This article just provides my take on the controversy raging amongst British lepidopterists over the significance of the appearance of the parasitoid Sturmia bella in the decline of the Small Tortoishell (Aglais urticae, Nymphalidae) in England.
In 1998 the first S.bella was reared from a Peacock (Inachis io, Nymphalidae) larva by a woman living in Hampshire. The specimen was determined by Dr Mark Shaw. Prior to this, during the 1990s, lepidopterists had noted a general decline in many butterfly species but towards the end of the 1990s the Small Tortoishell provided particular concern because it seemed to have almost disappeared from some parts of southern and central England.
The decline had been so rapid that in 2008 Butterfly Conservation commissioned a project to attempt to determine whether Sturmia is responsible for the decline in Small Tortoishells. It could all be a coincidence and it is quite a simplistic viewpoint but it is a fair question to ask when presented with the facts.
Dr Owen Lewis (Oxford University) is running and coordinating the project and Matt Smith & I volunteered to provide expert assistance with determinations and project design. Butterfly Conservation members have volunteered to collect and rear Small Tortoishell larvae and send any parasitoids and resulting data to Owen.
Across its world distribution S.bella is a parasitoid of a broad range of nymphalid butterflies, from our vanessids (Peacock, Small Tortoishell etc.) to danaids in the far east. It has even been reared from Speckled Wood (Pararge aegeria, Satyridae) in the UK so it seems clear that it will attack a broad range of medium/large butterfly larvae. A large number of microscopic eggs are laid onto the foodplant in the rough vicinity of feeding larvae and the egg is ingested by the host. For this reason when rearing larvae for the project it is important to collect foodplant from roughly the same location that the larvae were collected.
In March 2009 Owen presented his initial findings to the Upper Thames branch of Butterfly Conservation. I won’t go into the details because I don’t want to misquote Owen or appear to write-up what is an ongoing project. But I think a summary would be that the jury is still out in the main question but Sturmia is now one of the commonest parasitoids of the Small Tortoishell.
One interesting question was that when we know that Sturmia attacks all nymphalids, why has the Small Tortoishell been affected more than other common vanessids like the Peacock? Owen’s suggestion for this is that it seems that the Small Tortoishell’s phenology has worked to its disadvantage in that it seems perfecly synchronised with that of Sturmia. Other nymphalids are in their vulnerable larval stages at times of the year that result in less parasitism by Sturmia.
The project seems to be generating a lot of excellent tachinid host and distribution data, with the generous support of many enthusiastic Butterfly Conservation volunteers. Sturmia is by no means the only parasitoid of Small Tortoishell and we have noticed many of the common native species in the reared samples.
I think it is logical to conclude that high levels of Sturmia parasitism is having an effect on the Small Tortoishell population but it is impossible to know what the situation would have been had Sturmia not arrived in Britain. The decline of the Small Tortoishell was documented before the 1998 arrival of Sturmia and we have examined a lot of tachinid collections and have found no records prior to this date. Considering how large the fly is and how commonly the host is reared in captivity it seems very likely that Sturmia really did make landfall in 1998.
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!
I think it is vital that conservationists to make it clear to decision makers and the media that the primary reason that we find ourselves in this position is the same one that we have been preaching for the last 20 years – man’s adverse effect on the environment. 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.
Anyway, that’s my slant on things based on what I have heard – if anyone has any other thoughts or corrections please feel free to comment 🙂