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 moth-like in flight but 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. The main one in Yorkshire is polydama 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.

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.

Distribution

The Large Heath is a sedentary resident with one brood which flies from June to early to mid August.

It lives in a broad 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.

In Yorkshire it is currently only found on lowland peat moors near Doncaster and on wet heaths and marshy grasslands on the North York Moors. Restoration of the peat bog at Thorne Moors seems to be helping the species increase there. Large-scale peat-mining 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.

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

C Jones 04/01/21