Brown Argus

Aricia agestis

General Distribution and Status

The Brown Argus is widespread and fairly common in the south-eastern part of Britain; from Yorkshire in the east and Dorset in the west with scattered colonies elsewhere. There was a steep decline in numbers and contraction in range in the 20th century up to the 1970s attributed to habitat destruction through farming intensification which resulted in wiping out Common Rock-rose colonies, the larval foodplants. Since the 1980s set-aside schemes across the country have benefitted the species because it was able to colonise new sites containing alternative foodplants such as Doves-foot Cranesbill (Asher et al.). There was a big expansion in the 1990s in both range and abundance undoubtedly helped by the warm and dry summers in the middle of that decade although populations have declined slightly since then. The situation in Hertfordshire and Middlesex reflects what is occurring nationwide with peak populations during 1995-97 and a small decline since until 2017 when there was a resurgence which continued more spectacularly in the warm summer of 2018 (Wood, 2019). One example of expansion was at the Bayfordbury Estate where Rob Souter notes in his review for 1994-96 that the butterfly has 're-colonised the site very quickly ... possible that the recent hot dry summers may have allowed Geranium spp. to flourish, as grasses have suffered from desiccation' (Souter). However, 2020 and 2021 have seen abundance drop significantly (Wood, 2022).

UK distribution map
UKBMS Species summary (2020)

Habitat Requirements

The Brown Argus was associated with chalk grasslands and woodland rides but the recent expansion in range has seen it spread to other habitat types like gravel pits, roadside verges and field edges. It prefers short turf in sunny situations.

Larval Foodplants

Common Stork's-bill Erodium cicutarium, Dove's-foot Cranesbill Geranium molle.
Common Rock-rose Helianthemum nummularium on chalk.

Adult Food Sources

Common Ragwort Senecio jacobaea (6), Wild Marjoram Origanum vulgare (4), Common Fleabane Pulicaria dysenterica (4).

Historical Records

According to Gibbs' 1902 report Durrant found the butterfly at Hitchin. Foster's 1934 report describes it as 'usually abundant'. Some evidence of the decline came from Waterton's survey for 1970-1981 when he states that it has 'decreased since Foster's time in common with all the chalk-loving species due to loss of habitat and possibly climatic changes'.

Local Distribution and Abundance

As indicated on the map, the butterfly is well distributed in the Stevenage district but it appears to be absent in several areas. In the 2000s, Great Ashby Park harboured great numbers; for example, 40 on 31 July 2004 and again on 29 July 2005. However, numbers have plummeted at this site with no visit yielding more than two individuals since 2009. The best site now appears to be Frogmore Gravel Pit, where sometimes double-figure counts are recorded. The floodplain near the Knebworth Golf Course was also a good spot but now the area is too overgrown for the species to survive. More than half the records sent in for the butterfly up to 2017 reported one or two specimens only so most colonies were probably quite small. However, 2018 and 2019 were fantastic years and easily the best during the survey although no individuals were seen in the spring of 2018. The hot and dry weather in June might have provided ideal conditions for the immature stages resulting in a large emergence in July and early August. Specimens were still being reported well into September perhaps suggesting a third generation. In 2019, the species had an excellent start but numbers fell in the second brood which is unusual and the drop in numbers continued into 2020, probably the worst year since 2012, when there were only ten reports and nowhere was there more than one specimen seen at a site. 2021 was a similar year in terms of reports but eight specimens were reported from one of the best sites, Frogmore Gravel Pit, on 2 June. A very late individual in the same year was seen south of Watery Grove on 10 October, the latest ever in the Stevenage area. Generally, numbers have dropped significantly since 2019 perhaps partly due to the recent dry summers impacting the larval foodplant, Dove's-foot Cranesbill which was present in all the tetrads not supposedly holding Brown Argus colonies, at least in the 2000s (James), so open grassland areas with bare patches or short turf should be searched for the butterfly. There was some good news in 2022 when on 29 August there were sightings near St Nicholas Church (TL2226 not recorded since 2011), and between How Wood and Graveley (TL2428 first record in survey). 2023 was an even better year with 10 specimens reported in Weston on 26 July.

Transect weekly counts

Stevenage (South Fairlands Valley Park) transect 1993-2023

Records show that the species is seen in the majority of years since 2006 which was a bumper year. It is now mostly found at Shackledell Grassland and the surrounding grassland areas. Other areas like Millennium Wood became overgrown so the Brown Argus no longer occurs there. Like the Common Blue the annual cutting regime now being carried out at Shackledell Grassland has proved beneficial to the butterfly. Singletons were found in 1996 and 1997 at the site but thereafter shrubs and coarser grasses were taking over with just the occasional cut. Whatever the reason, the Brown Argus was not seen there for at least another 9 years. 2019 was the best year with four specimens seen on 29 July at Shackledell and the grassy area west of Peartree Wood

Knebworth Park transect 1996-2010 and 2017-2023

Very few were seen between 1996 and 2010 with five recorded including three in 2005. In 2017, the warm spring has undoubtedly helped produce the first reported sighting here at this time of the year. The best year was 2018 when eight were recorded including four on 21 August, all in the north-western section of Knebworth Park.

Knebworth Woods transect 2017-2023

All specimens were seen in the meadow at Norton Green Common and the top end of the open field south of the common. The highest ever local transect count was achieved here on 1 August 2018. Only one specimen noted since 2020 which is a concern.

Pryor's Wood transect 2000-2022

Only one record of this species - on 10 August 2003.

Life History

Earliest date: 20 April 2007 at Frogmore Gravel Pit
Latest date: 10 October 2021 at Watery Grove

The Brown Argus is a bi-voltine species with the second generation nearly always found in greater numbers. Eggs are laid singly on the underside of a leaf of the foodplant. Larvae eat the underside of the leaves resulting in visible patches seen from above and are often attended to by ants. Pupae are formed on the base of the foodplant but sometimes they are buried in ant hills. It overwinters in the larval stage.
More details on the UK Butterflies website

Behaviour/Observation notes

This species usually flies close to the ground and enjoys basking on sunny bare patches or on grass stems. As noted in the account for the Common Blue this species can be confused with the female Common Blue if the latter has minimal blue scales on its wings. The undersides of the Brown Argus differ in the spotting pattern in two ways: 1) the white-ringed black spot on the forewings near the body is absent and 2) the pair of spots nearest the inner edge of the hindwings is closer together which constitutes almost like a figure '8'. Good photographic opportunities can be obtained early in the morning when a number of individuals roost on tall grasses along with Common Blues (Riley).

Variations/Aberrations

There is some natural variation in the extent of the orange spots on the wings. A number of aberrations have been reported around the country and the one which is most encountered in Hertfordshire appears to be ab. snelleni where the black discal spots on the forewings are ringed with white - see photo on the right for specimen seen near Watery Grove on 2 August 2019.
Find out more on the UK Butterflies website

References

Reference	Details
Asher et al. (2001)	Asher, J., Warren, M., Fox, R., Harding, P., Jeffcoate, G. and Jeffcoate, S. (2001). The Millennium Atlas of Butterflies in Britain and Ireland, Oxford University Press, Oxford
Brereton et al. (2019)	Brereton, T.M., Botham, M.S., Middlebrook, I., Randle, Z., Noble D., Harris, S., Dennis, E.B., Robinson, A.E., Peck, K. & Roy, D.B. (2019). United Kingdom Butterfly Monitoring Scheme report for 2018. Centre for Ecology & Hydrology & Butterfly Conservation, British Trust for Ornithology and Joint Nature Conservation Committee. http://www.ukbms.org/reportsandpublications.aspx
Foster (1934)	Foster, Arthur H.(1934) edited by Hine, Reginald L. in 'Butterflies and Moths'. The Natural History of the Hitchin Region, pp. 120-140, Hitchin & District Regional Survey Association, Hitchin
Fox et al. (2023)	Fox R, Dennis EB, Purdy KM, Middlebrook I, Roy DB, Noble DG, Botham MS & Bourn NAD (2023). The State of the UK's Butterflies 2022, Butterfly Conservation, Wareham, UK. https://butterfly-conservation.org/sites/default/files/2023-01/State%20of%20UK%20Butterflies%202022%20Report.pdf
Gibbs (1902)	Gibbs, A.E.(1902) edited by Page,W. in 'Lepidoptera'. The Victoria History of the County of Hertford, Vol 1 pp. 148-153, Archibald Constable, London
James (2009)	James, Trevor J.(2009). Flora of Hertfordshire,, Hertfordshire Natural History Society, Welwyn Garden City
Riley (2007)	Riley, Adrian M.(2007). British and Irish Butterflies, Brambleby Books, Luton
Souter (1997)	Souter, Rob (1997). The Changing Fortunes of Butterflies on Bayfordbury Estate, East Hertfordshire, Transactions of the Hertfordshire Natural History Society Vol 33 Pt 1 pp. 43-55
UKBMS (2022)	UKBMS (2022). 2021 Summary of Changes table for the UK, Butterfly Conservation, the UK Centre for Ecology and Hydrology, British Trust for Ornithology, and the Joint Nature Conservation Committee, 2020 https://ukbms.org/sites/default/files/UK%20summary%20of%20changes%202021%20PDF%20vers.pdf
Waterton (1982)	Waterton, P. (1982). The Status and Distribution of Butterflies in Hertfordshire 1970-81, Transactions of the Hertfordshire Natural History Society Vol 29 pp. 111-119
Wood (2016)	Wood, Andrew (2016). Butterflies of Hertfordshire and Middlesex, Hertfordshire Natural History Society, St. Albans
Wood (2019)	Wood, Andrew (2019). Hertfordshire and Middlesex Butterflies February 2019 (covering 2018 records), Hertfordshire and Middlesex Branch of Butterfly Conservation
Wood (2023)	Wood, Andrew (2023). Hertfordshire and Middlesex Butterflies 2022, Hertfordshire and Middlesex Branch of Butterfly Conservation February 2023