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)