The Recovery Of The Small Tortoiseshell Butterfly

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Top down view of a Small Tortoiseshell Butterfly

Peter Martin writes:

During the autumn of 2013, the biggest surprise was the number of Small Tortoiseshell butterflies that I recorded in this part of North-east Hampshire. Please see pictures above and below.

This butterfly, which used to be seen in fairly large numbers, had been badly affected in recent years by a parasitic fly, Sturmia bella, which arrived here from the Continent due to global warming.

The fly lays its tiny eggs on the leaves of the food-plant which are then consumed by the Small Tortoiseshell caterpillars when munching the stinging nettle leaves. Grubs emerge from the eggs inside the caterpillars and start to eat them, leaving the vital organs ‘til last.

Due to attacks by this parasite, I only saw 8 Small Tortoiseshells in 2008, 11 in 2010, 8 in 2011 and 5 in 2012. In 2013, however, I recorded 88. A National Garden Survey for the whole of the UK showed an increase in numbers compared with 2012 of 388%. Locally, we have bettered this with over 1,600% improvement.

Was this due to weather or other conditions making 2013 a bad year for the parasite or could there be a long-term benefit? Only time will tell, but it would certainly be nice to see more of these beautiful insects in future years.

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Side view of a Small Tortoiseshell Butterfly

Picture credits: Wikipedia.

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