NORTHERN
BROWN ARGUS
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| male
mating pair female
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- 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!!
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