Small Tortoiseshell 3rd brood?

[Pete Eeles]

I was surprised to come across a web of 3rd instar larvae of Small Tortoiseshell today. Give that it's 9th September, I would tend to conclude that this is an unusual 3rd brood, but would appreciate any thoughts!

Cheers,

- Pete

[IAC]

Hi Pete,
I have seen some very late Small Tortoiseshell broods on only a few occasions, most notably 2011 when good numbers of Small Tortoiseshell pupated in late September emerging in mid to late October. They were joined by a very late number of Red Admiral. Both species I observed and photographed during a period of work on a short grass strip, field edge. The caterpillars were found pupating on the farm building walls in large numbers... I have never seen anything on that scale again, though late broods can be found from time to time. For this far north, Scottish Borders, partial second broods of Tortoiseshell and Comma, are, I would suppose, unusual, third broods...I just cant be sure. Small Tortoiseshell I have observed are really having trouble timing there broods and seem to be all over the place....I have seen oddness is all I can say really.

Iain

[Vince Massimo]

Hi Pete,

I have monitored many larval webs this year and, based on the development rates that I have observed, I think these could be third brood. They are late 3rd instar, so to get to this stage in their development, I estimate that the eggs would have been laid during the first week of August. This would tie in with the date when the second brood would be emerging (although I cannot rule out a first brood female still being active at that time). This season, eggs were being laid by around 21st March, with the first adults of the first brood emerging around 9 weeks later on 23rd-27th May. If their first eggs were laid in the first week of June and, allowing approximately 8 weeks for the second brood to emerge, this would take us to early August and potentially a mated female.

One other observation on this species this year is that, although I found hundreds of first brood larvae, I did not see a single second brood web.

Vince

[Pete Eeles]

Thanks Iain, Vince - that clears things up.

I am worried (again!) about this species, having not seen too many this year (or their larval webs).

Cheers,

- Pete

[bugboy]

Thanks Iain, Vince - that clears things up.

I am worried (again!) about this species, having not seen too many this year (or their larval webs).

Cheers,

- Pete

for me I saw good post hibernation numbers but I only saw decent summer individuals in damper habitats. I've only had one sighting on my transects this year (which to be fair is in central London) and on my local patch I found two larval webs, one from each generation. I was putting these observations down to the dry spring which caused nettles to become a bit desiccated in dryer locations. The best summer gen numbers I saw was at the North Stoke Large Tortoiseshell site, which as many of us will be aware is a damp meadow.

[PhilBJohnson]

As this butterfly species, needed to (had evolved to) "over frost-winter" as an adult in hibernation, one could assume that any third brood that did not make it to imago (adulthood), was wasted for genetic pass on to next generation and a second brood adult might over-winter in a wing damaged state.
This species is sensitive to climate change, appearing more sensitive to climate change than the Comma butterfly which appeared to have an ability, as a species, to better (more frequently) overlap it's generations within the British Isles.
I had assumed that Vince's study was based somewhere near the south coast.
I am sure Pete Eels new book explains something more or similar to this.
My thought was independent thus far.

[Neil Hulme]

Hi Pete/all

A couple of excerpts from The Butterflies of Sussex, reproduced with the kind permission of the author.

"Flight Period

The Small Tortoiseshell can be encountered at almost any time of year, and occasionally wakes from hibernation during warmer spells in even our harsher winters. The average flight period recorded between 2010 and 2014 was 15 January to 18 November, but significant numbers are seldom seen before the first warmer days of March. These overwintered butterflies immediately mate and lay eggs, persisting until early May.

The early summer (first) brood now emerges from late May onwards, building towards a peak in late June, and tailing off around early/mid July (some may hibernate). This flight now overlaps with the late summer (second) brood, which starts to emerge in mid July and rapidly achieves a peak in late July and early August. The majority of these butterflies will have hibernated by late September, with relatively low numbers seen thereafter."

As yet unpublished chart, based on Sussex-only data:

"Possible Reasons for Decline

The reasons behind the periodic crashes in the Small Tortoiseshell population are still poorly understood. Much has been written about the possible part played by parasitoid wasps and flies, and there is little doubt that their impact on the butterfly’s numbers can at times be profound.

Brachonid and ichneumon wasps, and tachinid flies, all have the potential to cause the death of large numbers of caterpillars. These insects generally lay their eggs on the skin of the tortoiseshell’s larva, or inject them into its body, and their grubs then feast on its living tissues. The tachinid flies tend to work to the same plan, by causing death before the caterpillar pupates, subsequently emerging as adults from its corpse.

However, the 21st Century has brought with it a new assassin from the Continent, in the shape of the tachinid known as Sturmia bella. This operates in a slightly different way to the other flies, by laying its eggs on leaves in the vicinity of Small Tortoiseshell and other nettle-feeding caterpillars. These are ingested whole and the resultant grubs feed within the caterpillar’s body without killing it, allowing it to pupate before delivering the coup de grace. The maggot then emerges from the chrysalis and leaves the crime scene on a silk-like thread.

The arrival of Sturmia bella to our shores cannot have helped the Small Tortoiseshell’s cause, and it may have been a contributory factor in influencing the butterfly’s fortunes on a local or even regional level. However, detailed studies of its potential impact have failed to account for the broader pattern of the tortoiseshell’s decline, at least in a decisive manner.

Sometimes the decline of a species will be due to a combination of factors, making the recognition of ‘cause and effect’ rather difficult; this may be the case with the Small Tortoiseshell. The female butterfly is very choosy about the type of nettle growth on which she will lay her eggs, particularly as a batch of 50 – 100 represents such a large single investment. Short, succulent, nitrogen-rich nettle shoots growing in full sunshine are selected, and these may have become more difficult to find in a warming climate, at the right time of year.

The comparison of phenological data for the Small Tortoiseshell in Sussex between the periods 1990 – 1994 and 2010 – 2014 has proven very revealing. The timing of the two broods has shifted forward by at least three weeks. This raises a number of potential issues, including synchronicity with its foodplant, in the required growth-form. However, the growing season will also have responded to climate warming.

Perhaps of greater significance may be the effect of a significantly earlier breeding cycle on the split in developmental routes taken by individual Small Tortoiseshells. With an increasing proportion of the first brood adults now emerging earlier, it is likely that a decreasing proportion will hibernate immediately. This potential polarisation of behaviour towards double-brooding may have increased risk in some way. However, such a theory requires testing and the reasons for the boom-and-bust of the Small Tortoiseshell may prove difficult to elucidate."

The risk I refer to (imposed by a phenologically-driven shift towards increased bivoltinism in the population) is almost certainly droughting of the nettle-beds in our increasingly warm summers. I believe we saw a spectacular example of this threat in the long, hot summer of 2018; Small Tortoiseshell and Peacock suffered stinkers. Far from the mystery proclaimed by many commentators in the media, I think that droughting of the nettle-beds provides us with a prime suspect. It also explains why I observed a much greater reliance (than normal) upon elm as a larval foodplant in the Comma, which also had a poor year in Sussex; the supply of succulent nettles had simply dried up.

A lifeline was thrown to these species in late July, with some long-overdue rain, which caused nettles to flush. This also facilitated a late season increase in the previously poor numbers of Red Admiral.

The increasingly early phenology of the Small Tortoiseshell does provide it (as in many other species) with the opportunity to experiment with additional broods (we saw second brood Peacock in 2018), and I have no doubt that Pete's larvae represent a third brood. This could be helpful to the species, but I'm sad to say that I believe that our beloved tortoiseshell is in dire trouble, at least in the warmer South of England. If my theories are correct, the species should be suffering to a lesser degree with increasing latitude, which is what we are observing.

BWs, Neil

[KeithS]

In my own small corner of North Kent, it has been an odd year for the ST, at least by my observations. Despite the late and vicious cold snap at the end of Feb 2018, this merely delayed the sight of larval colonies until mid-May. I found 9 decent-sized larval tents between first and third instar stages in and around the Cuxton area, which is about par for the course. I like to collect a few larvae from different colonies and bring them through in a cage, just to see what the parasite impact is. This year I lost only one out of 24 to a wasp, and none to Sturmia, which was very different to 2017, where Sturmia took about 80% of the pupae. Maybe the late cold snap knocked the Sturmia back. I let the adult STs go where I collected the larvae, hoping to see some second brood larvae. The nettle beds were pretty well cooked by the end of June however, and I found no trace of second brood larvae anywhere in my local area, for the first time since 2011. Curiously, five adults settled down in my garden shed in July, apparently to hibernate. I placed them in a net cage away from spiders in a cool store room and they still seem to be alive. They will drop off the mesh, seemingly inert, if the cage is disturbed, but if left alone for a day or so they eventually climb back up to the top of the cage. At no time from July onwards have I seen more than a couple of individual STs in any one month, but I have seen the occasional one on the wing even in December and January this year (it may be the same one of course).

Peacocks really suffered this year. I normally get at least half a dozen permanently in residence on the Buddleias in my garden, but I didn’t see one this year, with only a few sightings elsewhere throughout the year, mostly early on. As with the ST, I would have expected to find a least half a dozen larval colonies out and about, but I only found one at the start of June. The caterpillars seemed rather poor and weak, and indeed only three out of the 24 I took made it to adulthood, the rest succumbing to wasp larvae (I still have the spherical wasp pupae, just to see if and when the wasps emerge). It was a similar story to 2017 in terms of parasites, whereas prior to then most Peacock larvae I had under cover seemed to get through to adulthood. I get the impression that the warm January woke the Peacocks up early, only for them to caught by the snow and ice in March, with the few subsequent late larval colonies getting picked off by parasites.

It will be interesting to see what this year brings.

[MrSp0ck]

Has anybody worked out where the Sturmia spend the winter, speculation was in one of the "Browns" or a moth species, the fritillaries are probably too sparce spread in tiny numbers to be any use to Sturmia.

[peterc]

Following on from Malcolm Hull's observations of hibernating Small Tortoiseshells in his shed in Hertfordshire as featured on Friday's BBC Winterwatch programme there appears to be a 'lost generation' (second brood) since far fewer adults are now seen on the wing in late summer because supposedly many have already entered hibernation.

One would have thought that global warming would induce another generation, not fewer! In the Collins Butterfly Guide - The Most Complete Guide to the Butterflies of Britain and Europe by Tolman and Lewington it states that in the warmer parts of Europe the butterfly is bivoltine or trivoltine but what proportion are trivoltine? I understand that numbers are declining elsewhere in Europe and are there significantly fewer adults on the wing now in late summer? Might there be a 'lost generation' elsewhere in Europe too?

ATB

Peter

[bugboy]

One would have thought that global warming would induce another generation, not fewer! In the Collins Butterfly Guide - The Most Complete Guide to the Butterflies of Britain and Europe by Tolman and Lewington it states that in the warmer parts of Europe the butterfly is bivoltine or trivoltine but what proportion are trivoltine? I understand that numbers are declining elsewhere in Europe and are there significantly fewer adults on the wing now in late summer? Might there be a 'lost generation' elsewhere in Europe too?

ATB

Peter

I believe the prevailing theory in a nutshell is that hotter and drier summers in the southeast are causing nettles to desiccate so more of the summer brood larvae are failing to mature. This means there is a higher proportion of first brood hibernators are present in the following spring adults, and so their genes are being passed on in higher numbers and so by evolution it is becoming a single brooded butterfly in these areas. (personally I'm not convinced Sturnia bella has much to do with the decline since it's not species specific and we're not seeing similar marked declines in other Nyphalids) Ironically these warmer conditions also seem to be causing the usual single brooded Peacock to be bivoltine, second broods seem to be becoming more and more common in the south east. With it's slightly different phenology it perhaps manages to bypass the worst of the dry summers that are hitting the Small Torts. and our warmer, wetter autumns allow it to fit in the second broods on the recovered and still lush nettles.

[MrSp0ck]

Peacock larvae always seem to be at the same size, so all go through together, where you see Small Tortoiseshell larvae they seem to vary greatly in size, so are more at risk of losses from drought and parasites. the wasps and flies only have a short window to attack the Peacock larvae before they get too big.

[peterc]

Thanks bugboy and MrSpOck.

Yes I agree that the desiccation of nettles in the recent hot and dry summers as Neil has also indicated is a factor in the decline.

This theory leads me to think that there could be other reasons for the 'lost generation'. It is understood that female Small Tortoiseshells on emergence have immature eggs so there must be a long enough time window for the eggs to mature in readiness to be laid. It is possible that assuming that the females have accumulated enough nutrients including those from the male sperm in order for the eggs to be laid that if there are no or very few fresh young nettles to oviposit then the female may just 'give up and wait' until the following spring. After all, butterflies which hibernate as adults do so for only one reason - to produce a new generation in the spring.

On the other hand, is it possible that in mid summer drought conditions that Small Tortoiseshells do not attempt to mate and instead feed up for hibernation like Peacocks usually do later in the summer. Of course Peacocks seem now more likely to produce a second brood as already indicated.

Malcolm Hull's observations, albeit in a very short timescale, appear to show that a higher percentage of 'first generation' hibernators do not survive the winter resulting in fewer adults on the wing in the spring.

Another complication is the level of immigration from Europe. As you probably know, there has been a steep decline in Europe too which probably means that fewer individuals arrive here in the summer. Do migrants arrive here and enter hibernation almost immediately?

Has anyone here been monitoring numbers of second generation larvae in the wild (July/early August) and if there is a significant decline in the recent past?

A lot to learn ...

ATB

Peter

[KeithS]

Just wondered if the “success” of the Peacock in achieving regular viable second broods in the U.K. is having an adverse impact upon the Small Tortoiseshell. I had two late September second brood colonies of Peacocks on nettles in my garden last year. I took a few first instar larvae (around 20) under cover to raise them, just out of curiosity. Every single one yielded a Sturmia Bella fly. If late Peacock larvae are providing additional hosting opportunities for a parasite that then hibernates, then the Tortoiseshell could well bear the brunt of parasitisation the following Spring as they lay earlier than the Peacock, at least around my neck of the woods. Just a thought...

[MrSp0ck]

have people worked out the overwintering of S. bella, if its as a fly or pupa, or if inside an overwintering larva where it would be much easier. Red Admiral is becoming a more likely candidate for the overwintering, but has anybody had an S. bella out of a Red Admiral larva. If the peacock Bella rate is very high, it will help the fly in the spring generations if they breed and lay eggs for the owerwintering larvae to eat before they overwinter.

[KeithS]

I assume that Sturmia overwinters as the adult fly. All of the pupae that I gathered in mid-October produced adults.

[peterc]

Is Sturmia on the wing throughout the summer or are there peaks during the year? The very limited results on iRecord for this fly last year list 6 records in the UK (1 in May, 3 in August and 2 in September).

ATB

Peter

[PhilBJohnson]

Summer Redistribution to temperatures & precipitation, more suitable for larval food plant.
Continental Range Extended north
Butterflies could long distance travel, at a stage in their life cycle, when they didn't carry a species parasite.
If we talked about a migrating Small Tortoiseshell, might that mean across a border from England to Scotland?
It was thought that a hibernated Small tortoiseshell, generally, did not naturally, long distance travel, as low early Spring temperatures, were more suitable for local flight.
Might there be a loose comparison with a North African Painted Lady butterfly species, where I was told, many migrate, but some stay behind?
In places like Norway and Sweden, might they have higher counts of Small tortoiseshell in 2023, than they might of had previously?