Small Tortoiseshell Does a Mysterious U-Turn

Small tortoiseshell butterfly. Photo: Jorg Hempel, Wikipedia

The number of small tortoiseshell butterflies has increased dramatically at New Ferry Butterfly Park, from being in the doldrums during 2005-2012. This reflects the national trend, picked up in ongoing surveys by Butterfly Conservation, including their high profile Big Butterfly Count. This was carried out for 15 minutes at various locations 19th July - 10th August 2014. This year the big count involved nearly 45,000 people who spotted 560,000 butterflies, a tremendous recording effort. Look out for next year’s count and join in!

Richard Fox, surveys manager at Butterfly Conservation, said “The small tortoiseshell had a good year in 2013 and this seems to have acted as a spring board for the species enabling it to increase massively again this summer.”

Number of Small Tortoiseshell Butterflies Recorded Annually at New Ferry Butterfly Park 2003-2014

The small tortoiseshell butterfly is classed as a habitat generalist, that is, it is not too exacting in its habitat requirements. It is a strong flyer and is able to colonise new areas of suitable habitat. Their larval food plant, the common nettle, is abundant and expanding due to human activity. It is a mystery why the small tortoiseshell declined previously.

One factor was probably the arrival in UK in 1998 of the parasitic fly Sturmia bella, which looks like a hairy looking house fly. Sturmia bella has been able to survive recent milder climatic conditions and has had a role in reducing the survival rate of caterpillars.

Sturmia bella lays its eggs on nettles leaves and the grazing caterpillars inadvertently consume the parasite’s eggs. These eggs hatch out inside the caterpillars’ bodies and grow inside, avoiding the vital organs until they finally kill the caterpillar and emerge just before the caterpillar is due to pupate. Survival rate of infected caterpillars is reduced by 25% to 48%. Perhaps in the last few years there has been a re-balancing in the parasite/host populations; if the parasites kill too many potential hosts they reduce their future life chances too.

However, the small tortoiseshell has been declining further north in Britain where Sturmia bella has yet to colonise. Small tortoiseshell butterfly populations on the European continent, where Sturmia bella has been endemic for a long time, have also diminished further, so there are other unknown contributing factors to this recent decline.

It is difficult to tease out what factors affect a butterfly population with so many factors involved. These include weather at particularly critical times, habitat quality, influx of butterflies from elsewhere, predator and parasite population cycles.

All we can do at the Park to help the species is to provide plenty of large stands of nettles in sunny positions, which the caterpillars can feed on, and a host of seasonal nectar sources for the adult butterflies to use, and hope they take advantage of these. It is wonderful to see a recovery in this popular and readily recognisable butterfly.

Thanks go to all the volunteers who have worked at the park to increase nectar and larval food plants, and those who have added to the collection of butterfly records. These let us see what is happening to each butterfly species at the park over the long term.

Paul Loughnane