Brown Argus

Brown Argus Aricia agestis (Denis & Schiffermüller 1775)

Family: Lycaenidae, (The Blues)

Description

Photos by Martin Partridge

The Brown Argus has a warm, chocolate brown, ground colour to the upper wings, and striking white wing margins when fresh. The butterflies average wingspan is 28mm. There may well be a visible discal spot on each forewing, sometimes just black and sometimes with a white circle around it. The orange crescents (“lunules”) on all four wings vary in number an intensity of colour. The females of the species typically have more pronounced lunules and more of them particularly on the forewings. It is lively in flight showing unexpected shimmer of silver as it flies.

The males are often seen patrolling with a very darting movement over a small area of land flying low to the ground and landing often on prominent twig or piece of grass to survey his territory for incoming females. They perch usually with their wings wide open basking in the sun. If disturbed they take flight but often return to the same location. If another male or a female or another similar sized butterfly, Common Blue or Dingy Skipper comes into view, the male launches into aerial combat and the two head spiraling upwards before the intruder is chased off. In the early mornings they are often found perched on a prominent blade of grass wings closed. Where the populations are of a reasonable number they can be found roosting together. The adults feed on a variety of nectar sources including Common Bird’s-foot trefoil, dandelions, buttercups, Common rock-rose and ragworts.

The two species often inhabit similar places so caution is required with regards to identification, photographs are always very helpful, particularly underwing shots.

The best way to tell the Brown Argus and female Common Blue apart is to look at the underwing. The Brown Argus has two of its rear underwing dots in a figure of 8 or colon (:) arrangement. The Common blue has them in a continuous arc the two dots being in a line (. .) There is also a dot missing on the forewing of the Brown Argus when compared to the Common Blue. (see photos)

Separating the Brown Argus (A.agestis) from the Northern Brown Argus (A.artaxerxes) is now almost impossible in our region unless you are in a geographic area which is known only to have Brown Argus or Northern Brown Argus populations. In areas where they now may overlap in VC65 and VC66 only genetic analysis is likely to be able to tell them apart.

In the past there were a number of accepted methods to distinguish the two, but these are all now considered unreliable. These include using differences in the upper wing discal spots. The Brown Argus A. agestis (with black spots), Northern Brown Argus A. artaxerxes (with white spots) and Northern Brown Argus subspecies A. salmacis (with black and white spots) which was in the past recognized as coming from the Castle Eden Dene area in VC66 and stretching over to southern Cumbria.

White scales, and even the all-white discal spot, can occasionally appear throughout the Brown Argus range becoming more frequent as one heads north. Likewise, within the former A. salmacis range, a proportion of butterflies still show black discal spots. The number of white scales appearing in either group varies from a single scale to a white patch or even a complete ring of scales around the black discal spot (ab. snelleni Ter Haar). The Brown Argus tends to be more strongly marked than its northern counterpart, with more prominent orange lunules, and bigger black spots in the dot pattern of the underwings. The upper wing ground colour also tends to be a more reddish brown in the Brown Argus compared to a much darker brown in Northern Brown Argus. However, the range of variation means that individuals cannot be cited as having diagnostic features either in Yorkshire or elsewhere.

It is often noted that the Brown Argus is double brooded (Bivoltine) and the Northern Brown Argus is single brooded (Univoltine) but this is not a reliable indicator.

Habitat

The sedentary Brown Argus populations all seem dependent on common rock-rose, a plant of chalk and limestone habitats. Typical habitats are provided by ancient or semi-natural, unfertilised, base-rich grasslands, on the Yorkshire Wolds and in the northern Pennines. Both species appear to have lost out in the trend to fertilise most grazed grassland areas. Any sites still suitable are usually found on steeper slopes, which are too difficult to work, or in old quarries. The butterfly itself seems to be restricted to areas near the foodplant. Experiments and observations indicate that more succulent examples of the foodplant are chosen and eggs are usually laid on the darker more luscious leaves (Ellis 1997). These plants seem to be the ones which have found rather more nitrogen than the less luxuriant plants found on thinner soils.

The Brown Argus population which is moving at a pace up from the south utilises a number of different habitat types include for example, sand dunes, heathlands and grasslands on disturbed ground, where plants of the Geranium (Geraniaceae) family are found and used. The major ones being Dove's-foot Crane's-bill (Geranium mole), Common Stork’s-bill (Erodium cicutarium) and Cut-leaf Crane’s-bill (Geranium dissectum) grow. Interestingly in continental Europe the Brown Argus also uses plants of the Geranium family. The abundance of these suitable hosts plants has been aided by the setting-aside of land by farmers from the mid 1980’s allowing the growth of the Geranium family plants over vast new areas of the countryside.

Distribution

Brown Argus resident populations: These fairly small, and apparently sedentary colonies are found on the chalk grasslands of the northern Yorkshire Wolds (VC61) (c100-200m) in such places as Sledmere, Burdale, Fordon, Cowlam, Nine Springs Dale, Cottondale, Fridaythorpe and Thixendale. These populations are based on Common Rock-rose Helianthemum nummularium as foodplant and are usually single brooded (Univoltine) (May/Jul) but may produce a partial 2nd brood (Bivoltine) in good years (Aug/Sept).

Brown Argus colonisers: The bulk of the Brown Argus population in Britain has 2 broods (May/Jul, Aug/Sep) using Common rock-rose on limestone areas, Dove’s-foot Cranesbill Geranium molle (and other geranium species) on non-limestone soils, and Common Stork’s-bill Erodium cicutarium on coastal dunes. The first hint that an expanding population might colonise Yorkshire came in 1997 when a small outpost was established in a modified sand dune habitat at the tip of Spurn NNR. It is assumed that this resulted from immigrants flying across 5+ miles of open estuary from Lincolnshire. VC61 has also produced other sightings which appear to be colonisers, e.g. on the southern edge of the Wolds at Weedley Springs in 2000 [Sean Clough] and at Allerthorpe Common in 2002 [Christine Frost]. 1999 saw the first recently recorded VC63 records at Warmsworth (just west of Doncaster) where 6 were seen on 17/06 [Martin Roberts]. In 2000 further ‘arrivals’ were noted at Lindrick Common [Ben Key wood], Potteric Carr [up to 11 seen by Ken Woolley], Sprotborough [Dave Booth] and Limpool [Martin Greenland]. Since then, reports have increased annually. The butterfly also appears to have jumped northward to Fairbum Ings RSPB NR in VC64 where it was recorded on 20/08/2000 [Philip Stephenson].

The Brown Argus is now found across all five Yorkshire Vice counties and has crossed the River Tees into VC66 where it was seen for the first time in 2006.

Life Cycle

The Brown Argus populations on Common rock-rose can be single brooded (Univoltine) (May/Jul) but may produce a partial 2nd brood (Bivoltine) in good years (Aug/Sept). The Brown Argus colonies utilising Dove’s-foot Cranesbill Geranium molle (and other geranium species) on non-limestone soils has 2 broods (May/Jul, Aug/Sep). This is not however a hard and fast set of observations.

The first sighting of the Brown Argus in 2020 were made on 08/05 with two seen at St Aidans in VC64.

The last sighting in 2020 was at Oakhill Nature Reserve, Goole in VC63.

The eggs are laid singly, on common rock-rose leaves or on species of wild geranium. The Brown Argus tends to lay eggs on the underside of leaves, whilst the Northern Brown Argus places them on the upper surfaces. The female in looking for places to egg lay is seen flying close to the ground seeking out suitable foodplants tasting them with her feet until she finds a suitable place when she curls her abdomen to lay a single egg on the underside of a leaf.

Eggs appear as squat, whitish spheres, found from mid-June on low-lying sheltered situations in the south of the county, and from early July further north and higher up. Microscopic examination reveals that eggs have a very variable reticulated surface structure. Larvae hatch out after about two weeks, and crawl to the underside of the leaf, where they eat small areas, leaving the upper leaf surface intact. The affected surface of the leaf tends to turn yellow after a short time. Green woodlouse-shaped larvae are well-marked with darker green chevrons and black heads. Brown Argus larva have a green or dark purple dorsal line (down the middle of the back) but such lines vary significantly. The sub-spiracular line, (low down on both sides) varies from white, with purplish pink above and below the white, to white with virtually no colours on either side. In autumn, larvae of second, third or fourth instar hibernate at the bottom of the foodplant or under a leaf and become active again in early spring probably in March. Pupation takes place on or near the ground and ants may attend both larvae and pupae. Larvae are thought to attract the ants by ‘singing’ (ie producing a sound) and by the production of honeydew. Pupae may be carried off by the ants and guarded underground.

Photo above right shows an egg

Status

BROWN ARGUS: UK BAP status: Not listed.

BC priorities: National: Low. N of Eng: Uncertain.

Yorkshire: Medium

European status: Not threatened.

WORLD STATUS: Brown Argus: Resident, 0-1700m: 1 to 3 broods according to height, weather, and latitude (May/Jun, Jul/Sep or Apr/Oct). Present in 35 European countries: declined in 7, and listed as a Red Data Book species (RDB) in 9.

Aberrations

Over 40 Brown Argus variations have been described including ab pallidior Oberthdr with yellow upperwing lunules replacing the orange ones, or ab groof/Ver-Huell where the lunules are white.

ab. glomerata Tutt 1912 + ab. obsoleta Tutt 1912

A double aberration as follows: ab. glomerata: The submedian spots are drawn inwards towards the discoidal spot so as to form a semi-circle around it. ab. obsoleta: Missing spots on all four wings.

ab glomerata + portico-obsoleta

Martin Partridge 15/01/21