The Sturmia bella controversy

sturmia-bella-female-warburg-reserve-oxon-20050723-045This 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.

sturmia-bella-female-warburgOne 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 🙂

Click here to see a BBC movie about the Small Tortoishell and Sturmia bella

8 Replies to “The Sturmia bella controversy”

  1. i have found your article very interesting. I will certainly look for this fly in my garden. I have a 6ft high buddleia next to my kitchen window and I have never seen so many butterflies in my 47 years. I have so many tortoise shell I can’t count, red admiral and small whites. There are bumble bees and normal bees too. I have had visitors to my garden just to see the cloud of butterflies. This year must be a bumper year.
    I live in letchworth in herts.

    Alison Spary

  2. HI Chris,

    Most interesting article ! However, I have to disagree with some of it.

    I have collected part ‘nests’ of Small Tort at different times of year for the last few years. Finally reared to pupation in a large netted fish tank. In West Penwith, Cornwall.

    Going back to 2003 I was finding Sturmia pupae on their typical ‘ropes’ in about 5% of the pupae. Sometimes there were more than one Sturmia in a single butterfly pupa. ( there were a few parasitic wasps which showed a constant number over the years)

    Last year 2008, regardless of the time of year, the number of Sturmia infected Pupae had risen to about 85% by late summer…again many were multiple infections.

    Winter 2008-2009 we had a severe frost.

    2009 collections of larvae, three different dates this summer….. NO STURMIA, NO WASPS!!

    Today, Sept 4th 09 I shall be collecting probably the last larve. Large nests of Small Tort and Peacock in the garden. I trim the nettles back in stages to allow new growth to provide suitable food plants throughout the breeding season. I shall bring in both Small tort and Peacock and will update this message.

    It Is highly probable that there are other environmental effects on species loss, however the massive destruction of typically late brood, hibernating adult butterflies by Sturmia over the recent years is, I feel, a major contributor to their decline.

    Lets hope for more winter frosts !! Co-incidentally the frost eliminated years of White Fly from my Butterfly Greenhouse !!?

    Best wishes, KIF

  3. Hi Kif

    Many thanks for an interesting post – and don’t worry about disagreeing – that’s what good science is all about! :o)

    I fully understood that what I wrote was a bit controversial and a bit provocative but it was necessary to kick-start a bit more debate on Sturmia I think. My main problem was with the way certain articles have branded Sturmia as the *cause* of the decline in Small Tortoishell numbers across the UK. Before Owen Lewis’s research this ‘fact’ wasn’t actually based on any scientific analysis but on lots of anecdotal accounts – compelling though they were.

    I have no doubt that such high infestations would have some effect on ST populations but I stop short at actually blaming Sturmia for the decline. I reserve that dubious honour for the way us human’s have been abusing the countryside for decades. If anything I think Sturmia is the ‘last straw’ for a ST population that is both weak and fragmented.

    I think this is backed up by Owen’s initial findings because he found no significant difference in decline between areas where Sturmia was obviously present and those areas where Sturmia was not recorded. Also, there are many areas (Dorset comes to mind) where ST is still common and yet Sturmia must be there in high numbers. In addition, I understand that the ST population was already in quite rapid decline from about 1992/93 but Sturmia (a large and obvious fly) was only recorded in 1998 – it having jumped the Channel from its stronghold in mainland Europe where it hasn’t been causing any problems with vanessid numbers. Also, I feel that any parasitoid would not have a detrimental effect on its host (that would be suicide) as long as the host population is not already under a threat and balanced on a tipping point. With ST we could very well have hit a tipping point and Sturmia in some regions may have tipped it over the edge creating the obvious declines.

    It’s interesting that you have found no Sturmia bella in recent samples. Here in Berks and south Oxon I have found lots on a few local sites – sitting very obviously on Wild Parsnip flowers and looking very “Sturmia-ish”. I believe that the early frosts put pay to the asian Harlequin Ladybird expansion a bit because we have had many fewer of those but Sturmia should be well adapted to frosty conditions because it is common across most of northern Europe where they have much worse conditions than we do.

    Anyway, keep up the good work and let me know how those Sturmia are doing down there. It’s a fascinating story, no matter what the final outcome will be, and if nothing else it will give us a much better understanding of parasitoid/host relationships and population dynamics! :o) It’s also good to note that up in my garden at least we have had a lot more ST this year and my friends have all reported the same so perhaps they enjoy frosty winters and wet summers? Perhaps climate change has more to do with ST decline than parasitoids?

    Chris R.

  4. Hi Chris,

    I agree that population declines are usually caused by a number of factors, often including the usual anthropogenic causes.

    Because wet, mild winters seem to favour pathogens, viruses, moulds and fungi, I believe they are heavily implicated in population dynamics, and that this is one reason why a traditional winter such as that of 2008 has resulted in greater numbers of species such as Small Tortoiseshell in 2009 (in Sussex anyway).

    It has been suggested a number of moth species are also affected by S. bella. Are you aware of a definitive list of affected species? If so, I’d appreciate a reference to follow up!

    Best wishes,

  5. Hi Steven – thanks for the interesting comment. Sturmia favours nettle-feeding butterfly larvae but you are correct in that some other species have been used as hosts. In the butterflies we have a record for Speckled Wood and it seems that Grey Dagger moth has a few records too. But these records are rare, compared to the huge numbers of Small Tortoishells or Peacocks that are attacked. I think the records from non-vanessids tend to be from larvae that have accidentally eaten a Sturmia egg that was dropped on their foodplant by accident – Sturmia‘s parasitization strategy is to broadcast as many small eggs as it can on and around the foodplant. Tachinid larvae do not seem to have complicated mechanisms to control their hosts (unlike many ichneumons) and so they are not very host-specific – anything that eats an egg and is big enough to feed the Sturmia larva is eventually likely to produce a Sturmia fly. 🙂

  6. Hello, thank you for this website which I’ve found very informative. I’ve just cited you on my blog Science on the Land I’d be honoured if you read my words and if you let me know of any errors.

    Also, do you know the Centre for Biosciences Imagebank? They have many insect photographs, available free for non-profit uses, but nothing yet about Sturmia or any other parasitoid.

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