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Small Tortoiseshell

Photograph by: 
Peter Orchard

A familiar butterfly species but with very variable population levels year on year


 

  • Small Tortoiseshell: ups and downs

    Post date: Wednesday, 8 July, 2015 - 00:00

    The population levels of the small tortoiseshell (Aglais urticae) seem to vary year on year, almost cyclical. I can clearly remember in 2002 counting fifteen on ice plants in our garden at one go, all jostling for space and a chance to get at the nectar. However, by 2012 I wrote that I was concerned by the almost non-existence of small tortoiseshells anywhere. Last year and this they seem to me to be one of our most frequently seen butterfly species again.

    The reason for these ups and downs in numbers seems to be something of a mystery and Oxford University Zoology department are investigating what the reason(s) might be. One theory is that it is linked to the arrival from the continent of a small parasitic fly, Sturmia bella, in the late 1990's. The small tortoiseshell has a close relationship with the common nettle (hence the 'urticae' of the scientific name) and its caterpillars thrive on them. The fly lays its eggs on nettle leaves and the caterpillars consume them and the parasite then eats the inside of the caterpillar. This is now the most frequently recorded parasite of small tortoiseshell caterpillars killing 60% of them where present.

    It may be that as small tortoiseshell adult numbers fall because of the parasitic fly there are less hosts for the fly and so the fly numbers fall allowing more caterpillars to survive to adulthood and so the process continues.


     

  • Small Tortoiseshell in Dorset: what your tweets tell us ...

    Post date: Saturday, 4 May, 2019 - 19:08

    The small tortoiseshell is another butterfly species whose fortunes are closely linked to those of its nemesis, in this case a parasitic fly Sturmia bella. In good population years for the butterfly the parasitic fly has ample caterpillars to lay its eggs in to and so the fly prospers at the expense of the butterfly and so numbers of the small tortoiseshell fall meaning less caterpillars for Sturmia bella and so its population level falls allowing more butterflies to emerge and so it goes on. That said, the population of small tortoiseshells seems to have fallen in recent years and it is far less common than it once was; well that is so in our garden anyway and this is a butterfly very much at home in gardens where it is quite comfortable feeding on various cultivated varieties of daisies and other flowers.

    As a butterfly that hibernates they can be seen at almost anytime of the year as they will emerge on milder days in winter. Full emergence tends to come in late March and early April and then during May there is a hiatus whilst the second brood is awaited and they are then seen from June onwards. The weekly sightings in the Nature of Dorset database show this very well. There are a few reports from week 8 at the end of February until week 11 and then there is a surge of reports in week 12 at the end on March through until week 16 at the end of April. Following this there is a gap of seven weeks and then reports start flowing again from week 24 at the end of June on into the autumn. There are then odd reports during the late autumn and winter months.

    A good number of sites have reports of small tortoiseshell but interestingly in most cases just the one record. Only Portland and Radipole in Weymouth stand out with four reports and Lyme Regis and the wider Weymouth area three each. 


     

Common Name Small Tortoiseshell
Scientific Name Aglais urticae
Species Group Nymphalid Butterflies Admirals and Fritillaries
Status
Interest Level
1
Visabile
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Identification Notes
Primary Habitat
Preferred Environment
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Additional Identification Notes
Similar Species

The records for this species have been organised into reports, charts, maps and photos. Click a pic below to see the detail:

Notebook Distribution Map Sites List Some Charts Some Photographs Recent Records Guidance Notes

To see related species click here: 

Nymphalid Butterflies Admirals and Fritillaries