It looks as if this year is a good year for seeing another beautiful butterfly that migrates to Britain from the Mediterranean area. Back in May I wrote about Painted Ladies, now there are plenty of Clouded Yellows around too.
Migrants from North Africa and Southern Europe, Clouded Yellows usually appear in small numbers but every so often conditions come together for large numbers to make the journey, so-called Clouded Yellow years.
The vivid yellow shows up when they fly which allows them to stand out even at some distance. You might get a chance to see that the upper wings of the males have dark borders, probably the origin of the clouded name. Both males and females close their wings when they land and this hides the dark edges but reveals a prominent white spot enclosed in a dark ring, plus several brown blotches..
Some of the female Clouded Yellows seen on Alma Field might be confused for Large or Cabbage Whites when in flight because they have white upper wings and pale undersides to the forewings. This is the Helice variety, about a tenth of females are Helice, it is unclear why.
Clouded Yellows can be seen across England but they tend to congregate along the south coast, particularly in areas of calcareous grassland where they can find the right flowers for the adults and the right food plants for the caterpillars.
Even with the fluctuations, the numbers of Clouded Yellows seen has risen steeply in recent decades. There could be two reasons for this, a warming climate gives them suitable migration conditions and, unlike many native butterfly species, they can take advantage of modern agriculture. The principal food plant of the caterpillars is cultivated clover varieties, although they do feed on other legumes including common bird’s-foot trefoil, of which there is plenty on Alma Field.
There are often two generations in a hot summer, the first eggs laid in late May with the adults emerging to breed in August. If it is available, they will lay eggs on crops of Lucerne but, as this will be harvested as animal feed, it might not be a good choice for the second brood.
The caterpillars are camouflaged green with a pale yellow stripe along each side. When mature, the caterpillars attach themselves to a plant stem to pupate. The hind end is attached with a silk pad and the pupa is held upright with a silk girdle. The adult emerges in one to two weeks depending on the temperature.
There is some evidence that second generation adults migrate back south. Any that don’t, and any late caterpillars and pupae are unlikely to survive the winter, at present. As the climate of Southern England warms we may find these beautiful butterflies becoming permanent residents.
Ed Dolphin
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