Excitement builds as butterfly enthusiasts across the UK await the imminent arrival of a new species.
When it comes to the number of butterfly species, let's be honest, the UK is pretty impoverished when we compare ourselves to other European countries. Our species list of 59 resident and regular migrants is only bigger than Cyprus (53), Malta (41), Ireland (34) and Iceland (6), and is way behind the top of the league Italy which is home to an impressive 252 species.
So it's not surprising that the possibility of a 60th species is causing great excitement amongst butterfly watchers. Of course, if you are more of a ‘moth-er’, the addition of a new species to the British list is becoming a regular occurrence. At the last count, 35 moth species (excluding adventives), many previously unrecorded, have become established in the UK since 2000.
The butterfly species causing all the excitement is not one of our scarce migrants, like the Camberwell Beauty, or a butterfly like the Large Copper that went extinct decades ago. As a close relative of the Small White, you might think of it having humbler origins, but the Southern Small White has been doing something quite remarkable.
Until recently, and as its name suggests, this butterfly was confined to southern and especially south-eastern Europe, but it is now spreading rapidly in a north-westerly direction, at the rate over 100 km per year. It was first found north of the Alps in France and in Germany in 2008 and has since gradually extended its range and was first sighted in the southern Netherlands in 2015. In 2019 it was reliably recorded near Calais in France. So there is only the matter of the 22 miles of English Channel to cross before it arrives on our shores!
Of course, it may already be here, just not noticed amongst its cousins. So the race is on to be the first person to record this butterfly in the UK.
Identification is not easy, especially on the wing. This is made even more difficult by the fact there are differences between the sexes and between generations. Males and spring broods have fewer black markings than females and later generations.
The most obvious identification features are 1) the black patch at the wing apex extends as far down the outer margin as it does along the costa and 2) the outer edge of the main black forewing spot is often straight or concave.
These two features are best viewed in comparison to our other three similar whites, the Large, Small and Green-veined White, as in this series of photographs kindly provided by Kars Veling and Chris van Swaay of De Vlinderstichting, a leading wildlife charity in the Netherlands:
So where might you find a Southern Small White? South-east and eastern England are likely to be the best place to look, especially along coastal paths, but migrating insects can turn up just about anywhere. Remember to follow current advice to limit the spread of coronavirus, but it is well worth checking the ‘small whites’ that turn up in your garden.
Southern Small White caterpillars feed on candytuft. Whilst the native Wild Candytuft is more or less restricted to the Chilterns, where it can be found on bare, open ground on chalk, other species are grown by gardeners, particularly in rock gardens, including Iberis sempervirens, one of the butterfly’s known foodplants on the continent. In the wild, the Southern Small White is associated with dry stony grassland, but rock gardens with candytuft could provide an alternative habitat. In the Netherlands, the Southern Small White is also known to breed on flowering cultivated Rocket Eruca vesicaria in abandoned vegetable gardens and on Perennial Wall-rocket Diplotaxis tenuifolia.
On the continent, the Southern Small White has several generations each year and can be on the wing between March and October. In recently colonised areas, only a few individuals are usually observed in late summer or early autumn, but by the next summer, the butterfly is usually well established and quite widespread. So we recommend checking your ‘small whites’ throughout the season.
If you think you have seen one, do please send a photograph, as well as the date and location to @email. Photographs of both upper and underside are preferable because this butterfly can also be confused with the Small and Green-veined White.
Matt Berry of Greenwings has kindly offered a prize for the first verified UK record of the Southern Small White, to mark this once in a lifetime event!
Dr Sam Ellis
Follow Sam on Twitter: @SamEllisBC