This small butterfly is dark brown in colour with orange crescents towards the edge of the wings, those on the forewings are absent or very small, and the underside is pale (1). In Britain there is a noticeable white spot in the centre of the upperside of both forewings (1). The caterpillar measures up to 1.3 centimetres in length, and has a green body with a darker green line along the sides (3).
The flight period of this butterfly is variable, but usually occurs between early June and mid-August (4). This species has one brood a year; eggs are laid singly on the upper surface of leaves of the foodplant, and hatch after a week (3). The caterpillars hibernate at the bottom of the food plant or on the ground (4), and complete their development the following spring (3).
Recent genetic studies have shown that single-brooded populations once thought to be northern brown argus occurring in the Peak District, Yorkshire Wolds and north Wales are actually brown argus (Aricia agestis). Populations in Co. Durham once thought to be Durham argus (A. a. salmacis) are actually mainly A. a. artaxerxes(4).
Inhabits sheltered areas of free draining unimproved grasslands with patches of bare ground, supporting the main caterpillar foodplant common rock-rose, Helianthemum nummularium(4). Preferably there should be light grazing on the site (4).
In the UK this species is classified as Nationally Scarce and protected under Schedule 5 of the Wildlife and Countryside Act 1981, with respect to sale only (2). It has two subspecies; Aricia artaxerxes artaxerxes and Aricia artaxerxes salmacis (the Durham argus or Castle Eden argus).
There has been a serious decline in this species around the Durham area, and reductions have also occurred in southern Scotland. More surveys are needed to ascertain the species' status in this area, however (4).
Sheltered unimproved grassland with low grazing levels has been greatly reduced, and persists in fragmented patches. Loss and degradation of this habitat has greatly contributed to the decline of this species. In some cases inappropriate grazing regimes result in otherwise suitable habitat becoming degraded (2); a lack of grazing results in scrub invasion, and overgrazing excludes the species even if the food plant is abundant (4).
Many colonies in the north of England occur in nature reserves where management has successfully been altered for this species. A good example occurred at St Abb's Head National Nature Reserve, where in 1992 there was a switch from heavy grazing to selective spring and autumn grazing. The population of the northern brown argus subsequently increased dramatically (4). The Countryside Stewardship Scheme includes management prescriptions aimed at this species; private landowners receive grants for managing their land in these ways (4). The northern brown argus is a priority species under the UK Biodiversity Action Plan (2).
The UK Biodiversity Action Plan for this species is available at UK BAP.
Hibernation is a winter survival strategy characteristic of some mammals in which an animal’s metabolic rate slows down and a state of deep sleep is attained. Whilst hibernating, animals survive on stored reserves of fat that they have accumulated in summer. In insects, the correct term for hibernation is ‘diapause’, a temporary pause in development and growth. Any stage of the lifecycle (eggs, larvae, pupae or adults) may enter diapause, which is typically associated with winter.
Also known as ‘univoltine’. Referring to an organism which has just one brood each year.
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
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