Large Heath
Large Heath Coenonympha tullia (Müller 1764)
Family: Nymphalidae, subgroup Satyrinae (The Browns)
Description
Photo above left by P Simmons
Other photos by Martin Partridge
The Large Heath is a medium-sized brown butterfly with a slow erratic flight pattern. It is similar to the Small Heath, which as the name suggests is smaller, but they aren’t usually found together because the Small Heath likes dry areas and the Large Heath prefers wet areas. However, both fly together on Fen Bog (VC62).
The Large Heath looks greyer and more careful and considered in flight- it appears more directional and purposeful, with slower wing beats. However both species always perch with wings closed and also tilt their wings to absorb warmth. ln the closed-wing position Large Heaths usually have a distinctive cream band below their forewing eyespot and a line of spots towards the edge of the hindwing. Small Heaths are similar but plainer and less strongly marked, with only a hint of hindwing spots.
There are three subspecies in Britain, which occur along a roughly latitudinal cline in colouration from south to north. The subspecies more frequently observed in Yorkshire is C. tullia polydama, the intermediate form which often lacks the white pupil in its eyespots.
Habitat
The species is found in open areas of lowland raised bogs, upland blanket bogs, and damp acid moorland and where hare's-tail cotton grass Eriophorum vaginatum {the larval foodplant) grows close to cross-leaved heath Erica tetralix, which is usually the main nectaring plant. Larvae also use the tussocks of E. vaginatum to hide from predators. The congener E. angustifolium is sometimes used as a secondary hostplant.
On the North York Moors, there seems to be a preference for wet flushes, which also support bog myrtle Myrica gale and cranberry Vaccinium oxycoccus. On the Thorne and Crowle Moors area, although E. vaginatum is increasingly abundant, it is supplemented or replaced by other nectar sources, particularly common knapweed Centaurea nigra and to a lesser extent meadow buttercup Ranunculus acris, bramble Rubus fructicosus, white clover Trifolium repens, rough hawkbit Leontodon hispidus and water avens Geum rivale.
British colonies can exist up to about 500 metres; those on the North York Moors reach 150 to 200 metres, so they have to be very hardy.
Raised bogs often develop over shallow basins formed during the last Ice Age. After the ice retreated from northern parts of Europe, the landscape was littered with many depressions within a mantle of often impermeable glacial debris or till. These basins formed lakes and were colonised by a fringe of fen vegetation. In-washed material formed lake sediments which when mixed with dead and undecayed plant material eventually led to many lakes completely infilling. Classically, it was thought that this sequence of succession gradually led to the development of forest over the former lake however, it has been shown that this sequence of succession is rare.. Instead, as the lake basin is filled with fen peats and sediment, the plants at the centre are cut off from nutrients at the lake margins making conditions rather nutrient poor. Sphagnum bog mosses thrive in nutrient poor and waterlogged conditions such as these and can dominate in these situations . Sphagnum leaves are characterised by large and empty hyaline cells sandwiching much smaller photosynthetic cells. The hyaline cells act as water conducting systems, to allow Sphagnum spp. to cope with waterlogging, and create a large surface area for cation exchange. Cation exchange is the mechanism by which Sphagnum (and other plants) absorbs nutrients dissolved in water. Sphagnum’s unusual physiognomy means that the genus has a high cation exchange ability, swapping scarce nutrients for hydrogen ions. As a result, Sphagnum species gradually acidify their surroundings . High acidity favours Sphagnum over many other species further allowing Sphagna to dominate the infilled former lake. Sphagna grow from the top of the plant – the apices – and die at the base. As a result, dead organic material is left in a waterlogged zone to form peat; Sphagna are efficient peat formers. Water can move only very slowly through the peat as it has a low hydraulic conductivity (Ingram, 1982). In effect, it is difficult for rainfall to leave the site and waterlogged conditions are maintained.
Distribution
The Large Heath is a sedentary resident with one brood which flies from June to early to mid August, but it’s largely over by the end of July.
It lives in a broad circumpolar band from north-east France to Scandinavia, and eastward across temperate Asia to North America (where one to three broods may occur). In its northernmost areas around five percent of larvae may take two years to mature. Overall it is declining in Europe, and was one of the few British resident species listed as ‘vulnerable’ on the European Red List for butterflies in 2010. There are signs of a range retraction in England, most likely associated with historical loss of habitat and vegetational succession on unmanaged sites (Franco et al. 2006- Global Change Biology - see below for link to paper).
In Yorkshire it is currently found on lowland peat moors near Doncaster and on the upland wet heaths and marshy grasslands of the North York Moors. Restoration of the peat bog at Thorne Moors seems to be helping the species increase there. Large-scale peat-extraction finished in 2004 with a two-year restoration project beginning immediately. The process of raising water levels actually began in 1992, through small-scale management improvements at Crowle, and in the nature reserve at Thorne. Large Heath numbers at Crowle responded quickly as the hare's tail cotton grass recovered.
The species is on the southern edge of its range in the county, the line of which runs south-westwards from southern Yorkshire into central Wales.
Although colonies in Britain north of Yorkshire can be very large, with populations up to 15,000, the ones in Yorkshire are usually very small. However the species can survive well at very low densities and there may be colonies that haven’t been noticed yet. Mobility is generally low and adults stay within very small areas, although on the open areas of Thorne Moors the species is subject to wind-blow. lt also appears that populations exist in networks, with occasional mixing between adjacent colonies, but natural colonisation or re-colonisation seems slow.
The Large Heath was more widely distributed in past than today, especially in the Humberhead Levels, the valley of the River Hull, the North York Moors and the Pennines. Before the first Holderness Drainage Acts of the 1760s the Hull Valley was a huge marsh, mainly too wet to be farmed. The Humberhead Levels cover the floodplain associated with the rivers which run into the Humber near Goole, including the Trent, Ouse, Aire and Derwent, draining around one-sixth of England, and in the 1700s the Large Heath would have been found over all over the floodplain in large numbers. Based on its legacy distributional extent it was probably one of the most common butterflies to be found in Northern England prior to drainage.
Link to paper above:
Life Cycle
Pale yellow eggs are laid singly, usually low down in tussocks of hare's-tail cotton-grass. David Wainwright, when researching the species in Northumberland, wrote, 'Literature frequently states that eggs are laid on tussocks. This is by no means the case where my research was carried out. Roughly one quarter of ovipositions I observed were on tussocks, but by far the commonest situation was in sparse, dead vegetation close to the ground. Post-diapause larvae were almost invariably found in tussocks (often small ones), which may account for the widely held assumption that the butterfly selects tussocks on which to oviposit. The best habitat was found where small (often unhealthy looking) tussocks were present and where vegetation was sufficiently sparse and short to enable ground layers of mosses to be seen easily.'
The eggs hatch after about 15 days and the pale-striped green larvae feed by day and, in warm, humid conditions, also by night. In late September, in their third instar, they hibernate deep in the base of a cotton grass tussock. ln this position they seem to be able to survive being underwater or even encased in ice, but probably only for short periods. Drier winter refuge areas for larvae appear to be an essential habitat constituent.
Larvae typically awake in March, continue feeding, and finally pupate in late May or early June in their fifth instar.
The species flies from early to mid-June to early August, with stragglers possible to mid-August.
Status
UK BAP Status: BAP priority species
Section 41 species of principal importance under the NERC Act in England
Section 42 species of principal importance under the NERC Act in Wales
Northern Ireland Priority Species
Scottish Biodiversity List
Aberrations
The species is so variable that lots of different looking examples can be found. Over 20 aberrations have been described, including ab thornensis (Pilleau 1952), described from a Thorne Moors specimen which lacked spots and cream band.
With thanks to The Butterflies of Yorkshire edited by Howard M Frost and Dr Andy Suggitt
C atherine Jones 04/01/21