Decline of the Wall - fascinating article.

[badgerbob]

Many thanks Neil for your very detailed report and reply. I must confess, my larvae searches each year is simply me satisfying my 'hunting instinct' and trying to look ahead to see what sort of season the Wall is likely to have rather than any scientific reason!! I certainly hope however, to find a few near fully grown larva soon as it is not normally until well into March when I have seen them in this stage before. As you say, last year was far from a normal Wall season so there should be many aspects that could be different this year. What I should do is to visit a North facing slope to see if I can find any larva there and see what state they would be in, although of course, they would probably be in the same stage as a normal year 'South' facing colony would be in. Basically, it is Very Interesting and my hunting continues!!

[Neil Hulme]

Hi Bob,
Good luck, if anyone can find more it's you! If I can find time I will start looking for them myself. It would be a good idea to get a record shot of each, and without disturbing them, try to measure them. It would be useful to record any signs of recent feeding damage, and observe whether the caterpillar appears active.
BWs, Neil

[peterc]

I take back what I said about colder winters having possibly a more detrimental effect on third-brood larvae although of course it may still be true! You have convinced me, Neil

In Hertfordshire, and other inland sites, in the south, the decline started in the early 1990s. After the fine summer of 1990 numbers plummeted in 1991 which is not unusual probably due to the desiccation of the larval foodplant in the drought conditions in 1990. Since 1976, populations have always recovered to reach a new peak in the next hot and dry summer (e.g. 1983 and 1989). However, the hoped-for recovery in the good summer of 1995 did not materialise. I was looking at weather trends, at least in the south-east, and it appears that climate-change had an effect. There was a series of cold winters between 1976 and 1983, and 1984 and 1989 but the winters of 1990 until 1995 were generally mild and wet. Unfortunately though, there is no evidence of third brood individuals in 1976 and 1983 although there were probably some around. There is a record of a Wall on 19 October 1947 in Hertfordshire so the third brood is not just a recent phenomenon.

The consensus seems to be that third-brood specimens are far more likely to emerge in the warmer inland regions so I am now subscribing to the view that third-brood larvae are more vulnerable in the milder and wetter conditions in the winter to predation, pathogens or whatever especially if larvae are coming in and out of diapause. If it is a predator could it have spread north as a result of the warming climate? If numbers are increasing in parts of Sussex could it be that the Wall has adapted in that part of the world assuming that inland Sussex suffered a big decline in numbers previously?

I know this is a very simple hypothesis so I don't claim to be an expert - I'll leave it at that

An article regarding predation of Speckled Wood caterpillars which I am sure will interest a few of you:

http://onlinelibrary.wiley.com/doi/10.1 ... 432.x/full

A suggestion that increased feeding activity by caterpillars increases the risk of predation. What this tells me is that maybe the smaller early instar larvae (i.e. mostly from the third brood) will need to feed more and fatten up before entering or re-entering diapause. The more mature second brooded larvae, when coming out of diapause, will probably be far less mobile as their body reserves are already more or less adequate to last the winter. Being less mobile makes the larvae far less conspicuous to predators.

Bob, you deserve all the luck you can get with the larvae hunting.

Peter

[Jack Harrison]

...Wall on 19 October 1947 in Hertfordshire so the third brood is not just a recent phenomenon.

The consensus seems to be that third-brood specimens are far more likely to emerge in the warmer inland regions...

My early years were on the east Norfolk coast. I took Wall Browns for granted then. I certainly recall seeing a very late Wall, a quite small individual, in (my guess) late September/early October 1947 or 1949, both hot summers. My "bible" South suggested that third brood Walls sometimes occurred after hot summers.

An interesting note in South, perhaps dating from the first edition in 1906 but maybe from a later edition:

Although it [the Wall] now seems to be absent from certain districts in which it was once abundant, it may still be regarded as a generally common species in England and Wales...

This hints at a cyclical abundance so maybe we are simply going through a trough at the moment.

Jaack

[Neil Hulme]

Hi Peter,

My gut feel is that one of the problems potentially facing small, overwintering caterpillars from third brood butterflies is indeed likely to be increased predation, as the drive for them to become more active and feed during warmer spells within the winter will potentially lead to their exposure higher on grass stems. Again only speculation, but one of the main culprits seems likely to be small passerines such as Meadow Pipit. I've watched them feeding on unidentified satyrinid caterpillars on more than one occasion.

Best Wishes, Neil

[Pete Eeles]

Interesting observations and thoughts - what else would you expect from such an enlightened community So ... I wonder why some species overwinter in certain instars - and it's not always first or last. Presumably, there is some advantage otherwise they wouldn't. For those that can overwinter as a larva in any instar, I've not come across any analysis that compares the survival rate of different larval instars - but would be interested if anyone has seen such a thing.

Kind of related - most of our species overwinter as a larva:

http://www.ukbutterflies.co.uk/species_phenologies.php

Cheers,

- Pete

[badgerbob]

One little observation is that the Wall larva are always very slow and sluggish in their movement. Totally different of course to the adult butterfly!! The 2 I found this week were certainly in a feeding mode. The good thing about them being so slow in movement is that they generally stay in the same area for all their growth. Its only when they move to pupation that they leave their feeding area and become almost impossible to find again.

[Neil Hulme]

Hi Pete/all,

For those (numerous) species which overwinter as a caterpillar, there are a number of factors to take into account when looking at the degree of larval maturity which each species attains prior to diapause. Perhaps the most basic is the flight period of the adult butterfly (univoltinism versus variable multivoltinism is another). If a species flies later in the year (which itself is a function of factors such as larval food-plant availability, particularly for non grass feeders), it doesn't suit its individual phenology to be in its late or latest stage of larval development when it becomes active during the first period of prolonged, warm spring weather. It can't then sit around doing very little, and the chrysalid stage cannot last for greatly extended periods at this time of year, due to issues such as desiccation. So the caterpillars of these species must diapause at an earlier stage in their development.

For those species which potentially fly very early in the spring, such as Wall, Small Heath and Dingy Skipper, the larvae really need to be in an advanced stage of development at the end of winter. Although there might be a spread of sizes/instars, giving rise to a quite protracted flight period in the first brood, it cannot be too much of a spread, or some individuals will 'miss the boat'.

So the overwintering caterpillars of various species will range from immature to mature. Immature overwintering caterpillars of the majority of these species will not be facing the potential problems we have been discussing for Wall. These species have of course each developed their own larval survival strategies for the winter months. The univoltine Purple Emperor always overwinters as an early third instar caterpillar, and its strategy is to hunker down in crevices, branch forks and against bud thickenings - its size being perfectly suited to the job.

If we look at mutivoltine species, and even some of those which demonstrate a trend for adding further broods at the end of a long, hot summer, there still doesn't seem to be an equivalent issue (to that facing the Wall). The Small Copper has three and occasionally four broods on my patch. However, this species (irrespective of whether the caterpillar is third or fourth brood) will always overwinter in the early instars (1 -3), secure on a silk pad beneath a sorrel leaf.

If we look at the Dingy Skipper, which often emerges in early April here in Sussex, again there doesn't seem to be a similar problem related to climate change (habitat is another matter!), and it's doing very well in the South. Being usually single-brooded, there is the entire summer for caterpillars to reach maturity, before diapause. In some years the species is now producing a strong second brood. However, this is sufficiently early in the year for caterpillars to comfortably achieve optimum (mature) overwintering size.

Turning to the Speckled Wood, which has adapted to diapause as caterpillars of variable size, or as pupae, any potentially similar problems (to Wall) affecting some of these larvae (smaller ones?) will only be faced by a relatively small percentage of the entire population. Risk is therefore cleverly spread, which is probably one of the reasons why the species is thriving and expanding its range.

So is this 'developmental trap' a problem unique to the Wall in the UK, at this stage during the on-going warming trend? Perhaps not. The Small Heath is another declining, grass-feeding species which is producing an increasingly regular third brood in warm summers in southern England. Like the Wall, its overwintering caterpillar needs to achieve an advanced stage of development, prior to a very early emergence in warm spring weather. Although it has not contracted in geographical range like the Wall has, its decline has been very marked over the last twenty years. I suspect, and fear, that the Small Heath may also be advancing its phenology towards a similar 'developmental trap'. Its saving grace may be the tendency for a proportion of its first brood caterpillars to reach a mature stage and then diapause, thereby spreading risk.

If we assume that in the medium to longer term the Wall can recover, when further climate warming allows a larger proportion of third brood larvae to reach a healthy size before the diapause phase - what are the potential benefits to having overcome this 'developmental trap'? Irrespective of whether the Wall caterpillars are large or small, there will always be significant losses over the winter period. The caterpillars produced by first brood adults will always have a much easier time of it, and hence the second brood is often three times larger than the first. If a third brood becomes the norm, this gives the possibility of a second phase of population build each year. A build in population is not only beneficial in its own right. When numbers of a butterfly species begin to swell, their behaviour becomes more adventurous; conversely, small and decreasing populations drive more conservative behaviour. When numbers are high within any population, it is then that pioneering females head out seeking new areas of suitable habitat, allowing for an expansion in geographical range.

BWs, Neil

[Pete Eeles]

Thanks Neil,

When I questioned the survival rate of different larval instars, I meant within the same species; clearly, different species have adapted to overwinter in different stages. But I've not come across any analysis that compares the relative success of different instars of those species that overwinter as larvae. This would, in itself, give a clue as to why Wall larvae that are under-developed prior to diapause are at a disadvantage. And you might be on to something regarding Small Heath.

Cheers,

- Pete

[millerd]

For information regarding the habits of the Small Heath: here are my last sighting dates for my local patch (near Heathrow) for the last six years.

2009 - 1st October
2010 - 20th September
2011 - 3rd October
2012 - 8th September
2013 - 20th September
2014 - 3rd October

I would suggest that shows a pretty consistent third brood, though it's tricky to separate them into broods as such, as they are present continuously from May onwards.

Dave

[Neil Hulme]

Hi Dave,

Yes, that looks pretty convincing to me. As you say, it's sometimes tricky to differentiate broods, as starts and tails often merge, so it's necessary to look at abundance stats over the season to put approximate timeframes on each brood, but once you are in October (or close), that's got to be three.

BWs, Neil

[peterc]

Neil, interesting point you make about higher populations among adults generally being beneficial to a particular species. However, during the earlier stages like the larval phase, things may be different. There may come a point in the population density (per unit area) where it is disadvantageous to have such high numbers because they are more conspicuous to predators, susceptible to disease and so on but I am only speculating. This could explain the wildly fluctuating fortunes of the Holly Blue.

Referring to the Small Heath, it too suffered a decline in the early 1990s but came back well in 1995 and 1996. In Hertfordshire, although there was a steady decline since then, in the last 5 years or so it has made a small recovery. The fact that the annual changes in populations of the Small Heath seem far smaller than the Wall may have something to do with it. It can probably adapt better to changes to the environment or climate.

Peter

[Neil Hulme]

Hi Peter,

Yes, that might well be the case with larval abundance/density. Matthew Oates is becoming ever more convinced that the Purple Emperor caterpillar population he monitors every winter suffers from a higher rate of predation by tits when the larvae are more numerous. It would appear that a high strike rate by feeding tits might cause them to start hunting in a more focused manner. He also suspects that cold, hard winters drive a large part of the tit population out of the woods, and into gardens of the more urban areas nearby, thus releasing some of the pressure from predation.

BWs, Neil

[badgerbob]

13/1/2015.
In between the showers I called up for a quick Wall larvae hunt. In just over 30 minutes I managed to find 5. I couldn't relocate the 2 from last week, but they are probably hiding somewhere near. All were around 1.2cm today. 3 were found when I disturbed them on their grass stems and they dropped into the tussocks. The other 2 were spotted sitting on the grass and they stayed for photos and measuring. Several spiders were also seen in the tussocks and they could be the main threat to the larva at this time of year, apart from the weather perhaps. Last year I found one of these spiders with a Marbled White larva in its claws!! A moth larva was also found which had a totally different approach to keeping hidden. Whereas the Wall larva is extremely slow moving so it keeps hidden, the moth larva moved quickly to try to escape. A shieldbug and a ladybird also seen in the tussocks. 3 photos attached, one showing the size against a measure, and the 2 sitting on the grass. Both of the 2 on the grass had water droplets trapped in their hairs!!

[Neil Hulme]

Great work Bob! I'm impressed.
BWs, Neil

[badgerbob]

I have just finished (perhaps) my Wall Brown report 2014 for Sussex BC and I thought some of my findings may be of some interest here. As far as a 3rd brood was concerned it was likely that we would get one here with the earliest adult flying on March 29th. This is the only Sussex record for a March sighting and the earliest Sussex record since 1848 according to the Sussex recorder. The 3rd brood was much greater than anyone could have foreseen though, but the most surprising thing for me is that the 3rd brood went on longer than both the earlier broods.

1st brood 43 days
2nd brood 51 days
3rd brood 53 days

These dates are taken from the 1st and last sightings across the county of all broods, subject of course to more records outstanding.

The only other significant years for 3rd brood since I have been keeping records showed that in 2009 the 3rd brood lasted 30 days and in 2011 it was 32 days.

With 147 possible days when the Wall could have been seen in Sussex it is unlikely any other butterfly could have a record like this accept maybe the hibernators or perhaps the Speckled Wood.

[badgerbob]

11/2/2015.
With the weather being overcast today, and feeling a little milder, I just had 30 minutes looking to see the progress of the Wall Brown larva. The temperature was actually only 7 degrees so I wasn't very confident of finding too much. However, in that short time I found 4 larva, all feeding from 12cm to 27cm off the ground. Measurements were roughly 1.4cm, 1.5cm, 1.7cm and 2cm. This is from 1.2cm average from my last visit. The largest that was also the highest off the ground appeared to have a large head. I remember that Vince mentioned that this was a sign of a moult.

[Neil Hulme]

Well done Bob! This does seem to demonstrate that the less advanced caterpillars from third brood Wall take every opportunity to become active and continue feeding through the winter, whenever conditions allow. You are collecting some very valuable data.

It will be very interesting to see what you can locate in the winter, following the next season during which the species only produces two broods. If my theory holds any water, they should have reached their optimum over-wintering size prior to the (sometimes broken) period of diapause, and be far less active.

Best Wishes, Neil

[badgerbob]

With a lot of time on my hands at the moment due to doing my back in, all in the name of looking after butterflies!! The steep area of High and Over is in danger of being taken over by Cantoneaster so I am trying to clear the area to preserve it for the many rare moths and butterflies that thrive there. Unfortunately, in trying to pull some up I ripped a muscle in my lower back and am now recovering (I hope).
With this spare time I thought I would come back to this thread with some thoughts from the year. It must be said that numbers of the Wall Brown were down from the last few good years here. How much of this is down to the 3rd brood discussion from 2014 is of course debatable. I certainly do not profess to be an expert, just a casual and perhaps enthusiastic observer and what I found in the early Spring of 2015 would point more to the normal butterfly predation cycle that happens with most, if not all species. Between January and late March I was finding it very easy to find good numbers of larvae with my new record of 14 found in little more than 30 minutes!! Nearly every suitable grass tussock seemed to have a larvae in it. In previous years a find of around 6 would have been significant so I am quite confident that numbers of larvae was well up on previous years. As in previous years the sizes of the larvae varied quite a lot with some being 3 times the size of others, however, I did follow many through to near fully grown, which is when they vanish to go and pupate. With all these larvae I was quite confident of a good 1st brood, but although the 1st brood went on for some time and numbers were okay they were without doubt down a bit on expectations. Following the large amount of larvae it was then on to the hunt for pupa. The hardest stage by a long way to find. In the end I only found 2 which were both parasitised before they made it through. I do feel that this is the main reason for lower numbers of the first brood this year as well as the intermittent weather.
The 2nd brood is normally larger than the 1st, and this was the case again this year, although once again it was down on previous years. Of course the fact that the 1st brood was down this would be expected. The 2nd brood also suffered from some serious heavy rainfall that certainly killed off some of the butterflies. A couple of weeks into the 2nd brood and suddenly there were hardly any left until numbers built up again with fresh emergences. It then happened again with more torrential rain that knocked the numbers back again. This is where a regular transect way of counting would have helped get a bit more understanding of what was happening, as I do a count only a couple of times on a circuit I would only count the specimens on that day so any that had been killed off by the heavy rain just before would not be counted.
2nd brood larvae counting is also difficult as in this brood the larvae seem to be nocturnal, as are larvae of other members of the Brown family. I have never managed to find a 2nd brood Wall Brown larva!! In the early Spring the nights are too cold for the larvae to be active so they have to feed on mild days, whereas in the Summer there are far too many Skylarks and Pipits about feeding their young for it to be safe feeding during the day!
Unfortunately I was unable to do any research on my main North facing colony, once again due to the weather never being suitable when I had the time to do it.
Whether any of this waffle helps in the mystery of this species is debatable, but I hope it does just that and gets people debating it.

[essexbuzzard]

I have no particular expertise regarding the Wall,but it is a species i like,and your experiance regarding the early stages makes a most interesting read,thanks for sharing it with us.