Butterflies of
Britain & Europe
Black Hairstreak
Satyrium pruni
LINNAEUS, 1758
Family - LYCAENIDAE
subfamily -
THECLINAE
Tribe - EUMAEINI
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Black Hairstreak
Satyrium pruni, female nectaring
at bramble, Northants ©
Adrian Hoskins |
Introduction
The
Black Hairstreak is widespread across central and eastern Europe, but is absent
from the Iberian peninsula, the Mediterranean coast, northern France, and most
of Finland and Sweden. Throughout Europe it is considered to be a scarce and
very localised species, although it's habitat requirements do not appear to be
particularly specialised.
Beyond Europe it's range extends eastward through southern Siberia, Mongolia and
Korea to Japan.
It
is possible to confuse this species with the White-letter Hairstreak Satyrium w-album,
which lacks the black spots on the underside, and in which the white median line
forms a distinctive "W" shape. In continental Europe it can also be confused
with the Blue-spot Hairstreak S.
spini, the underside
hindwing of which bears a perfectly straight white line, and a blue spot on the
tornus.
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Black Hairstreak
Satyrium pruni,
antennae-dipping to "taste" nectar-source ©
Adrian Hoskins |
Habitats
In Britain this species
is a rarity, now confined to about 30 small woodland sites within a narrow strip
of land running diagonally from Oxford to Peterborough.
At these sites the
butterfly nearly always occurs as tiny populations, breeding on tall blackthorn
bushes Prunus spinosa growing in sheltered and sunny situations. These are typically on the
southern edge of a woodland, or in small glades, or along ride edges, but
colonies also exist in hedgerows or in scrubby meadows close to woodland.
Blackthorn hedges that
are trimmed are entirely unsuitable for this species. Almost all of the sites
where it survives in Britain are specifically managed for the Black Hairstreak,
by ensuring the retention of well established blackthorn hedgerows, and fencing
them when necessary to prevent the bushes being grazed by cattle or deer.
Although the butterflies usually oviposit on long established bushes, good
conservation also requires that new young bushes are encouraged, to ensure an
uneven age structure of blackthorn. Bramble and privet bushes also appear to be
significant elements in this species ecology, as they provide vital nectar
sources for the adults.
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Black Hairstreak
Satyrium pruni, female at bramble ©
Adrian Hoskins |
Lifecycle
The butterflies are
single-brooded, emerging in mid-June. The flight period is of
very short duration, no more than 2-3 weeks, and individual
butterflies appear to only live for about a week.
The brown disc-shaped eggs are laid singly on
blackthorn twigs, usually on the
sunlit side of the bush, at any height, and often close to forks
in the twigs. They are laid in June or very early July.
The caterpillars are fully formed
within the eggs by late July, but do
not hatch until the following March when the flower buds appear.
When very young
the caterpillars are dark brown, and rest on the unopened leaf buds.
They feed at first on the flower buds, moving onto the leaf buds
in April and May. As they
develop they undergo several changes in colour and pattern. When fully grown in late May
they are green, with pale diagonal
stripes on each segment, and prominent pink-tipped ridges along
the back. They feed diurnally, and rest fully exposed on the
upper surface of the leaves.
In late May the pupa, which is
black marked with white patches, and perfectly disguised as a
small bird dropping, is formed attached by a silk girdle to a
twig, or less commonly on the top of a blackthorn leaf.
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Black Hairstreak
Satyrium pruni, female at bramble ©
Adrian Hoskins |
Adult behaviour
The
adults emerge in the early morning, and shortly afterwards can
sometimes be found settled on blackthorn twigs waiting for the
wings to harden. Freshly emerged butterflies are brightly coloured
and very beautiful, but within a day or two they become faded and
worn, the red submarginal band quickly fading to pale orange, and
the rich golden brown ground colour fading to greyish brown.
Males tend to sit motionless for very long periods at the top of
blackthorn bushes to intercept passing females. I have not
observed any form of pre-nuptial ritual, and it is likely that the
butterflies copulate almost immediately after meeting. Copulation
usually occurs in late morning,
at the top of blackthorn bushes, but the butterflies will often
fly short distances when copulated - I have for example seen
photographs of copulated pairs nectaring at bramble.
In
hot and sunny conditions females walk about on blackthorn twigs,
usually near the top of tall bushes, laying their eggs singly on
or close to forks in the twigs. After laying several eggs they
take a period of rest, and this is followed by a period of
nectaring. The females descend regularly to nectar at privet,
bramble, dogwood, wayfaring tree and field rose, but are very
secretive and easy to overlook. They tend to descend more
frequently in warm muggy overcast conditions than in full
sunshine, and often spend several minutes sitting on a particular
bramble or privet flower. They also imbibe "honey-dew" -a sugary
secretion produced by aphids which coats the upper surface of
leaves, particularly blackthorn and ash.
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Satyrium pruni, female imbibing
aphid secretion from the surface of nettle leaves ©
Adrian Hoskins |
In late
afternoon, when the changing position of the sun causes the bushes
which they occupy to become shaded, they migrate across glades to
seek the last remaining sunlit areas. They roost overnight on the
upper surface of blackthorn leaves, amidst the tangle of growth
within the bush.
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