During the Second World War British military observers, scanning the English Channel for signs of enemy activity, were alarmed to see a huge yellow cloud making its way towards the Kent coastline. As the apparition approached it became clear that it was a huge swarm of Clouded Yellow butterflies. These powerful flying mustard coloured insects with black wingtips are one of several species that migrate from Continental Europe to the UK each Summer. The number of migrants varies greatly from year to year, depending on the species and factors such as the weather. Most of our UK butterfly species are not migrants and some exist in very local colonies, such as in specific meadows, hedgerows, trees or on grassy hillsides.
Dry hot weather in Spring in North Africa and the Mediterranean and southerly winds can drive successive generations of migratory butterflies northwards as they search for sources of nectar and food plants for their caterpillars. One fairly common migrant to the UK is the Painted Lady and in most years they can be seen on the wing in the Marlow area from late May to September, and even later in some years. There have been some notable large scale invasions by this large powerful flying butterfly. Last year the east coast of the UK saw huge numbers and there is memorable video footage from the Farne Islands, just off the Northumberland Coast, of large clouds of these insects. In most years the numbers in our area are fairly moderate but sometimes they can be a common sight in our gardens, particularly on buddleia flowers. Recent scientific research has revealed their remarkable life cycle which begins in Spring in Morocco in the foothills of the Atlas Mountains. The first brood of butterflies of the year has a strong instinct to fly northwards. Over three or four generations they reach Northern France and then make the hazardous journey across the English Channel. Such is their urge to keep travelling that even when they make landfall on the South Coast they continue on their journey, frequently making it all the way to the north of Scotland. The Painted Ladies arriving in the UK breed during the Summer, the caterpillars feeding on thistles and then undergo the usual metamorphosis into butterflies. Remarkably when the conditions are favourable these insects then make the return journey southwards heading back to warmer climes as the weather in the UK cools in the Autumn.
Most migrants, particularly the rare ones such as the exquisite Camberwell Beauty, and the Large Tortoiseshell (once native to the UK but now extinct) will be seen predominantly in coastal southern counties such as Kent. Other migrants such as the Red Admiral and the Large White will arrive to swell the native UK population and are common sights in the Marlow area. So the next time you see a Painted Lady or Red Admiral, especially if it is looking a bit faded or battered, it might be getting a well earned drink and rest after an epic journey!