Great British Lepidoptera

Identification-

Pale and Bergers Clouded Yellows are best distinguished by the shape of the leading edge of their forewings. This is curved in the former and straight in the latter, giving the Pale Clouded Yellow's wings an altogether more pointed look.

In addition, male Berger's Clouded Yellows have a slightly warmer and more intense yellow-ground colour, the orange spot on the underside is brighter, and the dark markings are less distinct.

However, all these characteristics vary within the two species.

Although the shape of the forewing is a fairly good guide, for 100% positive identification the butterflies would have to be taken as specimens and their detailed anatomy examined. Today, few naturalists have either the ability or inclination to do this, and so the identity of vagrants is often difficult to establish.

Another way around the problem of identification is to breed the two species in captivity, or, preferably, to watch egg-laying females in the wild. The Pale Clouded Yellow lays on Lucerne (Medicago sativa) and probably other Medicks and Meliots (Melilotus spp), as well as, reputedly on Clovers (Trifolium spp), whereas Berger's Clouded Yellow restricts its eggs to Horseshoe Vetch (Hippocrepis comosa). Both lay the typical long, bottle-shaped eggs of all Whites, and these turn orange or pink after a few days.

It is the caterpillar, however, that is really distinctive, and that confirms that these two very similar butterflies are in reality two different species. The caterpillar of the Berger's Clouded Yellow is very different, camouflaging itself to appear like the foodplant; Horsehore Vetch. The cylindrical green caterpillar has bright yellow lines down its body and along both flanks, each punctuated by a pair of black blobs in every segment either side of each line.

The chrysalis of all three Clouded Yellows that visit our shores have extremely similar, pale green colouring that are almost impossible to distinguish and are unlikely to be found in the wild.

The ecology and behaviour of the Pale and Berger's Clouded Yellow is also very dissimilar. The Berger's Clouded Yellow is more likely to be seen on southern chalk downs, where Horseshoe Vetch is abundant, whereas the Pale Clouded Yellow generally settles in Clover and Lucerne fields.

Of the two, the Pale Clouded Yellow is more docile, and spends much of the day hiding or feeding on Lucerne, making strong zig-zag flights only in sunshine.