Baffling decline of the small tortoiseshell butterfly

The fact that I was delighted this week to find a trio of small tortoiseshells and three red admirals on one of a dozen buddleia I’ve planted in my garden shows how low our expectations have sunk.

My spectacles are not rose-tinted when I remember several dozen late-summer butterflies supping on buddleia to gain energy for their winter hibernation in very similar garden habitat when I was a boy three decades ago.

The small tortoiseshell was once a common garden butterfly but has declined by 73% since scientific monitoring began in the 1970s. Perhaps gardens have become tidier with fewer crucial nettles for their caterpillars. But nitrate pollution has helped nettles proliferate and more gardeners plant late-summer fuel like buddleia (cut them back hard in April so they flower later, and are more useful to the butterflies).

So scientists are baffled. It was thought that a parasitic fly called Sturmia bella, which arrived in Britain in 1998, was decimating small tortoiseshell caterpillars. More recent studies suggest its impact is not significant, and small tortoiseshell declines have been linked to drought-stricken nettles.

The small tortoiseshell unexpectedly bounced back in 2014 when there were six times as many as during the worst-ever butterfly summer of 2012.

But this isn’t a natural boom-and-bust, for numbers are still down on past decades. And this year has been disastrous. A warm winter and cold spring was deadly for butterflies like the small tortoiseshell which hibernate in their adult form.

A brilliant August has helped the few tortoiseshells on the wing and Butterfly Conservation wants us to record sightings at www.gardenbutterflysurvey.org. When scientists are stumped, our only hope is to gather more data.