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. 

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. 

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. 

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. 

 Contributions from  Martin Partridge