The clouded yellow is a relatively common migrant species arriving in Dorset in variable numbers each year and in some years there can be a major influx with clouded yellow being seen just about everywhere along the Dorset coast.. Most arrive in late summer and autumn but it is not unusual to see them much earlier in the year. They originate from north Africa and southern Europe and it is amazing that these relatively small insects can travel such great distances and even cross wide expanses of water to get here. They do lay eggs here and ones laid early in the year may well hatch and add to the immigrant population but the later ones rarely survive the English winter and so there is no real British population.
The weekly reports show clouded yellow being seen in Dorset from as early as week 8 in February and small numbers are seen regularly from week 12 at the end of March right through until week 38 in mid September and then the number of reports climb and reach higher levels for about a six week period before tailing off. September and October are undoubtedly their peak season here.
So far there are reports from almost fifty different sites in Dorset with the coast from Hengistbury Head, through the undercliffs at Boscombe and Bournemouth and then around the Purbeck coast to St Aldhelm's Head seeing the most. Certainly, the bulk of reports come from coastal, or near coastal, locations. You can encounter a clouded yellow almost anywhere along the Dorset coast on fine days in October.