Silver-studded Blue

Silver-studded Blue Plebejus argus (Linnaeus 1761 )

Family: Lycaenidae, subgroup Polyommatinae (The Blues)

Description

Photos above by Iain H Leach

The Silver-studded Blue is very similar to the Common Blue though it is a very variable species with many aberrations and subspecies or racial forms across its range. However its flight is more fluttery and the butterfly usually keeps close to the ground.

The male upperwings are more strongly edged in black than in the Common Blue but have a similar white fringe. Females of the main (nominate) race/subspecies argus are usually brown with orange lunules on the upperwings similar to the Brown Argus. Both sexes have blue/green spots in the middle of some of the black spots found inside the orange lunules of the underwings (visible when the butterfly is seen with closed wings). These blue/green spots are the “silver” studs, which give the species its name.

Habitat

The natural habitat for this species is lowland heath but it is also found on chalk grasslands and sand dunes and strongly favours warm, south-facing slopes on sheltered sites. It uses a wide range of foodplants including heathers, heaths and gorses as well as Bird's-foot Trefoil (Lotus corniculatus). In the past the species appears to have thrived in areas where small-scale controlled burning of heaths was undertaken to rejuvenate growth because it does not do well on overgrown heathlands. In Yorkshire the lowland hearth it lived on was probably disturbed by man's activities such as turf cutting, and grazing.

Distribution

In Britain the species is mainly on heathlands south of a line from the Thames to the Severn, with a presence in East Anglia, Shropshire and Wales. Successful introductions have been made in Suffolk and on the Wirral.

The Silver-studded Blue probably disappeared from Yorkshire about a hundred years ago but in 2004 a colony was seen near Lindrick Common which was presumably due to a private release. It is a difficult species to conserve because its requirement for thinly vegetated soils (as caused by the small scale burning of heathland or by the erosion of soils) is not easy to replicate on a regular and long-term basis.

Also individual butterflies rarely stray more than 20 metres from their point of emergence so natural colonisation is slow, requiring a continuation of suitable habitat. In North Wales, where the species was helped to spread to the Dulas Valley, colonisation was at the rate of about one km in ten years.

Life Cycle

Eggs are laid singly on foodplant stems in areas of sparse vegetation, but they do not hatch until the following spring. The larvae are strongly associated with ants and usually pupate close to, or inside, ants' nests. The adults are then on the wing for only four or five days in colonies which vary considerably in size from tiny to very large, some with several thousand specimens flying together.

Generally there is only one brood between June and August but occasionally there can be two depending on height and latitude.

Status

UK BAP Status: Priority species

BC Priority: Medium

Protected in GB for sale only

European status: Not threatened

Aberrations

There are around 120 described aberrations across its range in Europe and beyond. In Britain three subspecies are recognised though some experts prefer to class these variations as racial forms rather than distinct sub-species. The main subspecies is argus which is thought to occur throughout the species' British range; with cretaceous found in Dorset and the now very rare caernensis in North Wales. A fourth subspecies, masseyi, has become extinct in from the mosses of Lancashire.

With thanks to the Butterflies of Yorkshire edited by Howard M Frost

Catherine Jones 10/11/20