Benjamin

[Benjamin]

With all the survivors now feeding this is the survival data for this year:

11/20 Nabokovians survived - very good compared to previous years. The wider sample is also pretty positive so a good winter for iris at my site.

19/20: 14/47 - 29.8%
20/21: 13/35 - 37.1%
21/22: 12/44 - 27.3%
22/23: 7/20 - 35%
23/24: 19/42 - 45.2%

The first L4 appeared some time around the 10th of April and several others were skin changing when I visited a few days ago. This is very early and in line with 2020 which looked like producing a May(!) butterfly before the weather collapsed. As we know, something usually happens to hold them back. Quite interesting that the lack of sun hasn’t slowed them - fairly warm weather and early leafing has got them off to a flyer. If we finally get some settled sunny weather and it stays warm then records could be under threat……

L4

L4

L3s

In terms of development they’re all fairly close together this year. I’m used to seeing a 4 week gap between the earliest and the latest, but this is due to some well used sallows being particularly late leafing. As nothing survived on these trees this winter, there are no late developers to create the usual gap. As mentioned previously, even when a big developmental gap occurs initially, it gradually reduces as late starters rattle through the instars much quicker in warmer conditions. By the flight period 4-6 week initial gaps seem to be down to just a couple of weeks in most cases. This is the kind of thing that happens (which I realise it very obvious but I find useful anyway!):

Development

[Benjamin]

I should just update my diary regarding small tortoiseshell and Sturmia bella. Having looked into their relationship closely a couple of summers ago I was amazed and excited to discover recently that the crucial missing piece of the puzzle (the overwintering habits of S.bella) was not missing at all!

Although it is virtually impossible to find any reference to this in the literature, and it was not mentioned in any of the papers that were published after the original Oxford Uni project, it seems that S.bella has been recorded as overwintering in large white pupae in the UK on several occasions since as far back as 2009. Having now corresponded with the original recorder of this behaviour it is clear that those directing the original study were made aware of this and were involved in identifying the emerging fly larvae and resulting puparia.

Why this behaviour wasn’t highlighted in the reports that followed I’m not sure. It seemed to me from the outside that the study set out to try to understand the potential impact the parasitoid might be having on the host, and that to do this a deeper understanding of how the ecology of the parasitoid and the host were intertwined must have been an obvious aim. The lack of knowledge of a crucial part of the fly’s ecology (overwintering strategy) seemed like a significant barrier to a comprehensive understanding of the situation. This ‘missing piece of the puzzle’ was one of the motivating factors that led me to spend so much time looking into this in the first place.

It now seems likely that large white is just one of many overwintering hosts for S.bella, and this combined with the fact the data was only recorded from a single locality may be why it was not considered particularly significant. That may be fair enough, but just knowing that the fly was able to switch from targeting nymphalids to pierids would have made a big difference to the way I went about my own study.

At the time of my study I was convinced the primary host would be a nettle feeding moth that overwinters as a part grown larva. There are a few common species that fit the bill and are often present amongst small tortoiseshell/peacock larvae when they are being targeted by S.bella females. These moth larvae must ingest S.bella eggs and would keep the developing fly larva locked up inside them until they attempt to complete their development the following spring/summer. This would explain why I didn’t find a single S.bella in any of the 1st brood small tortoiseshells that I collected from multiple locations across Sussex.

It now seems even more likely that this is occurring, but it isn’t as crucial a piece of the puzzle. S.bella clearly has a wide range of routes through winter and will no doubt utilise many of them each year. Any route may be more or less significant in a particular year or in a particular area based upon timings of available hosts and numbers present.

Although, as mentioned, I never found S.bella in first brood small tortoiseshell it is clear that this must occur. Any S.bella emerging from overwintering pierid hosts should time pretty well with first brood small tortoiseshell so I guess the reason I missed this in my sampling must simply be that at this time of year numbers are very small. Those that do emerge from pierid pupae early in spring will begin to build numbers using their spring hosts and will then be joined by subsequent waves being released from their later developing overwinter hosts as the season progresses. The picture is clearly one where S.bella builds numbers rapidly through the year from a very low base of overwintered individuals.

Although extremely interesting I don’t think this revelation impacts the conclusions I came to a couple of years ago. Small tortoiseshell struggles in areas where summer droughts cause nettle desiccation - this much at least is obvious. The role that S.bella plays in exacerbating the problem was always more open to interpretation, but it does seem clear that its impact will be most pronounced in areas where a significant percentage of the small tortoiseshell population is driven towards breeding opportunities along waterways and in reliably damp areas. It is in these areas that S.bella will be found in large summer concentrations having built numbers explosively from a low post winter base.

In wetter parts of the country where the summer generation of small tortoiseshell is able to fly widely throughout the landscape looking for suitable nettle growth, S.bella will often find itself playing catch up - searching the landscape for hosts. No doubt it will often find them, but with the population so well dispersed there is not the opportunity to build numbers like they can with concentrated populations.

So the outlook for small tortoiseshell in the south east is clearly pretty grim. Any gradual warming and drying of the summer landscape will lead to an increase in the ratio of poor to good years for this species. It will be increasingly driven to the ever shrinking ‘best areas’ where unfortunately it now encounters its new enemy - a generalist parasitoid that becomes more numerous throughout the year and exploits each subsequent generation of small tortoiseshell with increasing ferocity.

To me it seems unlikely that small tortoiseshell will be able to respond effectively by adapting its overwintering timing as overwintering conditions in this region are likely already far from optimal and will also worsen in the face of climate change. I would suggest, therefore, that there is little room for manoeuvre here.

But for now at least, we (in the south east) can still enjoy small tortoiseshell in modest numbers (especially if we know where to look), and can expect any run of unusually wet summers to produce the well known booms that the species is known for. The underlying trend, though, is pretty clear.

I will just add some photos I took a couple of years ago whilst collecting small tortoiseshell and peacock to analyse parasitoids. This was taken right in the middle of my site - extensive riverside nettle beds covered in peacock and small tortoiseshells. How the penny didn’t drop I’ll never know……

[trevor]

HI Ben,
Sad to relate that so far this year I have seen just two Small Tortoiseshells,
both on a farm at Halnaker, West Sussex, where they hibernate in the roof
spaces of the barns and outbuildings. In the past when ST's had a good Summer
the previous year, every Dandelion in a neighbouring meadow was host to an ST
fresh out of hibernation, usually at the end of March, early April. But not this year.

[millerd]

That is fascinating information, Ben. It accords with my own experience of Small Tortoiseshells locally, where sightings now are almost all confined to areas next to the River Colne where the nettles remain lush all year, even in the bouts of searing heat we have had almost every summer recently. Numbers have steadily dwindled year on year as you say.

However, how does this fit with the continuing excellent numbers of Peacocks? I know they breed slightly later, and use more mature nettles than the Small Tortoiseshells do, but a lot of their other strategies would appear to be the same. Certainly last summer produced amazing numbers of Peacocks, and many of them seem to have survived the winter. As for sightings, Peacocks outnumber Tortoiseshells by about 25 to one now.

Red Admirals also tend to lay on (sometimes very) small nettle plants, but I suppose that by not laying batches of eggs they are not parasitised in the same way. In any case numbers are always boosted by immigration from the Continent, so population fluctuations are trickier to analyse.

The other nettle feeder, the Comma, is also not straightforward. It appears to have two staggered parallel cycles of three generations every two years (though it's probably not quite as simple as that!) and the summer generation of larvae also feed on hops and elm so laying females can avoid dried up nettles if need be. I've not read anything about their parasitoids, but I assume they are targeted by the same ones.

A mix of observation and speculation, Ben - I'd be interested in any thoughts you have...

Cheers,

Dave

[Benjamin]

Hi Trevor - unfortunately that’s probably fairly representative of the wider Sussex countryside right now. Most of these areas just don’t have lush summer nettle beds to support a big second or third brood. Faced with poor quality nettles I guess any summer brood STs emerging in these areas leave in search of something better - further north or into the reliably wet areas where they meet high concentrations of parasitoids.

Hi Dave - I agree it’s really interesting to contrast the fortunes of the various nettle feeders. I think, as you suggest, timing is everything with peacock. Being single brooded they have no problem with nettle desiccation and avoid the worst of the parasitoid load.

As they complete their development before any nettle desiccation they are able to exploit nettles throughout the landscape and (as was once the case with ST) can therefore be found everywhere.

They also have a much easier ride when it comes to the parasitoids. A common parasitoid of spring ST and peacock is the tachinid Pelatachina tibialis - I imagine you know this but I’ll lay out my thinking anyway…..

This is a univoltine tachinid that overwinters as a pupa. Its emergence in spring generally seems to time with ST and certainly it is the most common parasitoid of spring ST. I found around 22% of spring ST larvae collected at L4/5 contained this parasitoid. This was across 5 sites in Sussex and totalled around 400 caterpillars. Some later emerging P.tibialis do use peacock (as pictured) but are found in significantly lower numbers. Of 300 peacock L4/5 caterpillars collected across 3 sites I found around 9% contained this parasitoid.

Pelatachina tibialis and peacock

It is clear that P.tibialis times best with ST but, as we would expect with a parasitoid largely reliant on just one or two species, it doesn’t over exploit its host and numbers seem to have settled at a manageable level. Nevertheless it is a clear win for peacock timing over ST.

It is the relatively recent arrival of S.bella that causes more significant problems however - especially in the damp areas where summer brood ST is increasingly forced to concentrate. According to the sampling in my area S.bella is still at relatively low numbers when peacock caterpillars are developing. When I looked at this closely I counted over 100 peacock nests along a 500m stretch of perfect riverside habitat. If just 5% of peacocks in this stretch host S.bella then many hundreds of S.bella will emerge from the peacock brood and time perfectly to exploit the summer generation of ST.

Peacocks can cope perfectly well with low level parasitoid loading such as this, even when combined with P.tibialis, but the knock on effect is devastating for summer ST. It is unfortunate that S.bella seems (in these areas at least) to have linked the fortunes of peacocks and STs - with greater peacock abundance leading to yet more misery for summer brood ST. When collecting L5 summer generation ST from this site S.bella loading was up to 75%.

This level of exploitation would be (I guess) unsustainable for a specialist parasitoid and it is because of the opportunistic nature of this generalist that it is able to operate in this way. It is effectively able to shift its load around depending on host availability and doesn’t pay the price for over exploitation as a specialist would. This is clearly a very novel situation, with climate change and the introduction of a significant new parasitoid combining to rapidly change the landscape for some of our most familiar butterflies - peacock has largely remained untouched by these significant changes, whereas ST has felt the full force.

As for the other nettle feeders I tend to agree with your suggestions. Certainly I see comma laying on elm here in Brighton more than anything else. I do occasionally find them on sallow when checking on PE cats and no doubt they use nettle when conditions suit. Same with red admiral to a lesser degree - I often find late summer RA on pellitory-of-the-wall. I think the point is that they have plenty of options available and can spread their risk - obviously batch layers have to be so much more fussy - a fussiness that drives ST out of the drying landscape and into parasitoid misery along the waterways.

[Vince Massimo]

Thanks for all of this, Ben, it's fascinating stuff. I have sent you a PM on a related issue.

Cheers,
Vince

[Benjamin]

Quick update from the woods….

It was April 24th yesterday and I couldn’t resist. What does it tell us? Not much - it’s spring.

The ongoing woodland management around my woods is more dramatic however. This has been one of my most productive iris rides over the 5 year period. Although initially quite shocking, the ride was darkening and narrowing every year and will now begin a period of regrowth that will soon see it bounce back as one of the best rides for summer butterflies.

In any other year I would have lost a significant number of my post-winter stable to this management, but this year the inhabitants of this particular ride had already been predated. I initially began the previous sentence with ‘luckily’ but quickly had a rethink. Hmmmm….
‘Luckily no lives were lost to the tsunami as the islanders had all been brutally murdered and eaten by a raiding party of cannibals the week before.’
No - I don’t think we can suggest the inhabitants of that particular ride have had much luck this year, but the population in general is doing rather better. Here are some photos of caterpillars from around the site….

The largest L4s are not far off L5, but they’ll need some warmth and settled weather to make the move. As it stands I’d say they’re nicely on course for a mid June appearance but as always the May weather could shift this one way or the other.

[Benjamin]

It’s been a busy time for hairstreaks. The majority of black hairstreak have now pupated and the white letters are not far behind - in fact I would say that the w-album frontrunners were ahead of the pruni this year.

As mentioned before in my diary it’s very easy to monitor large numbers of w-album down in Brighton. My general impression from several night time walks is that they were very numerous this year, but this could have more to do with the fact my knowledge of their favoured areas and the timing of my searches improves each year.

I always collect a few to observe in captivity and enjoy the process of guaranteeing their safe passage to adulthood. This year however, I’ve collected rather more, and I’ll explain why….

There’s a relatively small young elm right outside my house and over winter I was pleased, but not surprised, to confirm that it had received plenty of eggs. During the flight period the adults were seen every morning jinking around the hedges that divide the front gardens.

Despite a group of wood pigeons biting off the new leaf growth on a daily basis, the caterpillars on the lower branches survived well and were fully grown a week or so ago. I kept an eye on the tree as I wanted to try to follow their route to pupation so on the first warm day we’d had for a while I was very pleased to observe them beginning to descend. Some had already settled on some shoots growing from the base of the trunk.

It soon became apparent however, that this is a particularly perilous time for caterpillars that find themselves on street trees. I had hoped that they might settle in cracks in the bark, or stay very close to the base of the tree but unfortunately they seemed determined to stride out across the surrounding concrete desert. Having turned one around that seemed committed to a road crossing attempt, and with an army of school kids about to be released upon the Brighton pavements I decided I had to switch from observer to collector. I checked up and down my road and was amazed to find caterpillars descending on several of the elms - i have never seen this happen en masse before, but perhaps I haven’t been playing close enough attention.

https://youtu.be/9-W2KTub73c?si=KtkHoCuHfBTIF0EZ

I left the pupating caterpillars at the base of the tree alone, as they had at least successfully navigated the perilous descent, but the following day I was disheartened to find that they had all been predated. I think the problem here is that as they become prepupae they lose their ability to eliminate their UV signal and so suddenly become very visible to foraging birds. We know this because of course the opposite happens when using UV torches at night - the very visible caterpillars suddenly disappear as they stop converting UV light to visible light. If they had a large area of undergrowth at the base of the tree then clearly they would have a better chance of finding somewhere to pupate hidden from view, but with the limited options at the base of a street tree they seemed to be lining themselves up in plain sight - and they weren’t lasting long.

So for the next day or two I decided to collect anything on street trees that lacked any usable habitat at their base - and now I have a lot of white-letter hairstreak pupae and prepupae. They will of course all be released when they emerge.

I’m hoping that this patch of creeping thistle that I’ve planted in the front garden will prove irresistible to the local population. I tried to grow them from seed but it didn’t work so I dug a few up from my local SSSI. Just kidding - it was some waste ground - I think. Anyway - just some creeping thistle - most people try to poison them at great cost to everything else.

In other activity I came across a pruni prepupa that I guess hosted an ichneumonid. Not sure about this but the exit hole is the right kind of shape - but it does look a bit big. Anyway - I thought it made a nice photo in macabre kind of way.

I also came across a very beautiful freshly emerged speckled wood that was the highlight (along with bumping into Dave Miller) of a visit to Rewell Wood to see PBF. The PBF were nice of course but I don’t like the coppice coupe environment - I find it strangely depressing, but that might have been more to do with the hangover I was nursing.

To cheer myself up I later called in to one of the best Sussex downland butterfly sites where it is rare to ever see another soul. This site served up a heavenly mix of spring species and once again all was well with the world.

[trevor]

A fascinating item above Ben. I had never really thought about WLH larvae walking
down the tree trunk to ground level to pupate.
I assume the same goes for Purple Hairstreaks, and would help to explain why I have
never found an adult in my garden. There is a massive oak with about a third of it's
canopy overhanging my garden but the tree itself is beyond my fence! So the guy at
the back of me would get all the adults. I know this oak has a population of PH most
years as I have seen them darting around in the evenings.

PS .The Greens seem to have had a great season.

[David M]

Some great early stage images there, Ben. I suspect your hypothesis is spot on vis-a-vis the bases of the street trees. Good luck with the rearing.

[Benjamin]

Hi Trevor. Yes - purple hairstreaks will do much the same thing - although I have never observed this out in the field. I suspect that they descend at night so a UV search of the trunks of some well used oaks may well be fruitful. I suspect it is the case with big old gnarly oaks that a significant percentage of larvae find a suitable pupation site on their journey down and never make it to the ground.

Thanks David - TBH there’s not much rearing to do as they’re pretty much all ready to pupate. It does give me a good chance to check for parasitoids….

Yesterday I checked on the remaining PE cats spread around Southwater woods. I didn’t spend too long trying to relocate some of the L5s that have given me the slip because I can check on them easily in a few weeks time with the aid of my UV torch.

In summary - they are all now L5 with most of them a couple of weeks away from pupation. The most advanced will pupate next week and is likely to emerge (if it survives) towards the end of the second week of June. The others will follow in the second half of June. As always the weather can still play a part in shifting this a little one way or the other.

[trevor]

OOOH they look nice, plump, and juicy Ben.
Thanks for the update, can't wait!

[Benjamin]

Black-veined white in southern Britain.

With BVW popping up at sites around the UK, and headlines appearing that suggest climate change together with habitat restoration/improvements are rendering much of southern Britain suitable for the re-establishment of this species, I’ve been motivated to spend a few years taking a closer look.

The main papers that deal with this are the M J Carroll one from 2009 (climate modelling) and the recent Chris Thomas paper that is still undergoing peer review (I think).

They rely heavily on climatic modelling and the headline conclusion is that:

“Substantial areas of England are already expected to be amongst the climatically most suitable parts of Europe for this insect”

If this is true then all the other factors should only have to be reasonably good for BVW to thrive. When climatic conditions really are optimal, then all the other dials can be twiddled around however one likes (within reason) and the species in question will most likely bob along quite nicely. Twiddle them favourably and it booms, twiddle them unfavourably and it bobs along until boom time returns. Whatever these other dials happen to be for a particular species (weather, habitat provision, parasitoid load, predation levels etc) as long as the underlying climatic conditions are optimal then this tends to be how things play out.

At the margins of a species range however, where climatic conditions are the limiting factor the situation is far more precarious, and there may well be very limited tolerance for any of the other dials moving away from optimal. That much seems pretty obvious.

My problem with all this for BVW in southern Britain is that I don’t believe that climatic conditions are anything like optimal, and far from improving year on year with climate change I would suggest that overall they’re actually getting worse.

I can well imagine that if one was to stand amongst trial releases of BVW gliding around the never ending scrub of certain pockets of the South Downs on a sunny June day in the middle of a heatwave then it might be very natural to become rather excited about the prospect of a successful reintroduction - but this is not the time of year that presents the most significant challenges. Warmer, sunnier summers and potentially drier autumns will indeed suit BVW - dials that appear to be turning gradually in the right direction, but I’m not convinced the position of these dials was seriously limiting BVW in the UK to begin with.

Winter I think is more important, and this dial is turning the wrong way (warming) and spring is the most important and again, conditions for BVW are worsening.

Having now reared the species in considerable numbers for a run of years and experienced a variety of conditions I’ve come to the same conclusion that many more experienced breeders have long since known - excessive spring rain is a big problem.

Just some colour to brighten up this text……

The way to ensure good survival is to keep them dry and to expose them to as much sun as possible. Keeping them covered or even inside in a position where they still receive sun is very successful. The problem with this is that they develop too quickly under these conditions and are out too early in the year. The adults then have to contend with the back end of our spring weather and are also out in advance of some of their favoured nectar sources which encourages dispersal. The resulting eggs take longer to develop, are subjected to more frequent downpours (which surprisingly can actually wash them off) and have a lower success rate as a result. Keeping overwintering larvae artificially cold (a few degrees above freezing) is ideal, and allows them to be kept dry and to be brought out of hibernation at a much better time (May) but this is hard to achieve in captivity as good airflow is also required.

Rearing this species definitely feels like things are always on a knife edge - with good weather things can go very well but with poor conditions it can all be over very quickly. This doesn’t fill me with confidence.

Clearly BVW has to be able to deal with rain as throughout its range it must develop post-diapause in spring conditions to be ready to fly in the summer. The problems begin, however, with periods of persistent rain when the larvae are once again feeding. They are very prone to virus under these conditions and although I’m no expert in lepidopteran diseases/viruses, I would guess that it probably has something to do with sitting around for days/weeks on end in a damp communal web full of their own faeces.

Anyone who’s ever attempted to rear butterfly/moth species in captivity (especially when humidity is required and plastic boxes are used) will know that hygiene is everything. They cannot be allowed to hang around in moist conditions in close proximity to their own waste for too long or else disaster (in the shape of virus) will surely strike. Species that can be kept in dry conditions (when net cages can be used) are markedly more tolerant to poor sanitation.

What this species really needs to thrive, therefore (at least in terms of post-dispause developmental conditions), is an explosive spring where the regular downpours are quickly dried by a strong sun. The closer this period is to the height of summer, the better. A long cold winter followed by an explosive spring that leads quickly into a hot summer is optimal.

What we have in the UK is a warming winter and an ever earlier (and increasingly slow) spring. The point at which post-diapause larvae are emerging and more crucially are able to begin feeding (leafing time) is becoming earlier and earlier and moving further and further away from the height of summer. This means that post-diapause they have to endure an ever longer and slower development phase where they will regularly find themselves exposed to excessively wet periods where even when the sun breaks through drying takes so long that is is often not completed before the next wet period arrives. These are exactly the conditions that kill them. For this particular condition the dial for southern Britain must be close to, and moving ever closer towards the negative extreme.

Large tortoiseshell and Camberwell beauty suffer under our conditions in the same way (both live in communal webs for most of their development), but do at least have the advantage of developing closer to summer (when the sun is stronger and water is evaporated more quickly) as eggs are laid in spring so larvae are later. The advice when rearing these species is always to avoid excessive rain!

One other potentially important factor worth mentioning for BVW is predation. Nice plump L3 larvae will be one of the first food sources to appear on the scene post winter. However big a problem predation is for BVW (I have no idea) it can only get worse by emerging earlier from the relative safety of their hibernacula and extending the length of the vulnerable development period. Some previous studies have suggested that predation by birds is a very important factor so it is not encouraging that this would again appear to be a dial that is turning in the wrong direction.

It is true that you can find fairly convincing climatic matches around Europe where BVW is present but my guess would be that if we really looked closely they wouldn’t be flourishing in those areas, and I think they’ll really need to flourish to get a proper foothold back here.

As with all reintro attempts under marginal conditions the short term results will largely depend upon what the weather dial does over the reintroduction period, and if we’re fortunate then it could be that they establish quite well. My concern of course is for the longer term prospects where I feel the fact that our spring conditions are ever worsening will eventually take its toll.

Having said all that I’ve just been reading about Hutchinson’s Bank where it appears a thriving colony has now self-established after a single gravid female (presumably having popped in to her local agent de voyage and picked out HB from the many options) touched down a couple of summers ago. If it’s that easy then there’ll be no need for a reintroduction attempt and we can expect to be overrun with them within a few years.

Speaking of the reintroduction attempts - I’m certainly not in any way against giving these a really good go. There are some fantastically knowledgeable people involved (certainly with the official ones and perhaps too with the ‘less official’ ones) with teams of volunteers ready to get stuck into the fieldwork so at the very least these projects will be fascinating and highly informative.

On the positive side I would just say that it is often the case that rearing issues in captivity are not reflected in wild populations where females are able to select the most suitable locations for egg laying. In a year where the vast majority of my captive stock was lost to virus I am encouraged that a small sample of ‘wild’ individuals (don’t ask) developed without issue.

It seems like it’ll be a very interesting few years and if we end up with BVW thriving in the UK once again then I’ll be very happy to be wrong about the importance of spring conditions!

[Allan.W.]

Very interesting post Benjamin ! Just as a matter of interest with the high numbers that you,ve successfully reared ,what do you do with them all ! ..............surely they can,t all be kept for breeding purposes ?.I,d love to see BVW,S flying again in Kent after their absence of a century..........can,t see it happening naturally though. When Swallowtails turned up at St.Margarets (Kent) last season ,i saw a very "Large" White patrolling the massed Hawthorns there .......but it wouldn,t settle ....and eventually tore off towards Dover.
Allan.W.

[Neil Freeman]

An interesting series of posts recently Ben, particulalrlythe last one covering the Black-veined White.

Cheers,

Neil.