Saturday, 27 February 2016

Everpresent on evergreen - a rare moth in Newhaven?

Ectoedemia heringella: is this tiny moth ubiquitous wherever Holm Oaks are located within the Lewes and Seahaven areas?

When I discovered some leaf-mines on a Holm Oak in Newhaven last summer, I was pleased to have found evidence of a nationally scarce species. Further fieldwork has led me to re-evaluate my initial excitement.

Ectoedemia heringella is one of one hundred species in the Nepticulidae moth family found in the UK. Each species in the family is tiny and heringella measures only 3mm in length (wingspan 4.5-6mm). Many of the adult Nepticulidae moths are similar in appearance, making it too difficult to separate most species on appearance alone. Their small size means they are rarely seen in their adult forms, but it is easier to find evidence of their presence during their early life-stage as a larva. This is because they feed within leaves, eating the tissue between the upper and lower surfaces of the leaf. As they progress through the leaf they leave a frass-filled trail behind them. Like a meandering river which can be followed from source to sea, the mine begins small, often near the egg, which may be laid on the upper or lower surface of the leaf depending upon the species, and broadens as the larva grows. The trail is called a leaf-mine. Many moth species and other insects have adopted this successful strategy during their early stages, including some fly species (Diptera), some wasps (Hymenoptera) and some beetles (Coleoptera). The genus Ectoedemia can be roughly translated as meaning 'outside swelling' (ektos and oedema) and this presumably refers to the appearance of the mines they create.

Ectoedemia heringella mines on a host plant at Riverside in Newhaven

Ectoedemia heringella feeds on Holm (Evergreen) Oak (Quercus ilex). The moth was first discovered in the UK in 1996 in Middlesex, but the record was not confirmed until 2001 (see UK Moths website). The species is native to the Mediterranean region from southern France to Cypress, where it feeds on Quercus ilex and Q. alnifolia (see British Leafminers website). It is possible that the accidental introduction to the UK was made during the importation of the host plant in the early 1990s. The mine created by the larva is strongly contorted and distinctive. An infested tree often has several individual mines on one leaf. One could speculate about how many adult moths may emerge from a single tree and reasonably suggest a number in the tens of thousands, perhaps more.

Ectoedemia heringella mines - many on each leaf

When I look at an infested tree it is reminiscent of the Horse Chestnut (Aesculus hippocastanum) and the notorious Horse Chestnut Leaf-miner (Cameraria ohridella). That species has infamously spread across Europe since its discovery in 1985 in Macedonia and has regularly attracted media attention in this country, probably because of concerns about its charismatic host-tree as much as anything else. Everyone loves the conker tree - including ohridella! Ectoedemia heringella has remained well beneath the radar compared with it, presumably because the holm oak is not as rooted in our collective hearts as the chestnut.

E. heringella has made the news, such as in a University of Reading Whiteknights Campus blog post and, hysterically so, in this Mail Online effort. It should be argued, however, that the photosynthesising potential of an infested tree would not be significantly compromised; only the mined parts of each leaf are affected in this way and the leaf is still able to properly function where chlorophyll remains. It is a reasonable suggestion that a heavily-mined leaf may be shed by the tree, but unless a critical threshold is exceeded, the health of the tree should not be too badly affected. This view may be somewhat subjective, however, depending upon where your interests lie: vegetable or animal.

The mines appear on the tough oak leaves in November, at a time of year when the deciduous tree species are shedding their leaves. This allows the recorder to pick out holm oak trees, which are as conspicuous in the wider landscape at that time of year as the mines themselves are on individual leaves in the macro-landscape. Larvae feed through the winter months and pupate in a cocoon between the leaves from April and appear as adults in June and July. It is an easy species of moth to find.

The species is given a provisional status of Nationally Scarce B (Davis, 2012), which suggests that, at the time, it had been recorded in only 31 to 100 hectads (10km x 10km squares) in the UK. According to the NBN Gateway, heringella is found to the south of a line drawn from The Wash to the Bristol Channel and from Hampshire eastwards.

Colin Pratt FRES, in his Complete History of the Butterflies and Moths of Sussex, Volumes One (2011) and Four (2015), provides a fascinating account of the history of the species within Sussex. It was first recorded in 2009 and only about sixteen records had been reported by early 2015 (when Volume Four was published), all following intentional searches for the leaf-mines. The sequential pattern of incoming records does not suggest a specific origin and colonisation of Sussex from elsewhere and this has led Colin Pratt to suggest that the moth might be a genuine overseas immigrant as well as an accidental import on holm oak saplings. The closest locations in which it had been recorded to the Ouse valley and Seahaven areas were Saltdean, Telscombe and in the Ouse valley itself at Piddinghoe, so it was probably already present in the Newhaven area by 2014 when these records were made.

I first discovered the moth on 29th July 2015 at Meeching Down SNCI, locally known as The Union Field, which is located to the west of Newhaven. This was during a children's bug-hunt in which I was assisting the SCDA and Lewes District Council. I noticed the distinctive mines on a low bough of a single oak close to the A259 Brighton Road. When I revisited on my own I counted well in excess of 300 mines and there were probably significantly more beyond my reach. Such an infestation on a single tree suggested that it might be found on other holm oaks in the area and I added a leaf-mine search to my winter to-do list. I finally managed to do some more work this February.

I was aware of a handful of  holm oaks in Newhaven so, on 14th February, I started inspecting them. Starting at Castle Hill LNR, I inspected trees in Riverside, just a few metres from the congested one-way system, and in Church Hill. Widening my search I inspected a plantation of seven holm oaks growing around the boundary of the Church of the Ascension at the junction of Arundel Road with Steyning Avenue in Peacehaven. A further four holm oaks were inspected in the Southover High Street area of Lewes. Trees at every location that I inspected were host to heringella - and often in very high numbers.

As winter began giving way to spring I continued to search for host trees and found them in numbers in Lewes (Brighton Road, Broomans Lane, The Pells area, Lewes House), Swanborough, where a truly mightly oak was affected, Rodmell, the churchyard of St. Mary's at Tarring Neville and Poverty Bottom near Newhaven, where only a few mines were seen amongst a heavy infestation by another Quercus ilex-feeding moth, Phyllonorycter messaniella. Perversely, messaniella is considered to be a common moth (which it is), but this is the only location at which I have seen it on Holm Oak.

So to conclude, while Ectoedemia heringella may still warrant its nationally scarce status, it does appear to be locally abundant - if the numbers seen in the Lewes and Seahaven areas are anything to go by. There are several further Holm Oaks that I have spotted during my travels and made a note of to check at a later date. I suspect that the known distribution of this moth will grow with almost every tree that is inspected.

The Holm Oak at Church Hill in Newhaven - about 40 years old
Some of the seven Holm Oaks at the Church of the Ascension, Arundel Road, Peacehaven
The Holm Oak between St. John The Baptist Church and Priory Crescent at Southover High Street, Lewes
The magnificent mature Holm Oak near the Cockshut Road junction with Southover High Street in Lewes

The Drove, Newhaven
Avis Road, Newhaven
St. Mary's Church, Tarring Neville

Swanborough Drove
Swanborough Hollow/Lewes Road

Southover Grange Gardens

The Pells, Lewes

Castle Banks, Lewes

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