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.