Small Tortoiseshells

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Lee Hurrell
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Small Tortoiseshells

Postby Lee Hurrell » Thu Apr 15, 2010 12:30 pm

Hi all,

I thought this might warrant a separate topic.

My branch of BC (Herts & Middlesex) are reporting increased numbers of Small Tortoiseshell sightings this year, just as I have been in my diary (viewtopic.php?f=29&t=3984&start=20 - go to Thursday 8th April)

It has set me thinking...where have they been? And is anyone else seeing more than in recent years?

One thing that springs to mind is that the cold weather might have benefitted them in some way and my initial thought was that it may have killed off any parasites or disease that would have harmed them or the caterpillars which I think was the problem with their decline. But the sightings I am seeing now have hibernated as adults and so must have had a good last season LAST year, so where have they been? I didn't see any in Middlesex in the autumn....and only a few in counties outside the the south west. It might suggest that they lose numbers in hibernation through a warm winter but doesn't explain why there are no corresponding sightings from last year (well for me, anyway).

I've seen more so far this year in Middlesex than the whole of last year all round the country put together I think. It was a another Middlesex reporter's comment about seeing one at Horsenden Hill on Monday 12th April (as I had on Saturday 10th) being the first one he'd seen in Middlesex for years that set me thinking.

Any thoughts?

Cheers

Lee
To butterfly meadows, chalk downlands and leafy glades; to summers eternal.

jellyang
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Re: Small Tortoiseshells

Postby jellyang » Fri Apr 16, 2010 2:40 am

Here in my corner of Norfolk there has been good numbers of Small Torts for the last 3 years. They do seem to be emerging earlier each year?
It has been sunny but quite chilly here but The small torts have been the only butterfly apart from one lone Peacock that I have seen in the last week.


Angie

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Gruditch
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Re: Small Tortoiseshells

Postby Gruditch » Sat Apr 17, 2010 7:02 am

Hi Lee, just to give an indication of how bad the Small Tortoiseshell situation was. On my transect for 2008 the total was just 2, :( In 2009 that reached a much more respectable 46. With only two transects done, we have equalled the 2008 total already this year. Locally ( West Hampshire ) the Meadow Brown was the only species to have a dramatic crash in 2009. Hopefully just a blip, but it makes you realise that we should never take any species for granted.

Regards Gruditch

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Re: Small Tortoiseshells

Postby Dave McCormick » Sat Apr 17, 2010 12:26 pm

Well so far the past 3 years, here in Mountstewart, Co Down, Northern Ireland, the small tortoiseshells have been doing well, in the year (was it two years ago?) when there reports of declining numbers for various reasons (like parasiting etc...) therey were up in good numbers here, saw roughtly 25 in one day (as far as I remember it was around this)

I have only seen 8 this year in total so far (well 9 if you include a dead one)
Cheers all,
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Lee Hurrell
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Re: Small Tortoiseshells

Postby Lee Hurrell » Sun Apr 18, 2010 7:29 pm

Thanks Angie, Gary and Dave for your comments, it does seem that certain locations (NI, Scotland and Norfolk (maybe 'further out places?) had escaped the decline a little maybe?

I saw another one today in Kent, more in my diary later.

Cheers

Lee
To butterfly meadows, chalk downlands and leafy glades; to summers eternal.

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Re: Small Tortoiseshells

Postby NickB » Mon Apr 19, 2010 9:26 am

We had a bumper Autumn for ST's a couple of years ago around Cambridge, when few had been seen earlier in the year; last year was a fairly normal year - again, more it seemed later in the season. I have to admit that this Spring I have seen far more STs than usual (what IS "usual"?) - but is that because the season is later and they are all concentrated into a narrower window?
Or is that the natural cycle of their parasites is at a low? Whatever the reason, nice to see them!
N
"Conservation starts in small places, close to home..."

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Lee Hurrell
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Re: Small Tortoiseshells

Postby Lee Hurrell » Mon Apr 19, 2010 12:54 pm

Neil (Sussex Kipper) has kindly answered a few ST queries in his diary:

"It's great to see so many Small Tortoiseshells around this year and, like many others in the South, I'm seeing them in numbers (and locations) that I haven't done so for many years.

In response to some of the questions posed by Lee Hurrell (Sightings - 'Small Tortoiseshells' thread) - there may be a numbers of factors at work here.

Firstly, an explanation of your observation of 'a higher number of STs this spring than can be explained by the apparently lower numbers seen late last autumn'. The post-hibernator butterflies emerge in spring, mate, and their offspring are around through June, July and August. These butterflies mate and give rise to another emergence of adults, which are on the wing through late summer and autumn. Soon after these second brood adults start emerging (which may be over a quite prolonged period of several months), some individuals start tucking themselves away, well in advance of late autumn/winter, and largely un-noticed. So their 'disappearance' from the countryside is a greatly staggered event, with a relatively modest number being active and on the wing at any given time from late August onwards. This phenomenon can also be 'exaggerated' by the fact that we are now seeing a partial third brood of STs in the South, meaning an additional but small emergence of adults very late in the year. Also, (particularly further North and following long, hard winters such as 08/09 and 09/10) a few of the adults that emerge in mid summer (first brood) go straight into hibernation, rather than mate 'this' year. This is a good survival strategy at species level, in the same way that a small percentage of pupae of other species will 'hold back' from emerging until the following year - a little bit of 'insurance' against particularly poor years.

Whereas the STs from a 'previous' year have gone into hibernation over a very prolonged timeframe, when spring arrives they all emerge over a very short period - and are all on the wing at the same time. So this gives the impression of a much higher number than could be explained by the numbers seen at any given time through the preceding late summer/autumn. The same effect can often be seen with Peacocks, which start to tuck themselves away in August. With fewer species around in the spring, we also tend to notice these particular butterflies more. If over-winter survival rates are particularly high (my second point), the whole effect becomes even more obvious.

The over-winter survival rate of STs (and other species) is undoubtedly high this year. This will have been helped by the cold weather, which reduces losses through moulds, parasites, pathogens and some predators. The situation with ST may be even more complex. Although research into the effects of the parasitoid Sturmia bella is ongoing (see photo of a project 'release'), and Owen Lewis' team is yet to make a 'final judgement', this tachinid does seem to have exacerbated the decline of the species. We still don't know the over-wintering strategy of the fly, but being a 'warm climate invader', it might not have thought very much of the cold 08/09 winter! With the 09/10 winter being even colder, we might be seeing an 'upward spiral' and I'm optimistic that numbers might be even higher next year, perhaps returning to the level that many of us have not experienced since our youth!

However, in the longer-term, it is possible that S. bella might start developing a tolerance to colder winters, and we might have a long re-run of warmer, damper winters. This might result in another 'crash' in the future. This cyclicity has always, and will always, affect the fortunes of some of our butterflies."


Neil, that certainly does make sense about the prolonged entrance to and short exit timeframe from hibernation. I hadn't really thought of it like that but of course it must happen. I would have imagined they would fly for a bit though as it can be warm enough into October. It also explains why there there so many Peacocks about now of course.

Near me, the Commas are usually the first to go to bed, last year they all disappeared way before the Peacocks and Red Admirals.

The cold winter helping with parasites etc also makes sense.

What I don't get though is why I'm seeing ST's in places this year that I didn't last year but that I frequented. That does imply they were of course, maybe I just missed them in the short time they were about before hibernation!

Thanks again Neil, and fingers crossed for them this year.

Lee
To butterfly meadows, chalk downlands and leafy glades; to summers eternal.


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