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:
Impacts of climate warming and habitat loss on extinctions at species' low‐latitude range boundaries - FRANCO - 2006 - Global Change Biology - Wiley Online Library