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……