The Clouded Yellow is an unpredictable annual migrant to Britain arriving from southern Europe in spring and dispersing northwards, occasionally occurring in huge numbers - about once every decade. It may be expected in many habitats where there is an abundance of nectar sources.
It has rich deep-yellow wings with solid black borders, which, on the female, are dotted with yellow spots. There is a pale female form helice. It almost always rests and feeds with its wings closed.
The first arrivals are usually seen in May and June, with further immigrations possible between July and September. Numbers are supplemented by locally bred butterflies as it has a very rapid breeding cycle. It may be recorded until October.
Food plants are clovers, Lucerne, and Common Bird's-foot-trefoil.
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The rich golden yellow of the Clouded Yellow and bold black margins on the upper sides make it unlike any other British butterfly.
The pale "Helice" form of the Clouded Yellow could be confused with a Brimstone. All forms of Clouded Yellow and Brimstone rest with their wings closed, hiding the bold black margins. The key differences being the shape of the rear edge of the wings and the shade of yellow, Brimstone having a slightly greenish tint, particularly the females.