We have had two reports of Large Tortoiseshells this year. This is a butterfly which, though common in Southern England in Victorian times, is now thought to be extinct in the UK.The first one was seen by Anthony and Margaret Dobie on 20 April, at the bottom of the downs between Ulwell and Corfe Castle, to the north of Knitson. The second was spotted by Bob Ford on 8 May at the Warmwell gravel pits, near Crossways.
Both sightings have been verified as definitely a Large, not a Small Tortoiseshell. The key factors are the lack of the bright silver white mark at the tip of the wing (which would indicate a Small Tortoiseshell) and the four dots and blotches on the upper wings instead of the two small dots and a blotch of the Small Tortoiseshell. Also the studs along the bottom of the lower wing are purple instead of the bright blue of the Small Tortoiseshell. The Large Tortoiseshell is, as its name suggests, larger than the Small Tortoiseshell, with the female having a wingspan of up to 75mm, while the Small comes in at 56mm maximum.
Our Records Officer, Bill Shreeves, has been keeping track of Large Tortoiseshell sightings in Dorset over the years.
2006-2008 were the great years for Large Tortoiseshell. In 2005 there was the famous one in the bedroom of a Dorset Wildlife Trust Ranger on Brownsea Island on 15th July. In 2006 there were five sightings, mostly in Purbeck and along the south coast of the county, including the famous ‘beer can’ Large Tortoiseshell photographed in a Worth Matravers garden. The 19 sightings in 2007 were almost all in Purbeck or further along on the south coast from 28 January ([Portland) to 5 August (Flower’s Barrow). The seven in 2008 ranged from 27 January to 23 May, with one later record for 16 July; sightings ranged from Bridport to Portland and the Swanage area. Numbers since then have fallen away.
The origin of these butterflies is unknown. Weather back-tracking was consistent with migration from Europe for only a few. Several were breeding releases. It is possible some may have come from natural breeding in Dorset but no larvae have ever been found. There were at one time many sightings on the Isle of Wight.
The main foodplant for the caterpillar of this species is elm, with aspen, birch, poplar and willow also used. The Small Tortoiseshell uses stinging nettles.
If you see one, please try to get a photograph of it, and send your sighting in via our Online Recording Form.