Brown Argus  

Brown Argus Aricia agestis (Denis & Schiffermüller 1775)                         

Family: Lycaenidae, (The Blues)     Wingspan 28mm

Name origins (Etymology): Argus means having many eyes and in greek Aricia means 'blessed' or 'shining' and Agestis meaning rural or countryside    

Identification

This species has a big problem in that it often dismissed as a common blue female with which it often found together. Your first encounter of this lively little butterfly is a silver flash that is tricky to follow.  Once landed it usually opens its wings to reveal a warm, chocolate brown ground colour to the upper wings, and striking white wing margins when fresh.  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.

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, usually yellow,  including Common Bird’s-foot trefoil, dandelions, buttercups, Common rock-rose and ragworts and can very often be found nectaring alongside Common Blue  which can make ID tricky

Brown Argus & Northern Brown Argus

Its complicated!

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 Craven limestones and  Durham magnesian limestone plateau  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 county Durham 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 also now not a completely reliable indicator either!

Identification Tips : Chocolate & Orange!

Common Blue females causes the most problem as  both not only look alike but also tend to nectar on the same  yellow flowers in particular Birdsfoot trefoil and at the same times of year so it is a frequent question. However there are some good tips and when landed and wings open fairly easy to tell apart.  The Brown argus has no blue while the female Common Blue have a variable amount of blue scales on its wing near the base.  Also, the Brown Argus is that rich chocolate colour with orange lunules. Secondly with Common Blue the lunules are tending to be outlined in white  emphasising the black eye  while the  lunules on Brown Argus are plain bright orange. The black spot in the centre of the forewings is much reduced in Common Blue as are the orange lunules and much less clearly defined. The sexes can be determined by their abdomens Males being long and thin and protruding beyond the wings while females  shorter dumpier often with  larger lunules and more lunules on the forewing

There is a definitive method by look at the underwing but that is tricky as the wings need to be closed and you need to get very close.  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 below) NB very fresh specimens can have a bluish hue  like many species

                                 Brown Argus (male)                                                             Common Blue (female)

Habitats

 Sedentary (Rockrose) form: Unimproved calcareous grasslands 

Quite large colonies can occur on abundant rock-rose, a plant of chalk and limestone habitats.  Typical habitats are provided by ancient or semi-natural, unfertilised, base-rich grasslands found in very local areas on the Yorkshire Wolds and in the Craven Limestones and North PenninesThe insect has  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) and  more likely to survive drought which is common on these very dry soils. Occurance static or decreasing Abundance  increasing  but suffers in dry years  Sites are increasingly precarious

Habitat: Migratory (Geranium) form:  Sandy, dry,  bare or disturbed areas  often as a arable crop weed   

The Migratory form  arrived  in Yorkshire around the millenium and spread fast in just a few years and lives in a variety of habitats  feeding on Geranium species in particular Dove's foot Crane's-bill;  a pioneer weed in arable agriculture but also sandy soils, dry areas, woodlands, brownfields & dunes   Distribution and abundance continue to increase rapidly

Only  mis-identification stops this species being recorded more commonly that Common Blue and  certainly rivals Holly Blue on most sites. Brown Argus has adapted to a habitat niche of the spray  margins, especially headlands,  of  intensive agriculture and along with  a warming climate combined to benefit  this species possibly more than any other. The bulk of the population is now  the migratory form moving from  breeding in arable fields in spring  to  flowery places in  summer  including  gardens in rural areas.

Over most of its  european range Brown Argus uses Geranium species as  its foodplant. The majority of colonies are small and transient in and on the margins of arable crops,  but also sandy areas,  dunes, heathlands  and grasslands on disturbed ground.  The major geranium used  is Dove's-foot Crane's-bill (G. molle), but occasionally Common Stork’s-bill  and Cut-leaf Crane’s-bill all three have become arable crop weeds but especially Dove's foot over  the last 30  yeasrsThese annuals usually germinate in the autumn and overwinter and grow vigorously in more open winter crop like Oil seed rape 

Dove's-foot Crane's-bill (G. molle) is an annual with round downy leaves made of many leaflets. Each leaflet resembles a dove's foot having three 'toes'  each 'toe' ending in a  round lobe. Flowers are pinky/blue with 5 petals each with two lobes.  Often confused with Shining Geranium (G. lucidom) with 5 leaflets with red leaf stems and glossy leaves and small dark pink flowers. Both species thrive on disturbance as pioneer species and very invasive, see below.  This glossy species is not used as host.

Cut-leaved Crane's -billup to 60cm tall 'cloud' of heavily dissected leaves with narrow lobes sometimes purple edged and very small flowers.  Creeping and hairy  and  found on stony ground, wasteland etc.

Common Storks-bill:  A very hairy sticky plant  with a strong smell.  A plant  of bare sand  and dunes. Often confused with Herb Robert

Status :   Common and  widespread  with both abundance and occurance increasing rapidly as a benificiary of intensive agriculture

Distribution, Occurance and Abundance  trends  all reaching all time highs 

 History: This dynamic species has rapidly expanded northwards. Besides  driven by our warming climate another key  factor is the changes  in intensive agriculture 30 years ago.  Replacement of older  herbicides with new more selective herbicides in the 90's  along with the rise of winter crops like Oil seed rape have created a 'vacuum' for pioneer geranium species to thrive in during autumn and winter. Today a good number of geranium species have become a significant weed particularly on Oil seed rape  but also cereals. Spread from these spray margins into conservation belts and set aside  has helped geranium species take off 30 years ago and the butterflies have followed.

  The first hint that an expanding population might colonise Yorkshire came in 1997 when a small outpost was established in sand dunes  at the tip of Spurn NNR.  Warm summers in the 1990's allowed populations in southern and eastern England to build up leading to an advance northwards and westwards. By 1995 a few vagrants had reached north Nottinghamshire.  The colonisers spread coming north into Doncaster and Sheffield in 1999, the south end of  wolds and along the Aire valley in 2000  and then into the north of the  Vale of York in 2002  crossing the river Tees in 2006.  It is now widespread and common  and  often sometimes in gardens but particularly the open countryside in the lowland upto  about 150m. It has spread up the river valleys bringing it into contact with Northern Brown Argus in Wharfedale and it is known from  genetic testing  they have hybridised with Northern Brown Argus in Wharfedale but the consequences are so far unknown. 

 Abundance Scale =

There has been a  big increase in  population (abundance) of 3 fold in the last 10 years with significant increases in warm years the trend being increasing upwards.  The spreading north of the species is well  illustrated in the map time line.  In addition,  more abundance means they are being seen in more locations with a 70%  increase in occupancy. Overall occupancy of 5% of annually visited squares is likely a big underestimate as this small, fast, silvery in flight,  butterfly is overlooked or misidentified as a common blue. The bulk of the population is now  the migratory form moving from arable fields into  flowery places in high summer which may account for the  unusually lumpy graph for occupancy with hot summers increasing migration and more than doubling occupancy. 

 Where and When to find 

 Brown Argus sedentary  colonies populations are often larger than the geranium form, and found on the chalk grasslands as discrete colonies in the northern Yorkshire Wolds in such places as Sledmere SE9365, Burdale SE871631, Fordon TA057751, Cowlam SE975648, Nine Spring Dale SE868692, Cottondale TA024769, Fridaythorpe SE880578 and Thixendale SE834615.  In uplans areas they are single brooded (May/Jul) but now often produce a  2nd brood  in warm years (Aug/Sept). 2nd brood is somewhat smaller. Below  about 100m 2 broods are the norm.

The Brown Argus migratory  generally exists at much lower populations and are highly mobile utilising Dove’s-foot Cranesbill Geranium  on non-limestone soils and Common Stork’s-bill Erodium cicutarium on coastal dunes. It has 2 broods (May/Jul, Aug/Sep) and even a partial third.  The second generation  is generally twice the size of the first and in hot years can really get going with even a third generation being seen.  In 2022 the second generation was smaller with teh severe drought of that long hot summer. 

In 2022 the cranesbill form boomed on more moisture retaining mineral soils of lowland sites while rockrose colonies suffered from the previous years drought . It seems a paradox that none were seen that year till August suggesting what we did see have come from somewhere else, possibly neighbouring farmland where dove's-foot cranesbill can be abundant, but nectar poor. In 2023 this local movement was seen in our data with the number of sites  increasing from 9 to 27  and  abundance tripling at the same time and the best year so far.   More than half of our monitored sites recording individuals illustrating its great mobility.   This year they were  likely attracted to nectar on our managed sites as it was often seen nectaring alongside Common Blue. It is likely to disappear from these sites next year, as is it's habit. 

Life Cycle

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. 

Brown Argus (Male)

Brown Argus (female)

Egg on the underneath of Dove's-foot Crane's-bill

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 

 Contributions from  Martin Partridge