Parasite implicated in decline of small tortoiseshell butterfly
18 April 2011, by Sara Coelho
Wild populations of the small tortoiseshell butterfly fell sharply between 2003 and 2008. The species even disappeared in some parts of Britain. Now, with the help of volunteers, this has been attributed, at least in part, to the recent introduction of a parasitic fly that preys on the species.
Small Tortoiseshell butterfly.
The small tortoiseshell (Aglais urtica) is not just a butterfly. 'This is an iconic species in the UK, a familiar and much-loved garden butterfly,' says biologist Owen Lewis, from the University of Oxford. 'Butterfly enthusiasts were dismayed by its sudden scarcity, and anxious to know the cause.'
A possible culprit was soon identified: the parasitic fly Sturmia bella, known to kill small tortoiseshell caterpillars in continental Europe and recently introduced in Britain.
Lewis and his Oxford colleagues joined forces with scientists from Butterfly Conservation and the Centre for Ecology & Hydrology to work out if this parasite is indeed one of the reasons behind the butterfly's dramatic decline.
Sturmia bella looks like a hairy housefly and has a cunning way of infecting its prey. The adults lay their eggs on the nettle leaves that small tortoiseshell caterpillars eat. The oblivious caterpillars eat the eggs, leaving one or two fly maggots free to hatch inside their bodies, feeding on internal tissue but not harming the host.
The caterpillars mature naturally until their final moult. But 'at exactly this moment, before the pupa has hardened, the parasitoid larvae eat their way out, killing the host, and abseil down to the ground on a silk-like thread,' explains Lewis.
The team enrolled the help of amateur lepidopterists from all over Britain to compile data for analysis during the summers of 2008 and 2009. 'The volunteers were brilliant, and we could never have collected the dataset we needed without them,' says Lewis. 'Many were incredibly dedicated and rigorous about documenting their samples and observations and would make excellent research scientists!'
The volunteers collected 5008 small tortoiseshell and peacock butterfly caterpillars in 195 batches and reared them in captivity to see how many were affected by parasitoids. When they found one that was playing host to a parasitoid, they sent it back to the lab for identification. The peacock butterfly (Inachis io) was added as a control species; 'it has a similar biology, and is also attacked by Sturmia bella, but it has not declined in the same way,' says Lewis.
The results, published in Ecological Entomology, show that Sturmia bella was present in 26 per cent of the small tortoiseshell and 15 per cent of the peacock butterfly batches. The parasitic fly killed more small tortoiseshell butterflies than any other butterfly parasitoid, and survival was between 25 and 48 per cent lower when it was present.
'We found that where Sturmia bella occurred, caterpillars were more likely to die from parasitism than where it was absent,' says Lewis. 'So we know that it isn't simply substituting for and replacing natural parasitoids, and that more caterpillars are dying now that it has invaded the UK.'
But the jury is still out on whether the fly is the main reason for the 2003-2008 population crash. 'It seems certain that other factors are also involved and small tortoiseshells are known to have inherently variable populations,' Lewis says. General drought conditions, which reduce the suitability of food plants, are another possible explanation.
'The good news is that since we carried out our study, numbers of small tortoiseshells have recovered to some extent,' he concludes.
Sofia Gripenberg, Nia Hamer, Tom Brereton, David B. Roy and Owen T. Lewis. A novel parasitoid and a declining butterfly: cause or coincidence? Ecological Entomology (2011), doi: 10.1111/j.1365-2311.2011.01269.x
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