Population Decline

[Grizewood]

I came across an article in the Guardian discussing the decline in population or the Small Tortoiseshell recently. We actually noticed a marked increase in numbers this year after last year being almost a wipeout for everything. We live in South Cumbria.

There are two or three Buddleia plants close to the house and during a couple of weeks of the summer we really had this year, late July into August, the bushes were alive with STs, almost to the exclusion af everything else at first sight. For a couple of weeks we had 30 or 40 on the bushes all the day. We have several acres of woodland originally planted for coppice but harbouring large areas of nettle as well. This all probably helps, but the numbers really were startling.

I know all this is just local but at least the declise may not be universal.

Regards

[KeithS]

I have seen very few STs in my bit of North Kent since May - except for the five that settled in my garden shed in June, apparently to hibernate!? I moved them to a netted clay pot to keep them away from spiders and they still seem just fine, with no inclination to fly off at all during the summer. Locally, I think you are quite lucky, though I do wonder if my local STs have just gone into hibernation earlier, maybe to avoid the severe heat and drought conditions that would hit the usual second and partial third broods. Certainly haven’t seen any second brood larvae around here this year.

Any thoughts anyone?

[David M]

I have seen this article and certainly agree as regards the situation in 2018 (though it's heartening to learn that some localised areas have witnessed an INCREASE).

Small Tortoiseshells are usually fairly commonplace near me in south Wales but they have been pretty scarce all year.

It's hard to know whether this is as a result of the relationship the species has with its main parasite (like Holly Blue) or whether it's due to something different.

I expect them to bounce back, but long term we all have to show concern for all our native fauna and flora. It's under pressure like never before and fewer and fewer people seem to care.

[essexbuzzard]

Small Tortoiseshell seems to show the greatest fluctuations in numbers in East Anglia and the south-east. And I’m really not sure what happened to the ‘double brooded’ theory. There are people on here with far more knowledge than me but, to my mind, the so-called second generation simply doesn’t exist nowadays. Few, if any, are seen in autumn, but they appear in April, suggesting that first brood adults hibernate, at least locally.

[millerd]

In other words, they are starting to behave in the same way as Large Tortoiseshells do. Why this might be is a much bigger question.

Dave

[Neil Hulme]

There's plenty on this in The Butterflies of Sussex (pp. 27 & 199).

I believe that the decline of the Small Tortoiseshell is in some way linked to its unprecedented phenological response to climate change, which is more keenly felt in the South of England. When we analysed the flight periods (for Sussex) over two five year periods (which flattens the effects of annual weather patterns), twenty years apart (1990-1994 & 2010-2014), we found that the entire timeframe of double-brooding has shifted forward by an incredible 3-4 weeks over that 20 years - significantly more than for any other species (Sussex average 13 days for 40 species). This has not been identified by any other studies, which tend to use the national dataset, masking this effect by including data from all latitudes.

A much earlier emergence of the first brood (under conditions of rapidly increasing day length) is likely to have polarised behaviour in the South of England towards double-brooding, with correspondingly fewer individuals entering early diapause. The species has, in effect, probably started to 'put more of its eggs in one basket', which may have decreased the spread of risk.

I believe this risk, in our ever warming climate, is droughting of high summer nettle beds, leading to reduced breeding success for the second brood. The summer of 2018 is a good example; the common nettle feeders have suffered a terrible season, whereas so many other species have done well. That said, we may now see good emergences of Red Admiral and even second brood (triggered by the weather) Peacock, which will have benefited from late summer rain and the recovery of nettles.

As an aside, I believe that it has been the unsuitability of the 2018 high summer nettle stock which has driven the Comma to search elsewhere; I have seen an unusually high number laying eggs on elm.

If this theory (for the Small Tortoiseshell) were to hold true, the decline in this species should be less severe as one travels north, which is what we see. Small Tortoiseshell is likely to recover to some extent in and immediately after 'poorer' summers, but, overall, I believe this species is in BIG trouble.

BWs, Neil

[David M]

Many thanks for the response, Neil. I re-read the relevant page in Butterflies of Sussex and the theory seems absolutely plausible. Why else are they doing so much better in the cooler and wetter parts of the UK (like near me in south Wales)?

Similarly, in the south of France they usually only pop up at altitude (mostly hill-topping) where, obviously, conditions are more conducive to all types of vegetation, including nettles.

[Neil Hulme]

Hi David

I'll put something together for dispar, showing the raw data, but this will be a winter job; at the moment I'm seeing more Wall than I've seen since the 1980s, so I'm squeezing every last drop out of this season.

BWs, Neil

[Jack Harrison]

Pretty certain ST - judging my the fresh individuals seen in early July and then after a gap, again early September - was double brooded this year in my part on N.Scotland. This was not unheard of in the past but is unusual.

Summers in the south are probably getting too hot and dry.

What is happening in mainland Europe?

Jack

[David M]

I'll put something together for dispar, showing the raw data, but this will be a winter job; at the moment I'm seeing more Wall than I've seen since the 1980s, so I'm squeezing every last drop out of this season.

I fully understand, Neil. No rush, but when someone like you thinks this species is in 'big trouble' then it has to be taken seriously.

I fancied Small Heath would be next to fall into the 'developmental trap'; I never considered that the humble Tortie might go the same way.

[peterc]

I collated data for the 3 transects I walked in the Stevenage area this year and the figures make interesting reading.

The biggest declines compared to last year in percentage terms are:

1. Painted Lady -87%
2. Comma -86%
3. Essex Skipper -74%
4. Brimstone -71%
5. Small Tortoiseshell -68%
6. Red Admiral -67%
7. Peacock -58%

All the adult hibernators are on this list and had a bad start to the year possibly due to the cold weather in February and March as very few individuals were seen in April, the first month of the transect season.

The warm and dry conditions in May and June may have indirectly impacted further development and therefore fewer offspring produced.

ATB

Peter

[KeithS]

Last year was a really bad one for Peacocks in my North Kent location. The late, cold snap in March did not help, I'm sure. However, I found a small colony of 24 second instar (I think) Peacock larvae on a rather lonely strand of nettle, and brought them home to rear up safely caged in the hope of boosting the numbers. I got three rather small adults and 21 wasp pupae. I saw no other larval Peacock colonies this year, whereas normally I would expect to see quite a few. I kept the little brown, almost spherical wasp pupae, which still seem to be dormant, as I am interested to see what hatches out. I won't be letting them go, though...

[Neil Hulme]

With reference to my post dated 26 September 2018 in this thread, I've written more on this issue at viewtopic.php?p=140248#p140248
BWs, Neil

[PhilBJohnson]

Helping track a possible Lincolnshire, annual 2022 local population decline in Small tortoiseshell numbers.


Spring numbers of hibernated butterflies, thought to be down (2021-2022).

 Hibernated butterflies were thought not to have naturally migrated, in wilful numbers, in cooler Spring life cycle speed.

Small tortoiseshell, June 2nd 2022

June 2nd 2022, ST on Bramble blossom


Very Late May/Summer, first butterfly generation, after hibernated generation, possibly a small recovery in numbers. The first fresh one I saw was very near the end of May.



July 7th 2022 Second annual brood, after hibernated generation
https://youtu.be/-L4Q4y4-z1I
Video clip of social defence against predation. I was happy to submit this video clip, for a correctly credited UKB Small tortoiseshell species video, at this time of posting.

Then after a continued hot, dry period, how fast did this brood life cycle and how far north might it have gone?

#Migration (Temperature allowed/induced) #DriedNettleBed

“

The UK's new record-high temperature of 40.3°C at Coningsby, Lincolnshire, has been confirmed by the Met Office, following a rigorous process of analysis and quality control. 28 Jul 2022”


Referenced: Met Office, www.metoffice.gov.uk
online September 26th 2022


[PhilBJohnson]

I have not yet seen the first summer generation of Small tortoiseshell near Lincoln in 2024.
I thought I was not "well travelled", but we would expect to see them along the river banks, nectaring on bramble blossoms, looking quite fresh, near the beginning of June.
Numbers of over-wintered Small tortoiseshell appeared to be down this year (2023-2024) and because of average winter temperatures, reaching significant differences, we thought the winter had not kept those individuals as fresh, for March flight appearance & April, adult to egg oviposit.
If overwintered Small tortoiseshell were generally looking in poor condition, then average length of longest diapause could possible be worked out relating to winter seasonal temperature fluctuations, with a climate changed increased average winter temperature.
Also I thought, many weather influenced longer roosts with no daily flight, outside of longest individual diapause, might take it out of an individual, if it kept waking up for warm winter or early Spring flight, but made little seasonal progression, for it's life cycle.
Further north, a more distinct, quicker, progressive seasonal change in temperature, might have followed a longer single diapause.
It might have been noted that internally uncooled wooden structures became less helpful first, for a longer diapause.
Nettles were naturally found in temperate European regions. I once spoke to an American who was nettle stung by surprise who then said;
"where I come from, we don't have them there"
In Lincolnshire, we had place names, named after nettles. #Nettleton #Nettleham

Intrinsic to our something or somewhere

Hot weather, less suitable for nettle hydration
A summer butterfly generation (if it be first or second) might be remembered as having flown north, in long distance flight, by those remembering sitting on a coastal beach in England and having Small tortoiseshell butterflies incoming, flying past them to beyond.

Kind regards