Colias croceus, clouded yellow, is a small butterfly of the family Pieridae, the yellows and whites.
- Colias crocea f. mediterranea <small>(Stauder, 1913)</small>
All of these forms are usually considered synonyms of Colias croceus.
This species is primarily an immigrant to the UK, originating from southern Europe and northern Africa. In the UK they can be seen on the south coast almost every year in varying numbers, and regularly breed there. Occurrence in the rest of the UK varies considerably from year to year, to Belfast, Co. Down.
A truly migratory European butterfly, this species is famous for occasional mass migrations and subsequent breeding, which are often referred to in the United Kingdom as "clouded yellow years". Notable clouded yellow years include 1877, 1947, 1983, 1992, 1994, 1996 and 2000.
Habitat
These butterflies may live in any open area in the countryside, including downland, coastal cliffs and fields containing the caterpillar's host plants, at an elevation up to above sea level. The upperside of the wings is golden to orange yellow with a broad black margin on all four wings and a black spot near the centre forewing. Usually these butterflies settle with their wings closed, so the black margin of the uppersides of the wings is hard to see.
This species is similar to Colias myrmidone, Colias chrysotheme, Colias erate, Colias hyale, Colias alfacariensis, Colias caucasica, Colias aurorina.
Life cycle and larval host plants
Adults fly from March to October.