In 2009 a colossal migration took place. A wet winter, resulting in lush plant growth, caused a vast population to build up in the Sousse Valley in Morocco. Immediately after emerging in March, the butterflies began migrating. Most appear to have headed directly north into Spain, while others headed east, crossing the Mediterranean from Tunisia to Italy.

By mid May a vast 850 mile wide migratory front had built up stretching from Spain to Poland. For several days their passage to Britain was impeded by a low pressure system. Then in late May a change of weather brought warm sunny conditions to the UK, and aided by southerly winds millions of Painted Ladies crossed the channel. The influx included tens of thousands of faded individuals, and their offspring - lesser numbers of very fresh specimens that had bred and emerged in southern Europe.

Observers from all over England reported almost unprecedented numbers, e.g. 80 seen in half an hour at Newhaven on 25th May, 96 in less than 10 minutes at Granborough, Bucks; and a colossal 800 per hour at Strumpshaw Fen in Norfolk ! On 30th May, as the migration subsided, a timed census by observers at 180 sites from Cornwall to Skye produced a count of 20,000 butterflies. Using this data Professor Chris Thomas estimated that at least 15 million Painted Ladies were in Britain during the 2 hours when the census took place.

On arrival in Britain the butterflies soon began laying eggs, and before long huge numbers of Painted Lady caterpillars were being reported. In two fields at Port Isaac in Cornwall, observers discovered an estimated half a million caterpillars. Another vast breeding site was discovered near Ditchling in Sussex, where between 250,000-300,000 adults were estimated to have emerged on a 200x300 metres patch of thistle-covered hillside !