Northern Brown Argus
The Northern Brown Argus is one of only a few butterfly species that have primarily a northern distribution in the UK, occurring in Scotland, Cumbria, Lancashire and Durham. The populations in the National Park are at the southern edge of the British range and it may be encountered in limestone areas of the Dales from early June until mid August.
The prime sites for this species are sheltered areas of unimproved limestone grassland where light stock grazing allows the larval food plant, the common rock rose (Helianthemum nummularium), to thrive. Research has shown that a sward height of between six and ten centimetres provides females with the optimum egg laying habitat.
Historically there were relatively few records of Northern Brown Argus in the Dales but an increase in the amount of fieldwork by a number of lepidopterists in the 1990s led to a discovery of a number of new colonies. A comprehensive survey of all known colonies in the Dales and visits to a number of potential sites in 2002 resulted in the recording of 33 occupied sites. This apparent population increase is more likely to be due to increased observer effort than any real expansion the Northern Brown Argus population. The key strongholds for this species remain in the Ribblesdale/Ingleborough and Wharfedale/Littondale areas of the National Park with a smaller number of sites in other areas.
Monitoring work has shown that there have been declines in some populations in other areas of northern England and Scotland. The results from several monitoring transects in the Dales, and an assessment of the status of Northern Brown Argus colonies in 2013 concluded that the majority of sites within the YDNP were in appropriate management that would maintain a stable population.
Small pearl-bordered fritillary
The Small Pearl-bordered Fritillary butterfly is found in damp grassland, moorland and woodland clearings. Here the key food plants of this butterfly, marsh and dog violet are found. It is a single brooded species with the eggs laid close to the main food plants. The caterpillars overwinter in tussock vegetation and the adults emerge from the second week of June. The main flight period lasts until mid-July although small numbers may be seen on the wing as late as the beginning of August. It is believed that the lifespan of the adults is approximately two weeks.
This species is a widespread species in Scotland and Wales but there has been a large scale declines across much of the English range. In Yorkshire, the Small Pearl-bordered Fritillary is restricted to three main areas which are the North York Moors, the Craven Lowlands and Yorkshire Dales National Park. Survey work carried out in 2002 located Small Pearl-bordered Fritillary colonies at four sites in the Yorkshire area and three colonies in the Cumbrian part of the Yorkshire Dales National Park.
Further information can be found at the Butterfly Conservation website.