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OSB archive

What's eating the Small Tortoiseshell?

Pete Wilton

Owen Lewis of Oxford's Department of Zoology is investigating the decline of the Small Tortoiseshell butterfly (Aglais urticae).

Ahead of a report on his research project airing on BBC South's Inside Out [on 11 March], I asked Owen what might be behind the decline and how volunteers can help track the cuplrit...

OxSciBlog: How concerned should we be about the health of Britain's butterfly populations?
Owen Lewis: We should be very concerned. Butterfly populations respond rapidly to environmental changes and are a sensitive bellwether for what is happening to biodiversity more widely.

We have a fairly small butterfly fauna compared with mainland Europe – about 55 resident species – but many of these have declined dramatically over the last few decades. We know this because there is a long history of butterfly survey work in Britain, almost all of it done by amateurs (for example, the volunteers working for the charity Butterfly Conservation).

In general the specialised species have fared most badly: ones where the caterpillars have exacting requirements in terms of microclimate and food plants. Butterflies like the beautiful High Brown Fritillary used to occur in almost every wood in the southern half of Britain. Now they are confined to a few carefully managed localities in the south and west.

Fortunately, butterfly ecologists have worked out how to manage habitats to maintain the right conditions for these threatened species. Restoring them to their original distributions will be far more challenging: the modern agricultural landscape just isn’t suitable breeding habitat for most butterfly species, and the remaining islands of good habitat are scattered in a sea of hostile terrain.

OSB: Why is the Small Tortoiseshell decline a particular concern?
OL: Their caterpillars feed on stinging nettles and are (or at least were) one of the most widespread, abundant and widely-recognised butterfly species in the country. Most people will have seen the adult butterflies feeding on Buddleia bushes or Sedums in their gardens, or will remember keeping the spiky black caterpillars in a jar as children.

Unlike the habitat specialist species, we had thought that Small Tortoiseshell populations were holding up well. However, in the last 10 years there has been a huge slump in population sizes, particularly in the south of Britain. Overall, numbers in the south have been reduced by 50 per cent, but the situation is much worse in some areas, where a complete absence of sightings has been reported during the summers of 2007 and 2008.

The slump coincided with the arrival from the continent of a new species of parasitic fly called Sturmia bella in the late 1990s. The fly lays its eggs on nettle leaves, and the Small Tortoiseshell caterpillars consume the eggs unwittingly along with the leaves. The fly eggs hatch in the caterpillar’s gut, and the fly maggots then develop within the caterpillars, literally eating them alive and eventually killing them. The southern parts of Britain where Sturmia bella has been recorded are also the areas where the Small Tortoiseshell is faring most poorly. We want to work out if the parasite is to blame, or whether this is just a coincidence.

OSB: How can volunteers help in the study of this decline?
OL: We want to track the spread of Sturmia bella through the UK and establish how it is in affecting populations of the Small Tortoiseshell and its close relative the Peacock. We are asking volunteers to look out for the caterpillars on nettle plants this summer, and to collect some to grow up at home. Any butterflies that result can be released, and any flies or wasps that develop from them can be sent for us for identification, along with a complete datasheet with information on the numbers collected and where they were found. Full details on how to join in are available through Butterfly Conservation.

OSB: We seem to be seeing an increase in parasitism and disease across many insect populations, what factors may be to blame? Is climate change implicated?
OL: I’m not sure if parasitism and disease are on the rise in general, but climate change may certainly bring new threats. No-one knows how Sturmia bella got here from the continent, but many species are spreading northwards as the climate warms – and this includes new pests and diseases as well as their hosts.

Another theory about the decline in the Small Tortoiseshell is that it is linked directly to climatic changes: caterpillars seem to do less well in dry summers, like the ones experienced in the first few years of this century. It’ll be interesting to see if the wet summer of 2008 leads to a resurgence of Small Tortoiseshells in 2009. Let’s hope so!

Dr Owen Lewis is based at Oxford's Department of Zoology.

Image: Small Tortoiseshell butterfly. Credit: Jim Asher