Here’s my Lepidopteran of the Month series, back after a few months’ hibernation. You might choose to follow my ‘butterfly’ tag for other posts in this series. Today, in June, let’s admire the small tortoiseshell butterfly (Aglais urticae) which I didn’t get around to writing about in April.
It’s so familiar to people like me that a Britain without small tortoiseshells is hard to imagine. Yet this species is far less abundant than it used to be. The State of Nature report says that the small tortoiseshell’s population in Britain has declined by 77% in the last ten years. Yes, 77% in ten years. Shocking.
What does the small tortoiseshell’s decline tell us? That’s a good question because butterflies are excellent indicator species. These delicate, short-lived creatures, many of them quite choosy about their habitats, tell us what’s happening on the land. Here’s what the UK Butterfly Monitoring Scheme (UKBMS) says about butterflies as indicators.
In the case of the small tortoiseshell, fluctuating populations are quite usual. Often one region of Britain has had a good small tortoiseshell year while another region has had a bad one, but over time they’ve balanced one another. This butterfly has always bounced back. But a 77% decline? Oh dear, that’s not usual.
Adrian Hoskins FRES (Fellow of the Royal Entomological Society) at Learn About Butterflies tells us about the small tortoiseshell. If you scroll through his page there, you’ll find Prof Hoskins’ speculation about the small tortoiseshell’s decline in Britain.
No surprise that one idea is to blame the small tortoiseshell’s decline on climate change. Everything’s getting blamed on that! But the small tortoiseshell is tough, for a butterfly. It even overwinters as adults so that it can wake up in March or April ready to start ovipositing (laying eggs) on its larval foodplant, the stinging nettle (Urtica dioica).
Here’s a picture of small tortoiseshell caterpillars eating a stinging nettle. Professor Paul F Brain at Swansea University made this picture available at the Centre for Bioscience Images.
Phillipe Bricaire at Fleeting Wonders shows us more detailed pictures of small tortoiseshell caterpillars eating stinging nettles.
Here’s a film about the stinging nettle, featuring several of the creatures which eat it. Even a television presenter! The small tortoiseshell too.
As I said, the small tortoiseshell’s tough for a butterfly. In such a tough little creature, a 77% population decline looks as though something more than weird weather is bothering it. The ‘something more’ could be a fly called Sturmia bella.
Here’s a picture of a female S. bella on a flower tentatively identified as wild parsnip (Pastinaca sativa), shown with permission from Chris Raper at the Tachinid Recording Scheme. I’m not a great field botanist so do please comment on this flower if you know more.
S. bella is a dipteran (a true fly), of the kind called a tachinid (family Tachinidae). Mr Raper tells us that tachinids are parasitoids. That means that these cunning little flies invade other insects’ bodies, eat them from the inside, and eventually kill them. Not vegan then, eh? Dr Bricaire shows us pictures of S. bella emerging from a pupa.
S. bella was previously known in warmer parts of Europe and Asia but it’s been found in Britain within the last 20 years. Perhaps the changing climate suits it. Now that it’s here, it’s eating small tortoiseshells. It’s also eating other vanessid butterflies including the peacock (Inachis io), the red admiral (Vanessa atalanta) and the comma (Polygonia c-album).
Mr Raper explains that S. bella oviposits on the stinging nettle so that when vanessids hatch, its larvae will find their food source right there. In fact when the caterpillars eat the leaves, they swallow the parasitoids, which then eat the caterpillars from the inside.
But the the peacock, the red admiral and the comma are hosts for S. bella too and they’re not disappearing. Why?
When it oviposits, Prof Hoskins suggests that the small tortoiseshell might be putting itself in harm’s way. It oviposits in open areas such as farmyards and field edges, where the nettles might have been sprayed with pesticides, whereas the peacock, the red admiral and the comma oviposit mostly in woodland. So the story makes sense without mention of S. bella. It’s plausible that small tortoiseshell caterpillars, hatching on sprayed nettles, get poisoned there.
Here’s Mr Raper’s opinion about S. bella and the small tortoiseshell. He agrees that the small tortoiseshell might put itself in harm’s way, but he suggests that’s more by hatching at a risky time of year than by hatching in a risky place. He points out also that some of S. bella‘s hosts live alongside it in healthy populations.
Here’s another portrait of a small tortoiseshell. This time the insect sits on a flower which Finn Holding at The Naturephile tentatively identifies as ragwort (Senecio jacobaea). For the sake of whoever’s land this plant was on, I hope it’s not ragwort. Anyway, Alan Stubbs made the photo available at the Centre for Bioscience Images.
The Natural Environment Research Council (NERC) asked whether S. bella might be a culprit in the small tortoiseshell’s decline. After NERC funded science about this question, Sofia Gripenberg and colleagues produced this peer reviewed report. Being good scientists, they report the association they found between S. bella‘s presence in Britain and the small tortoiseshell’s decline, but they don’t jump to conclusions about whether or not the parasitoid is to blame.
I’ll give the final words to Mr Raper. He’s an expert on tachinids who contributed to the science I’ve just linked to.
Chris Raper says, ‘My feeling is that the Small Tortoishell was in decline due to other constraints – possibly a combination of climate change, habitat destriction and other environmental problems. When Sturmia arrived it provided an additional pressure on an already very weakened population but it is clear that, in Europe (where Sturmia has long been a common native fly) and in some regions of England where the population of Small Tortoishell is healthy, Sturmia does not cause a decline in Small Tortoishell numbers. Nor does it make sense for any parasitoid to wipe out a host – no host, no parasitoid! … To label parasitism as a major cause of the decline in Small Tortoishell would muddy the waters and provide the enemies of conservation with a convenient scapegoat.’