Large Tortoiseshells – exciting news from Portland

Orange butterfly with dark markings, with its wings spread
Large Tortoiseshell. Photo: Mike Gibbons

Evidence of Large Tortoiseshell eggs & larvae has been found on Portland – the first time they have been detected wild in the UK for 70 years!

There has been a lot of speculation about the initial origin of Large Tortoiseshells on Portland and we have thought for some time now that they must be breeding there.

Just too many have been seen in the same places for days at a time for them all to be primary immigrants.

With Elm being reported to be the favoured foodplant, the woodland at Pennsylvania Castle/Church Ope Cove seemed likely to be a good place to host a colony.

Will Langdon, who has been following the Large Tortoiseshell trail on Twitter decided to go to Portland on 14/06/2020 to have a look around and succeeded handsomely in his quest with the discovery of the first obvious signs of larval damage.

He tried the Church Ope Cove area and after a few hours searching he eventually found an old larval web, at the eastern end of the ruins of St Andrew’s Church (SY 69698 71117).

The identification was a bit of a challenge for Will as it was about 5-6m up, and while it looked perfect for Large Tortoiseshells, he couldn’t see any larvae in it, or anywhere nearby on the tree.

He tried climbing the tree, but couldn’t get far enough out along the branch to check fully so decided fairly drastic measures were needed to confirm things. He got some shears, climbed the tree, and cut the branch off (after making absolutely sure from the ground there were no larvae!).

This enabled him to see the original egg batch, and 3 sets of shed larval skins (2nd, 3rd and 4th instar he thought). By his own count, there were about 175 eggs, 55 2nd instar skins, 45 3rd instar skins, and 5 4th instar ones. He took photos which can be found here of some of these, the tree he found them in and the larval damage.

From what he has been able to glean from the internet and books he felt that it appears to be a classic spot for larvae – a warm, little clearing, sheltered on all sides by taller trees and about 5-6m up on a tree facing S/SW.

He searched the trees as thoroughly as he could from the ground with binoculars and could see no sign of any larvae – what with the size of the larvae seen in Weymouth at the start of the month and the hot weather, he thought that they had already pupated.

The damage was fairly obvious from the ground, which made him think that he didn’t miss any other webs in the areas that he was able to search properly.

There are also a few more elms in gardens off Church Ope road to the north, and on the road heading through Wakeham and north out of Easton. They’re planted as street trees on the side of the road at the latter location, so a little exposed, but nice and sunny, so might be good and worth checking. 

Action needed now 

A report of a fresh adult Large Tortoiseshell on Portland in Perryfield Quarry (not too far from the larvae-stripped elm at Church Ope Cove) has been received so others could now be on the wing.

We would therefore now ask and encourage as many of you as possible to go out looking for these butterflies on Portland and help to chart what will hopefully be their spread.

Please do ensure that when you are out searching you follow all the Covid-19 guidelines regarding social distancing etc and do not trespass on private land.

Any seen from now ought to be spending their time building up food & energy for a long hibernation. Observations of what they are feeding on & any other activities observed would be very useful.

Please email details of all your sightings of Large Tortoiseshells to our Records Officer for Dorset, Bill Shreeves at [email protected] and also enter your sightings on the Dorset branch recording site at:

https://www.dorsetbutterflies.com/sightings/recent-sightings.php 

 

Editor’s Note:  

The Dorset Group of Butterfly Conservation would very much like record their thanks to Will Langdon for all the information he has given us and for allowing us to use this information and his photos in this article.  Without it we would not have been able to pass this information to you.

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