Given the last few posts I'm almost wishing for a particularly austere winter to see if Wall Browns respond in 2015.
I'm definitely minded to pay particular attention to this species next year. I've yet to see one more than 5 miles inland off the south coast of Wales but it may be a good idea to factor in a few sorties on the hillsides north of Swansea next year to see if they are present.
Decline of the Wall - fascinating article.
Re: Decline of the Wall - fascinating article.
If the Wall is being adversely affected by unusually warm winter temperatures then why is it doing so well on the coast, where winter temperatures are usually warmer than inland?
Re: Decline of the Wall - fascinating article.
But they have cooler summers so the theory goes they are less likely to produce the 'suicide' third brood.Matsukaze wrote:If the Wall is being adversely affected by unusually warm winter temperatures then why is it doing so well on the coast, where winter temperatures are usually warmer than inland?
Some addictions are good for the soul!
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Essex Bertie
- Posts: 246
- Joined: Sat Jul 24, 2010 6:16 pm
- Location: Brentwood, Essex
Re: Decline of the Wall - fascinating article.
I see the suicidal third brood as being about the progeny of the second brood rather than the progeny of the third. This has been the problem in the southern half of the UK for perhaps the last 20+ years. The comparison is with the Wall's closest relative, the Speckled Wood which has a similar life cycle. It matters little to the Speckled Wood how it approaches autumn as it has the dual hibernation outlet of both larvae and pupae. It probably copes with the vagaries of the UK climate similarly whether it ends the summer having achieved 2, 2.5 or 3 broods at any given location. It also spreads its risk with the twin phase of each brood. With the Wall however, it doesn't have the same outlets and the push towards a partial third brood puts it in a difficult position. Yes, there will still be fully- grown larvae present that can enter diapause but this element could be relatively small and the greater proportion could be pupae that would perish in the winter in their entirety (this is the high-mortality scenario - the suicidal brood). If the third brood adult emergence is particularly weak, then there may only be a predominance of a few males and very poor breeding success. A short run of years of this dilemma may be overcome by a return to the more stable 2-brood state. but if this has been happening fairly consistently since about 1990, then local and then regional extinctions are the result. Some coastal locations appear to benefit the Wall with a cooling effect putting the breaks on development and keeping it closer to 2 broods. And as others have pointed out, coastal locations often afford primary habitat and long continuity of habitat that might encompass subtly different microclimates that allow diversity of phenology. But a contracting population would always retreat to the optimum habitat whether this be inland or coastal and irrespective of how climate change will affect this location. So some coastlines with good but homogenous habitat can become barren. This is so on the 'Dengie Coast' in Essex (15 miles of flat arable meeting sea wall) where a friend of mine counted 10,000 Walls on one day in 1976. Last year, I saw one and that was a surprise!
The apparent full third brood (mainly South Downs) this year is the next phase of the conundrum. What if there are no second brood progeny larvae left and any remaining pupae perish? We are then left with Neil's scenario of being totally reliant on the success of the tiny larvae from the third brood. (Sorry, I haven't read the Belgian Study so perhaps they are looking at Walls in a more advanced state than in the UK). This full third brood certainly seems to be a unique occurrence in Essex this year, perhaps on the South Downs key sites too. We await next spring with heightened interest.
The apparent full third brood (mainly South Downs) this year is the next phase of the conundrum. What if there are no second brood progeny larvae left and any remaining pupae perish? We are then left with Neil's scenario of being totally reliant on the success of the tiny larvae from the third brood. (Sorry, I haven't read the Belgian Study so perhaps they are looking at Walls in a more advanced state than in the UK). This full third brood certainly seems to be a unique occurrence in Essex this year, perhaps on the South Downs key sites too. We await next spring with heightened interest.
Re: Decline of the Wall - fascinating article.
Bob, I have re-read your previous post 
Bertie, a very different take on the subject suggesting the progeny of the second brood could be a factor of the suicidal third brood - I like it
Distribution maps on Richard Fox's article show the decline more prominent in the warmer inland areas of the country and the South Downs, which is quite near the coast . does not seem to be affected as much if at all, at regional level, by the third brood/lost generation phenomenon in the last few years (maybe longer). The fact that there were insignificant numbers of third brood individuals earlier in this decade in Bob's circuit in East Sussex perhaps supports this although the summers in 2011 and 2012 were too poor anyway to trigger a third generation. However we should be talking about decades (not years) of decline of the butterfly. It will be very interesting what transpires next spring after a full third brood emergence in East Sussex this year.
I somehow believe that a cold winter will be detrimental to the third-brood larvae - sorry Neil
I agree that it is not climate change per se which directly affects Wall populations but implications of it and how it affects ecosystems in general in the countryside.
Peter
Bertie, a very different take on the subject suggesting the progeny of the second brood could be a factor of the suicidal third brood - I like it
Distribution maps on Richard Fox's article show the decline more prominent in the warmer inland areas of the country and the South Downs, which is quite near the coast . does not seem to be affected as much if at all, at regional level, by the third brood/lost generation phenomenon in the last few years (maybe longer). The fact that there were insignificant numbers of third brood individuals earlier in this decade in Bob's circuit in East Sussex perhaps supports this although the summers in 2011 and 2012 were too poor anyway to trigger a third generation. However we should be talking about decades (not years) of decline of the butterfly. It will be very interesting what transpires next spring after a full third brood emergence in East Sussex this year.
I somehow believe that a cold winter will be detrimental to the third-brood larvae - sorry Neil
I agree that it is not climate change per se which directly affects Wall populations but implications of it and how it affects ecosystems in general in the countryside.
Peter
My website: http://www.stevenagebutterflies.co.uk
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Neil Hulme
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Re: Decline of the Wall - fascinating article.
Hi Rob (Essex Bertie),
"Yes, there will still be fully- grown larvae present that can enter diapause but this element could be relatively small and the greater proportion could be pupae that would perish in the winter in their entirety (this is the high-mortality scenario - the suicidal brood)."
Bearing in mind the timing at which any second brood larvae might make the decision to pupate (indicated by the timing of emergence of third brood butterflies when they do appear - see below), I can't see why those pupae would then arrest further development, and attempt to overwinter in this state, given the more-than-adequate temperatures for the pupae to mature and emerge. These pupae must be formed in early September (some even earlier) here on the supposedly less advanced and cooler south coast (although we have to take latitude into account, irrespective of 'coast versus inland'). I think that once the thermally triggered decision to pupate (rather than diapause) has been made, the die has been cast, and those individuals will go through to emerge, whatever the outcome. The species is programmed to diapause in the larval stage. It is for this reason that I think the problems associated with the "development trap" are more likely to effect the progeny of the third brood butterflies.
Looking at the timing of the 2014 third brood emergence here in Sussex, it had reached peak by 25th September (34 adults) at Mill Hill, with plenty of egg laying observed. Third brood butterflies would have started emerging here before 20th September. Bob counted 12 Wall on 19th September at Seaford, having seen some the previous day. We also have records of third brood Wall at Cuckmere Haven on 17th September, a pair at Lancing on 14th September, and a “pristine” Wall at Seaford on 7th September.
In a previous year (2011) when we saw a significant third brood, 10+ were seen at High and Over on 29th September, 11 (including a mating pair) were seen in the area on 23rd September, and 3 were seen at High and Over on 15th September.
Given these timings on the less advanced coastal area (hence the less frequent third brood), it seems to me that pupae arising from second brood butterflies (early September) will always have enough time to produce third brood adults, which can generate at least early instar larvae. I think it is these small larvae which might face problems during diapause. It would take a catastrophic collapse of the weather from mid/late September onwards (prolonged very cold conditions) to prevent the pupae from hatching – this being a period which has been getting gradually warmer.
Going on to Peter’s point that “I somehow believe that a cold winter will be detrimental to the third-brood larvae”, we shouldn’t forget that the majority of our species diapause in this stage. There is a very large body of historical data to suggest that cold winters are generally advantageous to our butterflies. From what I’ve witnessed over the years ... the colder the better!
I suspect that this thread will run for a while yet – it’s one of the most interesting I’ve seen on UKB. However, it’s time to get some Christmas shopping done (or I won’t survive the winter), so it will be a while before I can contribute again (she’s watching me).
BWs, Neil
"Yes, there will still be fully- grown larvae present that can enter diapause but this element could be relatively small and the greater proportion could be pupae that would perish in the winter in their entirety (this is the high-mortality scenario - the suicidal brood)."
Bearing in mind the timing at which any second brood larvae might make the decision to pupate (indicated by the timing of emergence of third brood butterflies when they do appear - see below), I can't see why those pupae would then arrest further development, and attempt to overwinter in this state, given the more-than-adequate temperatures for the pupae to mature and emerge. These pupae must be formed in early September (some even earlier) here on the supposedly less advanced and cooler south coast (although we have to take latitude into account, irrespective of 'coast versus inland'). I think that once the thermally triggered decision to pupate (rather than diapause) has been made, the die has been cast, and those individuals will go through to emerge, whatever the outcome. The species is programmed to diapause in the larval stage. It is for this reason that I think the problems associated with the "development trap" are more likely to effect the progeny of the third brood butterflies.
Looking at the timing of the 2014 third brood emergence here in Sussex, it had reached peak by 25th September (34 adults) at Mill Hill, with plenty of egg laying observed. Third brood butterflies would have started emerging here before 20th September. Bob counted 12 Wall on 19th September at Seaford, having seen some the previous day. We also have records of third brood Wall at Cuckmere Haven on 17th September, a pair at Lancing on 14th September, and a “pristine” Wall at Seaford on 7th September.
In a previous year (2011) when we saw a significant third brood, 10+ were seen at High and Over on 29th September, 11 (including a mating pair) were seen in the area on 23rd September, and 3 were seen at High and Over on 15th September.
Given these timings on the less advanced coastal area (hence the less frequent third brood), it seems to me that pupae arising from second brood butterflies (early September) will always have enough time to produce third brood adults, which can generate at least early instar larvae. I think it is these small larvae which might face problems during diapause. It would take a catastrophic collapse of the weather from mid/late September onwards (prolonged very cold conditions) to prevent the pupae from hatching – this being a period which has been getting gradually warmer.
Going on to Peter’s point that “I somehow believe that a cold winter will be detrimental to the third-brood larvae”, we shouldn’t forget that the majority of our species diapause in this stage. There is a very large body of historical data to suggest that cold winters are generally advantageous to our butterflies. From what I’ve witnessed over the years ... the colder the better!
I suspect that this thread will run for a while yet – it’s one of the most interesting I’ve seen on UKB. However, it’s time to get some Christmas shopping done (or I won’t survive the winter), so it will be a while before I can contribute again (she’s watching me).
BWs, Neil
Re: Decline of the Wall - fascinating article.
With relatively warm weather persisting this year until the end of November in Southern England, including Sussex, is it possible that the offspring of third brood adults (of which I saw several myself at High & Over and at Cuckmere on 21st September) might actually get quite a way into their development before chillier days trigger diapause? If so, next year's first brood might be relatively unaffected, assuming an "average" winter thereafter?
Dave
Dave
Re: Decline of the Wall - fascinating article.
Might well do, Dave. Over the last decade or so there has often been a mild winter followed by a cold spring so is it possible that a particularly mild spell in winter will trigger caterpillars out of diapause and therefore make them more vulnerable to predation or starvation? If we then get a cold spell of weather will the surviving caterpillars be able to enter another diapause? If we make the assumption that third-brooded larvae of the Wall are less developed than second-brooded ones when entering diapause there may well be a higher mortality rate for third-brooded individuals in the above scenario.millerd wrote:With relatively warm weather persisting this year until the end of November in Southern England, including Sussex, is it possible that the offspring of third brood adults (of which I saw several myself at High & Over and at Cuckmere on 21st September) might actually get quite a way into their development before chillier days trigger diapause? If so, next year's first brood might be relatively unaffected, assuming an "average" winter thereafter?
Dave
A lot of unanswered questions ....
Peter
Last edited by peterc on Mon Dec 29, 2014 6:40 am, edited 2 times in total.
My website: http://www.stevenagebutterflies.co.uk
Re: Decline of the Wall - fascinating article.
Well, it looks like I will have to try to find some larva/pupa to see how things are going!! If I get the chance I will have a go and report back. Might be a good excuse to get out the house next Thursday!!
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Essex Bertie
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- Location: Brentwood, Essex
Re: Decline of the Wall - fascinating article.
I welcome the Christmas pressie break – gives me a chance to think up an answer to Neil!
My understanding from the Maes et al Study is that they believe that the decision to diapause or develop is made ‘early doors’. So, whilst Neil’s Mill Hill 2014 scenario is fine in terms of the time line and weather that we had this September/October, this is a bit ahead of the general South of England scenario over the past 20+ years. I think the problem in the 1990s and 2000s has been with larvae that made the decision to pupate say in early September, but didn’t achieve pupation until the end of September and then failed to emerge mid-October onwards. I suppose that I exaggerated for effect the high pupal mortality scenario in my previous posting, but it is possible to see how a generation can lose a certain proportion with unfortunate timing. These might be sites that experienced a small partial third brood in the first week of October, or in 2014, this might be the situation at the north-facing sites of the South Downs – e.g. Steyning Rifle Range where there was ‘nearly’ a third brood emergence. Hats off to Bob if he is willing to conduct a pupae hunt at one of these sites!
Here’s another thought; The Speckled Wood has a slightly more northern distribution than the Wall Brown. We know that the Speckled Wood can overwinter in the UK in 2 states, but is there a point further north where it can only survive in the larval state? Conversely, being a slightly more southern butterfly, is there a climate where the Wall can also overwinter in the pupal state, and could such microclimates exist on some of the UK coastline where warm seas suppress hard frosts?
There is an example of a successfully overwintering Wall pupa in Acworth’s ‘Butterfly Miracles and Mysteries’ 1947, p160 third brood 1893, but no further details are given.
My understanding from the Maes et al Study is that they believe that the decision to diapause or develop is made ‘early doors’. So, whilst Neil’s Mill Hill 2014 scenario is fine in terms of the time line and weather that we had this September/October, this is a bit ahead of the general South of England scenario over the past 20+ years. I think the problem in the 1990s and 2000s has been with larvae that made the decision to pupate say in early September, but didn’t achieve pupation until the end of September and then failed to emerge mid-October onwards. I suppose that I exaggerated for effect the high pupal mortality scenario in my previous posting, but it is possible to see how a generation can lose a certain proportion with unfortunate timing. These might be sites that experienced a small partial third brood in the first week of October, or in 2014, this might be the situation at the north-facing sites of the South Downs – e.g. Steyning Rifle Range where there was ‘nearly’ a third brood emergence. Hats off to Bob if he is willing to conduct a pupae hunt at one of these sites!
Here’s another thought; The Speckled Wood has a slightly more northern distribution than the Wall Brown. We know that the Speckled Wood can overwinter in the UK in 2 states, but is there a point further north where it can only survive in the larval state? Conversely, being a slightly more southern butterfly, is there a climate where the Wall can also overwinter in the pupal state, and could such microclimates exist on some of the UK coastline where warm seas suppress hard frosts?
There is an example of a successfully overwintering Wall pupa in Acworth’s ‘Butterfly Miracles and Mysteries’ 1947, p160 third brood 1893, but no further details are given.
Re: Decline of the Wall - fascinating article.
Just for those that do not check out my PD here is a shot I took from March 3rd 2014. These 2 Wall larva were feeding together at High and Over, it just shows the difference in size. Both would have overwintered as larva but would probably have been very different sizes all over the winter. The strong 1st brood here at High and Over were emerging from April 14th until well into June. Although I find the larva quite easily the pupa are almost impossible to find so although I always hope to find an over-wintering pupa it is unlikely to happen. The other interesting thing is that the 2nd brood is always much larger than the 1st brood so one assumes that the mortality rate over the winter is much greater than over the summer!!
- Wall Brown larva. 3/3/2014.
Re: Decline of the Wall - fascinating article.
With the weather appearing very mild today I decided just to spend 30 or so minutes Wall Brown larva hunting on my usual hillside to see if there was any evidence of the butterfly suffering as this thread has discussed. Although it was quite breezy the slope where I was searching was protected from the wind. The first small area I tried drew a blank so I then moved to a lower part of the bank where I found several last year. In the same clump of grass where the 2 larvae on the photo in the previous post I found a very small Wall Brown larva. This was not much more than half a centimetre long but was very distinctly a Wall Brown. I then moved around a few more clumps before finding another one which was bigger and around 1 centimetre long. I also found another larva which was very tiny and was almost certainly a Marbled White. It looked very much like a tiny version of those from last year.
Obviously this little survey doesn't prove anything but I was surprised to find these 2 larvae quite easily, and this early in the year, so just maybe signs are good for another good Wall Brown season? The only other signs of life in the tussocks were a few spiders and some really small frog hoppers.
Obviously this little survey doesn't prove anything but I was surprised to find these 2 larvae quite easily, and this early in the year, so just maybe signs are good for another good Wall Brown season? The only other signs of life in the tussocks were a few spiders and some really small frog hoppers.
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Neil Hulme
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Re: Decline of the Wall - fascinating article.
Good work Bob! These must be larvae out of third brood Wall, bearing in mind that second brood caterpillars would have had bags of time to reach final instar. There would be no sense in a second brood caterpillar entering diapause early, when conditions remain highly favourable, just to enter the winter period without optimum fat reserves on board.
I'm particularly interested in what they were doing. Where were they positioned in the grass (high/low)? Was there any sign of feeding damage on the grass stems (or adjacent stems) on which they were seated? If you haven't recorded this I'm afraid you'll have to ask Pen to turn the dinner down, and head back up there with a torch.
I'm sure you've marked their location and by tracking them through the winter (and the others you will surely find!), you might very well provide some highly valuable data. Keep up the good work.
BWs, Neil
I'm particularly interested in what they were doing. Where were they positioned in the grass (high/low)? Was there any sign of feeding damage on the grass stems (or adjacent stems) on which they were seated? If you haven't recorded this I'm afraid you'll have to ask Pen to turn the dinner down, and head back up there with a torch.
I'm sure you've marked their location and by tracking them through the winter (and the others you will surely find!), you might very well provide some highly valuable data. Keep up the good work.
BWs, Neil
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Essex Bertie
- Posts: 246
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- Location: Brentwood, Essex
Re: Decline of the Wall - fascinating article.
I'm amazed that you can find such small larvae. Very well done, Bob.
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Butterflysaurus rex
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Re: Decline of the Wall - fascinating article.
Bob, I must try my local spot where I got lucky again, if I find any larvae at this location the signs would look very good for a healthy first brood population. However getting out there to look is not easy for me at the moment!
It's all completely true - except for the small comb, the very large magnifying glass and the tweezers.
ATB
B'saurus
I've seen Bob in action, he has eyes like a hawk, with his small comb (for the grass) a very large magnifying glass & a pair of rubber tipped tweezers he never fails.Essex Bertie wrote:I'm amazed that you can find such small larvae. Very well done, Bob.
It's all completely true - except for the small comb, the very large magnifying glass and the tweezers.
ATB
B'saurus
Re: Decline of the Wall - fascinating article.
Hi Neil. The first Wall larva was about an inch above ground laying along the blade of grass. The larger larva was higher up the grass, about 3 inches. Signs of nibbling was evident in the area. The largest larva was a similar size to the first larva I found last year at the end of January. This one would have been from a 2nd brood as there was no 3rd brood to talk about in 2013!!
Hope this helps, and yes, of course I will continue to monitor!!
And yes, Pen might let me out with her tweezers!!
Hope this helps, and yes, of course I will continue to monitor!!
And yes, Pen might let me out with her tweezers!!
Re: Decline of the Wall - fascinating article.
This is highly interesting stuff, Bob.
I'm amazed at how you can locate larvae in deepest midwinter.
I'm amazed at how you can locate larvae in deepest midwinter.
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Neil Hulme
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Re: Decline of the Wall - fascinating article.
"I've seen Bob in action, ......, with his small comb ...... a very large magnifying glass & a pair of rubber tipped tweezers he never fails". I used to go to those clubs in London!
Hi Bob,
"The largest larva was a similar size to the first larva I found last year at the end of January. This one would have been from a 2nd brood as there was no 3rd brood to talk about in 2013!!"
I would exercise caution in assigning the (recent) larger larva to the second brood, on the basis that it is similar in size to the smaller, probable second brood cat you found in early 2014, as the 2013 second brood ran very late (precluding any third brood), leaving far less time for all of that batch of larvae to develop before the onset of winter. A late 2013 second brood cat could easily be in a similar state of maturity to an early 2014 third brood cat. I think it's important to look at the size of any overwintering larvae in relation to the timing of either the second or third brood adults. In fact I suspect that this is precisely where difficulties for the species might arise.
If you can find a small cat covered in mould I'll give you a fiver!
BWs, Neil
Hi Bob,
"The largest larva was a similar size to the first larva I found last year at the end of January. This one would have been from a 2nd brood as there was no 3rd brood to talk about in 2013!!"
I would exercise caution in assigning the (recent) larger larva to the second brood, on the basis that it is similar in size to the smaller, probable second brood cat you found in early 2014, as the 2013 second brood ran very late (precluding any third brood), leaving far less time for all of that batch of larvae to develop before the onset of winter. A late 2013 second brood cat could easily be in a similar state of maturity to an early 2014 third brood cat. I think it's important to look at the size of any overwintering larvae in relation to the timing of either the second or third brood adults. In fact I suspect that this is precisely where difficulties for the species might arise.
If you can find a small cat covered in mould I'll give you a fiver!
BWs, Neil
Re: Decline of the Wall - fascinating article.
Bear in mind also Neil that the 2nd brood went on for roughly 2 months and the 3rd brood for over 1 month so larva from the ova laid at the front end of each brood will vary in size from the latter ova from each. Basically, I don't think it would be easy to say which brood any larva is from with any certainty!! Once again the above picture shows the possible difference in size of larva that come from the same brood. What I would really like to find is an overwintering pupa. Highly unlikely as there probably isn't one!! Also the pupa is by far the hardest stage to find even in the main time to find them.
I hope last night they went deep in the roots as there was a heavy frost here.
If you fancy having a search for some come over in a mild spell.
I hope last night they went deep in the roots as there was a heavy frost here.
If you fancy having a search for some come over in a mild spell.
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Neil Hulme
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- Joined: Fri Feb 08, 2008 8:27 pm
Re: Decline of the Wall - fascinating article.
Hi Bob,
I think you are spot-on in highlighting the potential difference in the size of Wall caterpillars from either second or third brood butterflies, and I believe this is a key issue to consider in trying to determine the problems that the species appears to be facing. However, I also maintain that the size of any over-wintering larvae will usually tell us which brood it hails from, IF we consider this in relation to the timing of either the preceding second or third brood adults. The images you showed us of caterpillars taken on 3rd March 2014 MUST be caterpillars derived from second brood adults, as there was no recorded third brood in 2013. Without seeing the beasts in the flesh, I am fairly confident we are looking at a fourth instar (near full grown) and third instar caterpillar.
Going back to your recent find, I feel that these MUST both be third brood caterpillars, irrespective of the fact the largest is a similar size to the smaller cat of 3rd March 2014. From your description they sound very much like second and third instar larvae. Bearing in mind that the 2014 second brood flew from (at minimum - BC Sussex website) 8th July to 10th August, I find it inconceivable that any caterpillars from that batch would not have comfortably achieved 4th instar status before the diapause period (which may of course be broken diapause, dependent upon weather conditions and stage of development). As I said in my last post, there would be no sense in a second brood caterpillar entering diapause very early (e.g. third instar), when conditions remain highly favourable, just to enter the winter period without optimum fat reserves on board. I just can't buy that. This stage of the development (reaching 4th instar) will only require c. 3.5 - 4 weeks in favourable conditions, so there was ample and more time for all 2014 second brood larvae to reach this optimum over-wintering stage (hence a strong third brood, triggered by warm weather).
Returning to your point about the quite variable size of caterpillars from either second or third brood adults, this is an important issue, and we are always going to be talking about size range and percentages. However, this variability will be far more important in caterpillars produced by third brood adults, than by second brood adults. Earlier second brood caterpillars will reach the 4th instar stage and (if they are not thermally induced into pupating and producing third brood adults) go into the diapause phase. Later second brood caterpillars will lag behind, but still have plenty of time to fully catch up in good years. However, even if they get caught out by rapidly deteriorating autumn weather, the vast majority will still comfortably achieve 3rd instar. Even this slightly less favourable scenario will probably result in a relatively good survival rate if my theory holds any water (of course it could be a load of old tripe!).
If we look at the situation where a strong third brood has occurred, the spread of caterpillar ages becomes more influential. Early third brood caterpillars should be able to reach at least 3rd instar size, and in favourable years, 4th. However, a potentially significant proportion of later third brood caterpillars could get caught out by rapidly deteriorating autumn weather, leaving them to enter the diapause phase during the 2nd instar, or in the worst case scenario, 1st.
This is where I see the problem potentially lying. These smaller caterpillars can be left facing the winter period with low fat reserves, causing them to become more active during a subsequently warm and humid winter. It might be susceptibility to moulds, or pathogens, or predation, but I suspect that mortality rates are likely to soar.
I agree with Dave Miller, who suggested that the increasing length of favourable autumn conditions might see an ever increasing proportion of third brood caterpillars achieving 4th or near 4th instar size, before environmentally triggered diapause. I suspect that this will be the saving grace for the Wall in the medium to longer term. To me it looks like evolution in action, with a potential long term benefit being balanced by shorter term suffering.
It is important to emphasise that we are all speculating (which is entirely healthy), and we are not going to know the real situation until more research is conducted. Which is where Bob comes in. If I can find time to get over there and help find a few more, I will.
BWs, Neil
I think you are spot-on in highlighting the potential difference in the size of Wall caterpillars from either second or third brood butterflies, and I believe this is a key issue to consider in trying to determine the problems that the species appears to be facing. However, I also maintain that the size of any over-wintering larvae will usually tell us which brood it hails from, IF we consider this in relation to the timing of either the preceding second or third brood adults. The images you showed us of caterpillars taken on 3rd March 2014 MUST be caterpillars derived from second brood adults, as there was no recorded third brood in 2013. Without seeing the beasts in the flesh, I am fairly confident we are looking at a fourth instar (near full grown) and third instar caterpillar.
Going back to your recent find, I feel that these MUST both be third brood caterpillars, irrespective of the fact the largest is a similar size to the smaller cat of 3rd March 2014. From your description they sound very much like second and third instar larvae. Bearing in mind that the 2014 second brood flew from (at minimum - BC Sussex website) 8th July to 10th August, I find it inconceivable that any caterpillars from that batch would not have comfortably achieved 4th instar status before the diapause period (which may of course be broken diapause, dependent upon weather conditions and stage of development). As I said in my last post, there would be no sense in a second brood caterpillar entering diapause very early (e.g. third instar), when conditions remain highly favourable, just to enter the winter period without optimum fat reserves on board. I just can't buy that. This stage of the development (reaching 4th instar) will only require c. 3.5 - 4 weeks in favourable conditions, so there was ample and more time for all 2014 second brood larvae to reach this optimum over-wintering stage (hence a strong third brood, triggered by warm weather).
Returning to your point about the quite variable size of caterpillars from either second or third brood adults, this is an important issue, and we are always going to be talking about size range and percentages. However, this variability will be far more important in caterpillars produced by third brood adults, than by second brood adults. Earlier second brood caterpillars will reach the 4th instar stage and (if they are not thermally induced into pupating and producing third brood adults) go into the diapause phase. Later second brood caterpillars will lag behind, but still have plenty of time to fully catch up in good years. However, even if they get caught out by rapidly deteriorating autumn weather, the vast majority will still comfortably achieve 3rd instar. Even this slightly less favourable scenario will probably result in a relatively good survival rate if my theory holds any water (of course it could be a load of old tripe!).
If we look at the situation where a strong third brood has occurred, the spread of caterpillar ages becomes more influential. Early third brood caterpillars should be able to reach at least 3rd instar size, and in favourable years, 4th. However, a potentially significant proportion of later third brood caterpillars could get caught out by rapidly deteriorating autumn weather, leaving them to enter the diapause phase during the 2nd instar, or in the worst case scenario, 1st.
This is where I see the problem potentially lying. These smaller caterpillars can be left facing the winter period with low fat reserves, causing them to become more active during a subsequently warm and humid winter. It might be susceptibility to moulds, or pathogens, or predation, but I suspect that mortality rates are likely to soar.
I agree with Dave Miller, who suggested that the increasing length of favourable autumn conditions might see an ever increasing proportion of third brood caterpillars achieving 4th or near 4th instar size, before environmentally triggered diapause. I suspect that this will be the saving grace for the Wall in the medium to longer term. To me it looks like evolution in action, with a potential long term benefit being balanced by shorter term suffering.
It is important to emphasise that we are all speculating (which is entirely healthy), and we are not going to know the real situation until more research is conducted. Which is where Bob comes in. If I can find time to get over there and help find a few more, I will.
BWs, Neil