Sussex Butterfly Conservation : Species

Butterfly Species in Sussex

Female Brown Argus nectaring on Birdsfoot Trefoil – photo by Martin Kalaher

Brown Argus

Aricia agestis

Description

For the county, as a whole, this is a relatively uncommon species, as its principal larval food plant is Common Rock-rose, and this wildflower is only found naturally on chalk downland. Where we find good-quality chalk turf we should be able to find a colony of Brown Argus, but as many of these colonies are rather small, a fair amount of searching may be required to find just 2-3 individuals. In some years, if the conditions are very favourable, a few downland colonies may be large, and a count of 125 at Chantry Hill, in 2017, was notable, and even more memorable was a huge count of 1000+ at High and Over, in 2023.

Away from the South Downs, this species lays its eggs on a wide variety of wild Geraniums such as Dove’s-foot and Cut-leaved Cranesbill, and also Common Stork’s-bill. These native plants are very widespread, but suitable plants are often very scattered, and the Brown Argus colonies they support are usually small. This is a species that seems to be rather good at surviving in very small numbers, in many types of marginal habitat.

It is a diminutive butterfly, and it can be easily over-looked. An experienced observer will recognise the characteristic silvery flash of the wings, in flight, and will be aware of the possible confusion with Small Blue. When perched, a Brown Argus and a “mostly brown”, female Common Blue can look very similar (they are both small brown butterflies with orange markings on the wing margins). In the field, the behaviour of the territorial male is probably the best guide to identification. He sits proudly on a grass head or a seed head and defends his territory with great purpose, checking out or seeing off any butterfly that comes within range. More often than not, he will also sit there quietly and allow a close examination of a camera lens!

Between 2006-2024 the “average first sighting” was May 5th (range April 21st to May 29th) and this first brood flies until late June. The more numerous second brood flies from early July to mid-September. In the 20th century this species was firmly double-brooded, but in this millennium we can now see a fairly regular third brood between mid-September and early November. The 2006-2024 “average last sighting” was October 14th with a range of August 22nd and November 14th. In most years it has a total flight period of well over six months.

Where to find

Anywhere, where there is Common Rock-rose on the South Downs. Reliable sites include Chantry Hill at TQ085123, High and Over at TQ509013, Levin Down at SU887130 and Steyning Downland Scheme at TQ168112.