11 March 2009

Volunteers are urgently needed to help discover why populations of the Small Tortoiseshell butterfly are plummeting.

The Small Tortoiseshell was once a common sight in gardens throughout the UK. Over recent years it has a suffered dramatic decline, particularly in South East England where its numbers are down by over 80 per cent since 1990.

Scientists are becoming increasingly worried that the parasitic fly Sturmia bella could be responsible. This newly-colonised parasitoid fly lays eggs on the nettles that the Small Tortoiseshell's caterpillars feed on. It is suspected that the caterpillars are consuming the eggs which then hatch and develop inside them as they reach the cocoon stage, killing the host.

Butterfly Conservation is working with the Department of Zoology at Oxford University to investigate the severity of the problem. Dr Martin Warren explains why your help is needed: "In order to prove the link between Sturmia bella and Small Tortoiseshell mortality we need to look at trends in butterfly population and sizes across the country. We need volunteers who can collect and rear Small Tortoisehell caterpillars, then report back about how many are dying from parasitism. We are deeply concerned about the decline of the Small Tortoiseshell and are desperate to find the causes".

If you can help, please visit Oxford University's website for full instructions on collecting and rearing Small Tortoiseshell caterpillars and details of how to report your findings.

For more information on the Small Tortoiseshell, see our species page