
It's difficult isn't it?
Best wishes
Safi