Brown Argus (Aricia agestis)
Peak District local information
When to see: June-August
Where to see: Limestone grassland
Caterpillar food plant: Common Rock Rose (Helianthemum nummularium)
Peak District status: The specific identity of the Peak District population of this butterfly has been the subject of much debate, with opinion varying as to whether it should be regarded as northern brown argus (Aricia artaxerxes) or a single-brooded (univoltine) variety of brown argus (A. agestis). Genetic research now appears to confirm the latter. However, given the distinctive nature of the population and its isolation from other populations in southern England , it is usually recorded as ‘brown argus (Peak District race)’. At the national level, the ‘normal’ brown argus is expanding its range from the south through the east Midlands and is already established in south Derbyshire, and it may reach the Peak District in the next few years.
The brown argus is another limestone speciality, and is restricted to the White Peak . It has been recorded from localities widely distributed across the area, although the overall population seems to be relatively small. Its preferred habitat is sunny dales, hillsides and quarries containing unimproved limestone grassland where its larval food plant, common rock rose (Helianthemum nummularium) grows.
It emerges at the end of May or early in June, which is the month of peak emergence. It remains on the wing into July and August and is sometimes recorded in early September. Some of these late individuals have a very fresh appearance and represent a partial or overlapping second brood.
The sexes are similar in appearance but need to be distinguished carefully from the brown female form of the common blue. These usually show some blue on the body or inner part of the wings, a feature lacking on the brown argus. The common blue is also 15-20% larger and shows slight differences in the spotting pattern on the underwings.
Many recorded brown argus sites are located in the Derbyshire Dales National Nature Reserve, Derbyshire Wildlife Trust and Staffordshire Wildlife Trust nature reserves, and Plantlife’s Deep Dale reserve. All or most of the rest lie within Sites of Special Scientific Interest (SSSI).
Limestone grassland is a national priority habitat under the UK Biodiversity Action Plan and a considerable amount of effort is invested in conserving and extending the area occupied. This involves removal of invasive hawthorn scrub and optimising grazing regimes to maintain short, species-rich swards. Common rock rose is a frequent component of a particular type of calcareous grassland that is common in the Peak District and the future of this – and suitable habitat for the brown argus – seems assured. This conservation work is funded and carried out by several agencies including English Nature, Peak District National Park , Derbyshire Wildlife Trust and the Countryside Stewardship Scheme.