Late summer butterflies

Red Admiral, Painted Lady, Small Tortoiseshell and Peacock (known collectively as vanessids) must be amongst our most familiar butterflies and can regularly be seen taking nectar from late flowering summer plants such as marsh thistle, devil’s-bit scabious and buddleja
All four species occur in the Outer Hebrides. Red Admiral and Small Tortoiseshell are the ones you are most likely to see. Painted Lady is a long distant migrant than can occur in their thousands in good years but is hardly recorded at all in other years. The Peacock is less predictable but perhaps under recorded here and is maybe a recent colonist or re-colonist. For all of them, the nectar they consume in late summer and early autumn is vitally important in helping them survive the winter. It’s all about building reserves of body fat.
Two of our species over-winter here as adults and they need those reserves to survive their hibernation during the cold months. As temperatures drop in autumn Small Tortoiseshell and Peacock butterflies seek out sheltered hibernation sites in woodpiles, sheds and other outbuildings and even inside our houses. The reserves laid down in late summer keep them ticking over until temperatures start to rise again in spring.
For the other two species, Red Admiral and Painted Lady, the reserves are for more immediate use. Both species are long distance migrants and they need energy for their return migrations. The earliest sightings of these species are from late march and early April. These individuals may have been carried here by strong winds directly from overwintering sites in North Africa. Later sightings in June and July are probably from intermediate generations that bred somewhere in Europe. The arrival, here, of most of these butterflies in spring is an example of a “multi-generational long distance migration”. They may have had one, two or three generations en route before they get to us.
What happens to those Red Admiral and Painted Lady butterflies we see in August? It used to be thought that many of these butterflies, seen in the UK in late summer, simply died. For them, it was a one-way trip. We now know differently. Entomologists using vertical looking radar have been able to look at the movements of insects to and from the UK. This type of radar is very sensitive and has been used to look at movement of plant pests such as aphids as well as larger insects. Radar studies on the movement of butterflies such as the Painted Lady and Red Admiral have given us the real picture.
In 2009, one of the biggest Painted Lady migrations of recent years, it is estimated that 11 million of them arrived in the UK. Some of these made their way to the Outer Hebrides. Over the UK as a whole they had a successful breeding year and an estimated 22million left the UK in autumn. Flying at heights between 200m and 1200m they headed for North Africa. Winds at these heights are 4-8 times faster than at ground level and butterflies can reach speeds of 100kph (approx. 60mph) and accomplish flights of 1000km with ease. If you are lucky enough to spot a Painted Lady nectaring on a marsh thistle in late summer, just think, it’s next stop for food might be in North Africa.
Robin Sutton