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NORTHERN BROWN ARGUS



If any one butterfly typifies the northern counties of England it is perhaps the Northern Brown Argus. This delightful and spirited butterfly has colonies on the limestone slopes on both sides of the Pennines and is always a delight to watch, not least because of the attractive sites it inhabits. In County Durham they call it the Durham Argus, but here in Cumbria we call it by its proper name!

It is perfectly engineered in size and colour for our cooler climes and sensibly only tries to fit in one brood per year, unlike the Brown Argus of the south, which has two. A freshly emerged specimen looks immaculate in its chocolate and orange livery and this dark upper side to its wings helps it to absorb heat when the morning air is chilly. If it gets hot (as it often does, despite claims to the contrary!) it will fold its wings and the light undersides will reflect heat - it is this colouring that gives the butterfly its characteristic flutter of silver as it flits from plant to plant in the early July sunshine.

Females are generally more heavily marked on the upper side with orange, especially on the forewing, and have wider bodies, as do the females of most species for egg carrying purposes. Although texts suggest the undersides of both sexes are similar, direct comparison of a mating pair shows that the male is bluer in tone and the female more heavily marked (see photo).

The food plant is Common Rockrose, a plant of short limestone grassland that occurs in Cumbria on the limestone at the head of Morecambe Bay (Humphrey Head, Whitbarrow, Hampsfell, Scout Scar, Arnside Knot) and the head of the Eden Valley (Smardale, Waitby). However, not all sites supporting rockrose have this butterfly by any means.

Although not yet fully understood, it would seem that the Rockrose must be in the right conditions for egg-laying, probably with a southerly or westerly aspect for the site. Uncultivated and ungrazed sites appear to be best where the grasses do not become too tall, although the species does like some longer grass in sheltered spots where it spends the night hanging from the stems.

The Northern Brown Argus suffers heavily from parasitism - Dr. Mark Porter of Edinburgh University found that up to 80% of the larvae studied at a Whitbarrow site carried parasites. The species appears to have its own host specific parasite, an ichneumonid wasp Hyposoter notatus, which therefore in itself merits conservation for its rarity. This lays its eggs inside the caterpillar of the butterfly, gradually growing and developing by consuming, and eventually killing, the caterpillar from within! There is even another small parasitic wasp which lays its eggs in the larva of Hyposoter as it develops within the Northern Brown larva - isn't nature amazing!!