Clouded Yellow

Colias croceus — the clouded yellow — is one of the most familiar migratory butterflies in Europe and a flagship species of European Lepidoptera migration biology. The species is part of family Pieridae (the whites and yellows, including cabbage whites, brimstones, and other familiar garden butterflies). Adults are 5-6 cm wingspan with bright orange-yellow wings carrying broad black borders (males) or black borders with cream marginal spots (females); the underside is dramatically different — pale lemon-yellow with small silvery and reddish spots and one prominent silver eye-spot in a pink ring on each hindwing. The species' annual migration is multi-generational, similar to the painted lady (Vanessa cardui) and the silver Y moth (Autographa gamma): northbound migrants from southern overwintering grounds (Mediterranean basin, North Africa) arrive in temperate Europe in spring, breed, and produce 1-3 northern generations through summer; autumn southbound migrants return to Mediterranean overwintering areas. No individual completes the round trip — the migration cycle requires 3-5 successive generations annually. Most years see modest clouded yellow numbers in northern Europe; occasional 'clouded yellow years' produce massive northbound waves with hundreds of millions of butterflies arriving in successive pulses. The most famous recent clouded yellow year was 2000 in the British Isles, when billions of butterflies arrived in multiple waves and the species was abundant from southern England to Scotland and into Iceland — the largest documented clouded yellow migration in modern UK history. Caterpillars feed on Trifolium clovers, lucerne (alfalfa), bird's-foot trefoil, and other legumes, making the species a significant pollinator of agricultural meadows. The species is particularly affected by climate change: northern populations are establishing earlier in spring and overwintering at higher latitudes than historically.