Why is the Small Tortoiseshell D

Many people have noticed dramatically reduced numbers of small tortoiseshell butterflies (Aglais urticae) in the UK over the last few years. Butterfly Conservation has become concerned that a newly-colonised parasitoid fly, Sturmia bella, may be causing widespread mortality to small tortoiseshell caterpillars in southern Britain, providing a possible explanation for the butterfly's decline. To investigate the effects of Sturmia bella, We are hoping to collect data from as many localities as possible across the UK, and we are linking information on the frequency of parasitism from Sturmia bella and other parasitoids to information from the UK Butterfly Monitoring Scheme on trends in butterfly population sizes. This work is being carried out in collaboration with Butterfly Conservation.

New! Click here to read our 5-page report on the results so far, thanks to the efforts of our 2008 volunteers.

New! Click here to watch a 7-minute BBC 'Inside Out' Film on the Small Tortoiseshell research, featuring Martin Warren and Owen Lewis

You may also be interested in newspaper articles in the Independent (29 March 2008) and in the Guardian (27 April 2009) highlighting the decline of the Small Tortoiseshell butterfly.

Small Tortoiseshells have declined by approximately 50% in the past decade
Sturmia bella now has a wide distribution in southern Britain, at least as far north as Merseyside and Doncaster.
Small Tortoiseshells have declined most dramatically in southern Britain but has been declining in northern England and Scotland too.
Sturmia bella is now the major parasitoid for Small Tortoiseshells (but not Peacocks)
Sturmia bella is mostly recorded from caterpillars collected in late summer
Sturmia bella is an additional cause of mortality to the larval batches it parasitizes and may now be competing with the native parasitoids.
The overwintering strategy of Sturmia bella in the UK is still unknown.
There is no evidence that local decline rates reflect the presence of Sturmia bella, but sample sizes are small

A fuller summary of the results is available here.

We are asking volunteers to collect batches of wild larvae of Small Tortoiseshell and Peacock butterflies this summer, and to rear them to determine how many die from parasitism. Emerging butterflies can be released into the wild, and any emerging parasitoid pupae or adults should be sent to us for identification. Please follow the links below for further information on taking part (Word documents):