Apologies, I’m a little late in putting this one up!
Peter Martin writes:
If you have seen Small Tortoiseshell butterflies earlier in the year, they will have been the ones that have flown after winter hibernation and may have, therefore, been damaged and rather less colourful that those that you see in July.
The females will have laid 60 to 100 eggs on the tender leaves of Stinging Nettles, growing in sunny spots, early in May. These will have hatched out after about 10 to 14 days and you may have seen the dense silken webs that the black and yellow caterpillars spun around the leaf tips to live communally until they made their last skin change. Some of the beautiful gold or brown chrysalides have been found as far as 60 yards away from the nearest nettles, suspended from other plants or materials.
A few butterflies from this generation may emerge towards the end of June, but July is the month to watch out for them, You may come across the Small Tortoiseshell almost anywhere, with so many patches of Stinging Nettles being left in sunny spots by the “conservation-minded”.
Their upper wings have a mixture of orange, yellow and black, with dark edges usually showing some blue. The underside forewings are lighter than the hindwings, which provide a darker camouflage when the butterflies are resting with their wings closed. When open, their wingspan is about 45 mm (1.8 inches). You may see them posing with their wings open, whilst nectaring on Devil’s Bit Scabious, Field Scabious, Small Scabious or other flowers. The over-wintering generation, which flies from late August onwards, often nectar on Hemp Agrimony, Buddleia, Michaelmas Daisies and Sedum.
In Scotland, in the past, the Small Tortoiseshell has been known as the “Devil’s butterfly” or the “Witches butterfly”. People in other parts of the British Isles have called them the “Tortoiseshell fly” or the “Nettle Tortoiseshell”.
Small Tortoiseshell butterflies were once very common, but I have seen very few in the last few years. The reason for this is that a parasitic fly called Sturmia bella lays its eggs on nettle leaves and, unwittingly, these are consumed by the caterpillars, which are then eaten by the emerging fly grubs at this or the chrysalis stage.
Butterfly Conservation and Oxford University have started a research project to investigate whether Sturmia bella is the main reason for the butterfly’s decline. Initial results have revealed that, at least in the southern half of the UK, 60% of the caterpillars were killed wherever it was present.
Picture credit here.