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.
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