The Large Tortoiseshell butterfly which was thought extinct in the UK is making a comeback in Dorset.
Few have been recorded since the 1970s and it was thought this species had been lost to the UK.
Since 2018, however, there have been sightings on Portland every year including this spring. These will most likely be the adult butterflies which have overwintered and are now coming out to breed and lay their eggs. The eggs are laid on branches at the top of tall elm trees and will themselves hatch into caterpillars, then pupate and subsequently emerge as adults later in the summer.
The caterpillars eat the elm leaves leaving just the central stalk which attaches the leaf to the tree. Dutch Elm disease has been responsible for the decimation of elms in the UK, and hence also this species. However, there is an elm grove behind Church Ope Cove which appears to be resistant to disease. Evidence of old caterpillar skins and egg cases were seen there in 2020 by Will Langdon.
There are some unconfirmed reports that this population originally came from a captive breed and release in 2018. However, this butterfly has also started to be seen in other areas along the south and east coasts of the UK, as well as Belgium and The Netherlands – suggesting there is also probably migration in from the continent.
Don’t miss Will Langdon’s superb article about how he found breeding proof and his climbing exploits on the elms in the next issue of the Dorset branch newsletter, which will be out soon.