Identify UK Lycaenid? (Aricia agestis)

[Nick Greatorex-Davies]

A very good identification feature for separating Aricia agestis
(Brown Argus) from both Polyommatus icarus (Common Blue) and Plebejus
argus (Silver-studded Blue) that has not been mentioned, is that the
white-ringed black spots at the top edge of the underside of the
hindwing form a colon (:) in A. agestis. This is pointed out in Jeremy
Thomas's excellent little Hamlyn guide "Butterflies of the British
Isles".  In the other two species the white-ringed black spots more or
less form a 'D' shape. With agestis this 'D' shape is interrupted /
incomplete because one of the spots 'drops' below one of the others to
form the 'colon' (however very occasionally the lower 'colon' spot is
missing altogether). This is very clear in Keith Edkins photo of the
underside of his specimen.

I have found this a very useful identification feature in the field.
It is not easy to see the absence of forewing ventral surface
sub-basal dots that are found on P.icarus and, to make life difficult,
one or both of these dots are absent on some icarus specimens too!

Aricia agestis has expanded it's range (as Neil Jones pointed out)
considerably northwards and eastwards in England during the last
decade. For example it can now be seen almost anywhere in
Cambridgehire - even in gardens. Not many years ago it was a rare
butterfly in the county. It's ability to use annual Geranium species
coupled with the generally warmer, and some years dryer, summers of
recent years appear to be the main reasons for it's recent spread,
perhaps helped for a time by the advent of set-aside where annual
Geranium species (e.g. Dove's-foot Cranesbill (Geranium molle),
Cut-leaved Cranesbill (Geranium dissectum) and Small
flowered-Cranesbill (Geranium pusillum) are often abundant. In
captivity agestis larvae do extremely well on all these three species.
About three years ago I wrote an article on the spread of the Aricia
agestis in the Butterfly Monitoirng Scheme report to recorders which
gives a lot more imformation on the spread of this species in the UK
and looks at possible causes. I am happy to send a copy of the article
to anyone who might be interested - I intended to publish it (and
might yet - with a bit more analysis of data) more in the public
domain, but  ......

Regards
Nick



Mr J Nick Greatorex-Davies
(Butterfly Monitoring Scheme co-ordinator)
NERC Centre for Ecology and Hydrology
Monks Wood
Abbots Ripton
Huntingdon
Cambridgeshire PE28 2LS  UK

Tel: (+44) (0) 1487 773 381
Fax: (+44) (0) 1487 773 467
E-mail: ngd at ceh.ac.uk