Butterflies and moths overwintering in the Newhaven Fort tunnels

Our third year of surveying overwintering insects and bats in the tunnels at Newhaven Fort started on 14th December. The Fort management team kindly allows us access to the tunnels which are closed to the public at a time when the Fort itself is closed for winter maintenance. 

Peacock in diapause (Vanessa io)

Of the few butterflies which spend the winter in diapause, species like the Red Admiral (Vanessa atalanta) tend to prefer the cover of thick vegetation rather than a subterranean hibernaculum. They are the last species on the wing in most years and often one of the earliest seen the following year. We have never found Red Admirals in the tunnels. Peacocks (Vanessa io) and Small Tortoiseshells (Aglais urticae) are often seen on a warm morning in February or March basking on the bare ground around rabbit warrens, having emerged from within the holes. The Newhaven Fort tunnels offer a similar refuge and we find both species in small numbers here.

Bloxworth Snout (Hypena obsitalis)

Of all the insect species found overwintering within the tunnels, we are most interested in the Bloxworth Snout (Hypena obsitalis), a moth which is in the process of colonising coastal Britain from the south. It feeds as a larva on Pellitory-of-the-Wall, a nettle which grows in walls and crevices, hanging on along the edges of things - a bit like the Bloxworth Snout itself. The Newhaven Fort tunnels are emerging as one of the best spots in the UK known for this moth.

Bloxworth Snout (Hypena obsitalis)

Another notable moth we find is a well-known overwinterer, The Herald (Scoliopteryx libatrix). This species feeds on poplar and willow and there are a few of these trees in the northern edge of the Fort and Castle Hill Local Nature Reserve. This species has been seen in single figures in previous surveys. It is a common but beautiful moth which looks like an autumn leaf.

Eristalis tenax hoverflies clustered beneath a paint blister

In addition to these core species, we usually also see good numbers of the Twenty-plumed Moth (Alucita hexadactyla) which feeds as a larva on the abundant honeysuckle growing at Castle Hill LNR, a few Common Flat-body (Agonopterix heracliana) and a few individuals of the Fleabane Smudge (Digitivalva pulicariae), which are curiously found deep within the tunnels far from an obvious point of entry. All the other moths and butterflies we see are found fairly close to the open air, in the company of bristletails (Petrobius maritimus), various woodlouse species, slugs and snails, several species of midge and lots of spiders. Their prerequisites appear to be a stable temperature and some ventilation with protection from the weather, predators and pathogens. Some light intrusion to indicate changes in day length is probably also an important guide to the right time to re-emerge the following Spring.

So to 14th December 2016. Five of us ventured underground on what was a sunny winter day with a moderate southerly breeze blowing warm air across the English Channel from the continent. This weather pattern had been in place for several days prior to our visit and followed a period of colder weather with a number of overnight frosts. It's a shame the frosty weather hadn't lasted until our visit as this would have allowed an interesting comparison with previous years' surveys, which have always taken place during mild weather, but the mild weather did allow a direct comparison this year with the previous two. The thermometer read 13°C, which is pretty warm for a clear December day. Temperatures in the tunnels ranged between 12°C and 13.5°C. Here's what we saw:

Peacock (Vanessa io) (17)
Small Tortoiseshell (Aglais urticae) (9)
Twenty-plume Moth (Alucita hexadactyla) (67)
Herald (Scoliopteryx libatrix) (31)
Bloxworth Snout (Hypena obsitalis) (10).

Five species is the fewest we have seen since we started surveying the tunnels, yet we saw more individuals overall and beat our previous record of 112 by quite a margin: this time we saw 135. The reason for this was a significantly greater number of Twenty-plume Moths (December average is 40) and Herald (December average 7.5). It is misleading to place too much relevance on only a few years' data, but this December we saw a 67% increase of Twenty-plume Moths and a more than 400% increase of Herald.

The value of regular surveys grows with the accumulation of comparable data from year to year and we are beginning to see some interesting patterns emerge now that we are into our third year of monitoring. Observing overwintering insects also reveals a surprising insight into different species' behaviour. Predictably perhaps, warmth-loving butterflies tend to be in a much deeper state of diapause than moths, which are more tolerant of cooler temperatures. The Bloxworth Snout is particularly tolerant of low temperatures, often entering a shallow state of torpor rather than proper diapause. This ability to remain active at times when predators or competitors may be less active might offer a selective advantage as it radiates northwards into new territories, but it might consequently also be more at risk of severe cold snaps. If more people were able to inspect overwintering sites each year it would almost certainly help improve the understanding of how weather and climate influences the population dynamics not only of moths or butterflies but also all those species associated by a complex web of interaction.

The reasons for the increases in some species this December might be that the weather was good when the adults we saw were progressing through their earlier life-stages; it might be that predators and pathogens were fewer; it might be that previous years were influenced by adverse conditions at the time and we are now witnessing a regression to the mean. Whatever the reasons, the other species we saw this time were at much the same levels as in previous surveys. Questions arising from the collected data are exactly what is hoped for. Some of these questions might be answered if we can continue to collect data in future years.

A cluster of Herald (Scoliopteryx libatrix)

March in the moth trap

I ran my moth trap on 21 nights during March, slightly up on the average number of nights over the last ten years, which is 17 nights.

Despite starting the month with some wild weather and enduring the ferocity of Storm Katie on the night of 27th, the nights during March were generally quiet and chilly, with a mainly northerly flow to the weather. This is the probable reason for the slightly depressed numbers of moths recorded. March, like the first two months of the year, can be painfully slow on the relatively treeless parts of the Downs where I operate my trap. Species and numbers seen were both below their ten-year averages, with the exception of the March Orthosia stalwarts Common Quaker (Orthosia cerasi) and Hebrew Character (Orthosia gothica), which were about a third higher than usual, but still very low in number compared with many more wooded locations. Early Grey numbers were low, despite an exceptionally early record I made on 4th February, and began to appear only during the final week of the month, which suggests a late emergence locally, along with the two Orthosia species Small Quaker and Powdered Quaker, both of which were absent during the month.

There was a similar pattern to the butterfly sightings, with numbers only starting to improve towards the end of the month as the sunlight strengthened and the winds became milder and lesser in strength. Most Small Tortoiseshells were seen basking around rabbit holes, as is the March norm, but they were beginning to wander on the wing during the last two days of the month. With warmer temperatures forecast next week, we should all see a welcome increase in numbers and species.

Ectoedemia heringella (New Holm Oak Pigmy) dominated my fieldwork efforts. This Nationally Scarce species has now been recorded in 59 locations in Newhaven, Peacehaven, Seaford, Lewes and the villages between.

There was a bonus record of the willowherb-feeding Mompha epilobiella, which was seen during a visit to Charleston Farmhouse within one of the bedrooms. At first sight I thought I was witness to the destruction of the natural fibres making up the fabrics and other textiles on display at the house, but relief came with a closer look - so hold off with the pheromone traps!

Hebrew Character (Orthosia gothica)

Common Quaker (Orthosia cerasi)

Early Grey (Xylocampa areola)

Species summary, March 2016
32 species recorded (10-year average is 25), 150 individual moths and butterflies (10-year average is 186).

125W Robinson trap at home (17 species)
Parsnip Moth (Depressaria heraclei)  2
Common Flat-body (Agonopterix heracliana)  6
Brown-spot Flat-body (Agonopterix alstromeriana)  1
White-shouldered House Moth (Endrosis sarcitrella)  1
Light Brown Apple Moth (Epiphyas postvittana) 6
Common Plume (Emmelina monodactyla)  6
March Moth (Alsophila aescularia)  2
Double-striped Pug (Gymnoscelis rufifasciata)  1
Oak Beauty (Biston strataria)  4
Common Quaker (Orthosia cerasi)  41
Clouded Drab (Orthosia incerta)  2
Twin-spotted Quaker (Orthosia munda)  2
Hebrew Character (Orthosia gothica)  40
Early Grey (Xylocampa areola)  3
Chestnut (Conistra vaccinii)  1
Angle Shades (Phlogophora meticulosa)  2
Silver Y (Autographa gamma)  2

Other garden obs at home (5 species)
Small White (Pieris rapae)  1
Brimstone (Gonepteryx rhamni)  2
Peacock (Aglais io)  1
Oak Eggar (Lasiocampa quercus)  2 larvae
Large Yellow Underwing (Noctua pronuba)  2 larvae


Fieldwork (12 species)
New Holm-oak Pigmy (Ectoedemia heringella)  mines
Golden Pigmy (Stigmella aurella)  mines
Holm-oak Pigmy (Stigmella suberivora)  mines
Bordered Carl (Catoptriche marginea)  mines
Brown Oak Slender (Acrocercops brongniardella)  1 mine
Garden Midget (Phyllonorycter messaniella) mines
Common Flat-body (Agonopterix heracliana)  1
Common Cosmet (Mompha epilobiella)  1 - seen inside Charleston Farmhouse!
Bulrush Cosmet (Limnaecia phragmitella) larval feeding signs on bulrush
Peacock (Aglais io)  4
Small Tortoiseshell (Aglais urticae)  12
Comma (Polygonia c-album)  2

Back into the Motherlode

A rich seam of hibernating moths and butterflies runs beneath the wintry land at Newhaven Fort and its stunning and embracing neighbour, Castle Hill Local Nature Reserve. They stand together, arm in arm, overlooking the Ouse estaury and facing the full force of the bracing English Channel winter. The historic tunnels are dark, damp, ventilated, mild and these conditions are stable. The temperature within the tunnels is consistently between 1°C and 2°C warmer than the ambient outside temperature. Any adult insect seeking an hibernaculum would increase their chances of surviving the winter as they fly into the tunnel entrances or ventilation shafts here. Buried alive each autumn to be reborn the following spring. We ventured into the dark, damp shafts twice during the 2015/2016 winter. This was the second winter of what we hope will be regular annual surveys. This is an account of what we found.

Outside and in: inside the southern counterscarp gallery

Outside and in: outside - the area close to the southern counterscarp gallery

Our first survey during this winter was made on 15th December 2015. The outside world was halfway through experiencing the warmest December on record and I was interested to learn whether the mild conditions had affected the number of hibernating insects within the tunnels. It had not; we saw very nearly the same number of insects as we had during the previous December. So does day length rather than temperature drive adults into hibernation, or is there a complicated juxtaposition between the two? All good tunnel surveys should generate more questions than you begin with - it's part of the journey.

Two new species were added to our list: Acrolepia autumnitella (Bittersweet Smudge) and Amblyptilia acanthadactyla (Beautiful Plume). Acrolepia autumnitella feeds as a caterpillar on bittersweet or deadly nightshade. The tiny caterpillar feeds within the leaves and creates characteristic pale blotches. As an adult, it remains small at about 6mm long, but its inconspicuous appearence is betrayed under magnification, revealing a macroscopic beauty in its brindled patches and pale cross-lines. There are two broods in most years, in July and again in October, when the adults hibernate and emerge the following spring to produce the next summer brood - exactly what our individual was doing. Photographs and further information about this and all the species described below can be found by following the links.

The Beautiful Plume (Amblyptilia acanthadactyla) is a common but spectacular little moth, which appears to have become more common during recent decades. It feeds in its early stage on a range of herbaceous plants and, similar to autumnitella, the adult flies in July and again in September, when it seeks hibernation.

Although total numbers were similar to those in 2014, there was an interesting variation in the numbers of individual species. The Bloxworth Snout (Hypena obsitalis) was present in almost the same numbers (thirteen this year; fourteen in 2014), as were Small Tortoiseshells (Aglais urticae), with six counted both this year and last. The Twenty-plumed Moth (Alucita hexadactyla) was more numerous with 47 seen this year compared with 32 last December. This intriguing little moth somehow chooses a resting place which, despite the pitch dark, often provides a cryptic background, for example a pebble-dashed wall. This makes it a challenging species to count with a headlamp, but perhaps the greater number seen this year suggests we have got our eye in better than we had during our first visit last year.

The Peacock (Aglais io) was seen in fewer numbers this year than last

The greatest contrast in numbers was seen in the Peacock (Aglais io). In 2014 we counted 33 individuals; this year we saw only twelve - nearly two-thirds less. Perhaps this was because fewer adults had survived this year to the point of hibernation? Perhaps last year's number will prove to be unusually high? More questions requiring an answer! Jean Jacques Rousseau once wrote that patience is bitter, but its fruit is sweet. This I am learning. We saw a total on ten species and 112 individuals during this survey, which has proved to be our best result to date, but only just greater than the very similar result obtained in 2014 (eight species, 109 individuals).

Curled autumn leaf: The Herald (Scoliopteryx libatrix)

Our second and final visit of this winter was made on Tuesday 2nd February. Why do we survey twice each winter? I don't know whether there is any value in this just yet, and a good enough reason might emerge during the coming years. For now, a second survey enables an interesting comparison with the earlier winter survey - a before and after or early- and late-winter evaluation. One clear benefit of a second visit is that it fulfils a need which grows within me during a long period which is otherwise absent of moths and butterflies. So the flame of personal gratification is perhaps the strongest attractant. But there is also a fascination that many of the individuals discovered during December have not move a single millimetre during the five or so weeks between our visits. There is a pattern emerging to this: certain species appear to enter a deeper slumber than others. For example, the butterflies - those poster boys of hot summer days - are regularly found in the same state in February as they were in December. The same may be said of the stunning Herald (Scoliopteryx libatrix), which hangs like a curled autumn leaf from the tunnel walls. Also apparently less active is the tiny micro-moth Digitivalva pulicariae (Fleabane Smudge), which inexplicably appears to seek the same section of deep tunnel a long way from the outside world and in which no other moth or butterfly species has so far been found.

A reluctant slumber? The Bloxworth Snout (Hypena obsitalis)

More active is the Twenty-plumed Moth, which will often flutter back into activity under torchlight. But by far the most restless species is the one we are the most interested in: the Bloxworth Snout. During the four surveys we have performed, the Bloxworths have proven to be sensitive to light, taking to flight as torchlight falls upon them and flying a short distance before settling down again. In February 2015 we counted only five of the fourteen individuals we had seen during our first visit in December. It seemed a fair suggestion that they were more tolerant than other species of cooler temperatures - a behaviour which is often witnessed in the outside world during the early and late winter, when they are seen around doorways - and that the Bloxworths emerge from a comparatively lighter slumber ahead of some other overwintering species. Does this convey some ecological or evolutionary advantage? Would this winter's second survey suggest the same behaviour? As well as one stand-alone survey could suggest this, yes it did: only seven of the thirteen individuals counted in December remained and the others were not seen. Of the ten species seen on 2nd February, only the similarly active Twenty-plumed Moth was seen in noticeably fewer numbers: 28 individuals compared with 47 seen in December. The numbers of both these species had roughly halved. Only patterns which emerge in future tunnel surveys will provide more robust data to these initial findings, but it is one of the more interesting findings we have made to date. Until December, patience.

One of the Bloxworth Snouts seen in February 2016

Another of the Bloxworth Snouts seen in February 2016

 

The Herald (Scoliopteryx libatrix) - a spectacular moth which is associated with Poplar and Willow

  

Overwintering insects in Newhaven

One of the surveyed areas at Newhaven Fort

There are not many places on this Earth where I have been buried alive and yet at the same time been able to see the far horizon. The tunnels at Newhaven Fort are one such place. On 17th December I had the privilege of accompanying underground the Custodian of Newhaven Fort, Steve Watkins, Lewes District Council Rangers Dan Fagan and Thyone Outram and Tony Hutson from the Sussex Bat Group. The purpose was to survey the accessible areas for hibernating bats and Lepidoptera. Along the way we saw only two Brown Long-eared Bats (Plecotus auritus), plenty of spiders, including many Amaurobius (probably fenestralis) which were active in the shady, damp, mild habitat. I didn't attempt to identify the spiders, but I was able to record a pleasing total of 110 moths and butterflies.

Two butterfly species were seen: Small Tortoiseshell (Aglais urticae) (6) and Peacock (Inachis io) (33) - two of which are pictured above. Six moth species were seen:

Case-bearing Clothes Moth (Tinea pellionella)  8 larvae
Fleabane Smudge (Digitivalva pulicariae) 3

Common Flat-body (Agonopterix heracliana)  5
Twenty-plume Moth (Alucita hexadactyla)  32
Herald (Scoliopteryx libatrix)  9
Bloxworth Snout (Hypena obsitalis)  14

This initial survey is hoped to be the first of a regular annual survey and some interesting data was collected to compare with what we see during future visits. The Case-bearing Clothes Moth (Tinea pellionella) larvae (pictured right) were, like most of the invertebrates we saw, close to the entrance and exit of a tunnel. They are likely to be feeding on natural fibres from rabbits or possibly birds nests, of which there are plenty of both within a short distance of where we saw them.

The only species we saw which was deep within a tunnel was Digitivalva pulicariae - the Fleabane Smudge (pictured below), and the three we saw were close together in a location which was roughly beneath the moat adjacent to the main car park. It is likely that they fed during their larval life stage on hemp agrimony (Eupatorium cannabinum), which has a fairly dense population on the adjacent Castle Hill Local Nature Reserve. How and why did they retreat to such an inaccessible part of the tunnels?

Fleabane Smudge (Digitivalva pulicariae)

Common Flat-body (Agonopterix heracliana) and Twenty-plume Moth (Alucita hexadactyla) are both very well known to hibernate overwinter as adults and to emerge when the day length and temperature increases the following spring. Both species have visited my moth trap on warm winter nights. It was no surprise to find these species within the tunnels, nor The Herald (Scoliopteryx libatrix) - one of the better known hibernators which, in its mimicry of a curled autumn leaf, could easily be mistaken or altogether missed.

Twenty-plume Moth (Alucita hexadactyla)

The Herald (Scoliopteryx libatrix)

Each of these species was a welcome sight during the lean winter months, but the value of this survey, the benefit of having made the effort to survey the tunnels, was seen in the fourteen Bloxworth Snouts (Hypena obsitalis) that we counted. This is a remarkable number.

The Bloxworth Snout in Sussex

Back in November, when the Newhaven Oleander Hawk-moth was making headlines, following its discovery by Steve Watkins and his colleagues, we (Thyone and me) had seen two Bloxworth Snout in the entrance tunnel to the Fort - the same location as the Oleander . They were at rest on the wall, a male and female. I suggested that the tunnels might offer a suitable hibernaculum for the species and, although we had previously decided that we should survey the tunnels, we made this species our main subject of interest. We were well rewarded. I would first like to place this in context.

I saw my first Bloxworth Snout in March 2010. It was an adult which had emerged from hibernation in the basement car park at Southover House in Lewes. It fluttered into my face as I walked out of the door, but came to rest for me to identify from a photograph I was able to snap.

Since 2010 I have seen it as a singleton on a further three occasions: at home in November 2012, when I saw one at rest outside of my kitchen door, at home again in July 2014 (an example of a second brood emergence?) and then up at the Fort in November 2014.

Using Colin Pratt's Complete History of the Butterflies and Moths of Sussex (2010) as a reference, the Bloxworth Snout was first recorded in Sussex in 1983 - in a garage at Houghton Green near Rye. One further 20th century record was made in 1997 at Littlehampton. A further five records were made between 2000 and 2009 before my moth flew into my face in Lewes. That adds up to a total of eight moths seen up to 2010. These records have led Colin Pratt to surmise that the moth, previously considered to be a scarce immigrant from continental Europe, was resident in the county by the turn of the century, if not before. All these sightings were singletons.

Between 2010 and 2013 the moth was recorded a further twelve times. Again, each of these reported sightings were of single specimens. This means that a total of twenty Bloxworth Snouts had been recorded in Sussex throughout its entire history in the county to the end of 2013. The brace that Thyone and I saw on 26th November this year was, it seems, the first recorded occurrence of two individuals in the same place at the same time.

The species is clearly becoming more widespread and a more regular visitor to Sussex-based Lepidopterists. This brings us up to date and the available data (excluding any records made during 2014) suggests that the species has a strengthening toe-hold along the Sussex coast. This was the situation as we entered the tunnels on 17th December: a total of 22 known adults previously recorded in the county.

This background helps to place the fourteen specimens we saw in the tunnels into context. It is a significant record which hints at the actual number of adults which may be present elsewhere in the tunnels at Newhaven and indeed along the Sussex coast. It is difficult to estimate how many of the fourteen moths were immigrants, but it is reasonable to suggest that most, if not all, are native to the cliffs at Newhaven, where they would have fed as larvae on either pellitory-of-the-wall (Parietaria judaica) or other Urticaceae species such as nettles. For those who are interested in climate change and the impact upon invertebrate species, the Bloxworth Snout would be a fascinating species to study and I suspect it will be recorded more frequently and in increasing numbers in the future.

I don't have that much data for records outside of Sussex, but I am currently aware that only Guernsey has returned greater numbers of the Bloxworth Snout, where the species is regularly seen in and around fortifications similar to those on the cliffs at Newhaven. I would strongly encourage the reader to seek out and survey any tunnels that you may gain safe access to, particularly in a coastal situation, in order to add to the knowledge of the Bloxworth Snout and other hibernating species. I will look forward to continuing this work in Newhaven - and I would happily join you elsewhere if you would like the company!

Many of the moths and butterflies seen were at rest near vertical shafts close to the open air, like this Bloxworth Snout