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.
Surrey butterfly enthusiast Tony Hoare said: "They used to be the most common butterfly in my garden. Now they have virtually disappeared."
The fact that losses are greater in South East England suggests that the problem may be linked to climate change.
Another suspect is the tiny parasitoid fly Sturmia bella,first noted in Britain in 1999. Its arrival from Southern Europe is either the result of climate change or an unwitting introduction from caterpillars brought in from Europe.
Butterfly Conservation has launched a research project with Oxford University’s Department of Zoology to determine the extent, if any, that the fly is to blame.
Sturmia bellaeggs are increasingly found 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.
Eventually little flies burst out of the cocoon, slightly reminiscent of the film Alien.
Dr Martin Warren, Chief Executive of Butterfly Conservation, said: "We are deeply concerned about the decline of the Small Tortoiseshell and are desperate to find the causes".
He added: "Butterflies are great indicators, telling us much about climate change and the state of the environment. However, there are times like this when we need experts to interpret what they are telling us. In this case there may be a serious message about unexpected consequences of global warming."
Hundreds of Butterfly Conservation volunteers are expected to help the Oxford research team by collecting batches of Small Tortoiseshell caterpillars.
Dr Warren said: "It is only after we get to the bottom of this that we can try to devise a strategy to save the Small Tortoiseshell."