The Brown Argus, Aricia agestis
The Brown Argus is a slightly uncommon butterfly that can be found in small numbers in Hadlow Parish – but seek and ye SHALL find – especially in the late summer, July to September!
The first impression of a Brown Argus is usually of a chocolate brown small butterfly, that looks quite a bit like a female Common Blue in some ways, and it moves and has a definite jizz of a Blue Butterfly. I would say it is usually a clearer and perhaps richer brown in contrast to the female Common Blue, with a clear black mark in the centre of the upper fore-wing.
The two sexes are very similar, but the orange spots on the front wing of the males do tend to get a bit smaller and fade out as they get close the the front edge, or margin, of the wing. In the males the abdomen tends to project out beyond the hind wings, whereas in the females it is shorter. Males are most likely to be seen, but this Brown Argus perhaps has bright enough orange patches on its wingtips, stretching around to the front margin of the fore-wing, to be a female. Note the short abdomen.
When you see the underside of the wings the feature most commonly quoted is the arrangement of the black spots – on the underside of the hind wing these do NOT form a circle in the Brown Argus, and the two at the top are often referred to as forming a colon. Another feature is the lack of a spot nearer the body than the half-way mark on the underside of the fore-wing, and unusually this appears to be quite clearly exposed in this photo.
In the UK the Brown Argus is confined to the southern half of the country, with some colonies in North and South Wales and Cornwall, but few northwest of a diagonal line drawn from Dorset to Yorkshire in England.
There may be Brown Argus butterflies in various grassy areas of Hadlow Parish, but they are not very common and seem to be not easily found. The best places to see them are the Bourneside Meadow, the Fairlawne field margins, the Styles Place Reservoir bank and the meadows and field margins on either side of the River Medway.
In the UK, the Brown Argus is no longer confined to southeastern chalk or calcareous grasslands as it used to be, and is now apparently spreading to other less specialised grasslands (such as the ones in Hadlow Parish – fantastic for us).
The Brown Argus is found across the whole of Eurasia as several different subspecies.
In the south of the country the adults fly in two generations, the first from mid-May to the end of June, the second from July to September, although there is a small amount of overlap. Each individual butterfly only lives for about four days, but can still be relatively mobile, and on average may travel up to 200 m, up to 10 times more than Adonis or Silver-Studded Blues for example – a slight tendency to wander that may have facilitated its recent range expansion in the UK.
A virgin female will perch on grass heads and a male will soon arrive to mate with her. Once mated, the female may reject further approaches from other males by bending her abdomen up in the air, as in this “tableau” here:
The male in this picture looks very worn and tired. Poor chap!
The females lay their eggs with great care on their chosen food-plants. Within the bounds of historical understanding it has been thought that the traditional egg-host and caterpillar food-plant in the relatively cool UK has been the Common Rockrose, but as the climate has warmed, the butterfly has added Dovesfoot Cranesbill and some other Cranesbills and Common Storksbill to its UK range of chosen egg-hosts and caterpillar food-plants. Of these plants, the Cut-leaved Cranesbill is by far the commonest in Hadlow parish. The key to this choice appears to be in the behaviour of the egg-carrying females – what triggers her to choose which plant – and could this be temperature-dependent?
Once the eggs hatch, the caterpillars “windowpane” the leaves of the food-plant from the underside, leaving a clear membranous window in the upper epidermis. Well worth my while looking out for in future years! As in many other “Blue” butterflies, the caterpillars are extremely attractive to ants especially just before pupation. Ant workers appear to live longer when fed on the caterpillars’ secretions, and the caterpillars may (to some extent) be protected by the ant activity around them.
This female (note the short abdomen) was seen south of the Medway, on the edge of an arable field below East Lock, in August 2019, so is from the second generation of adults this year.
This Brown Argus should be a male, judging by the slight fading out of the orange spots on the top of the forewing as they approach the front margin of the wing, and the projecting abdomen. It will also be a second generation adult, as it was seen in August.
As late as the 1990s the Brown Argus (a very picky butterfly) was thought to rely almost entirely on lush plants of RoAckrose, Helianthemum nummularium, and it suffered greatly as these areas were ploughed up or fertilised. However in this decade the butterfly appeared to adapt in the UK to using a couple of its alternate hosts known to be used in Europe – Dovesfoot Cranesbill and Common Storksbill – and extended its range to new habitat where these plants can be found, as well as extending its range over 100 miles northwards. One of the important factors facilitating this change is thought to be generally warmer temperatures, which allows the butterfly to thrive on these alternate hosts. Another factor has been agricultural “setaside”, which caused a great increase in these plants. These factors together mean that this butterfly can now be found in Hadlow parish. A generation (30 years) ago, it could not!
There is a sister species, the Northern Brown Argus (Aricia artaxerxes) found in northern England, northern Wales and Scotland. This is a rarer butterfly and is regarded as threatened in the UK. DNA evidence indicates some cross-pairing in the boundary zone where both species meet has occurred in the past, and the current northward shift of distribution of the Brown Argus, if maintained, may increase the frequency of hybridization in the future.
There are many other species of Argus, Aricia, in Europe and Asia, with complicated taxonomic relationships.
Predators and parasites
Even though the caterpillars are tended by ants, at least five species of parasitic wasp and one species of Tachinid fly manage to get access, and attack the poor things! One advantage though – research has suggested that parasitism has been somewhat less in the newer areas that the butterfly has recently colonised.