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