Wednesday, 10 February 2016

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

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