Butterflies of
Britain & Europe
White-letter Hairstreak
Satyrium w-album
KNOCH, 1782
Family - LYCAENIDAE
subfamily -
THECLINAE
Tribe - EUMAEINI
introduction
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habitats |
lifecycle |
adult behaviour
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White-letter Hairstreak
Satyrium w-album, male,
nectaring at bramble |
Introduction
The English and
scientific names
of this species both refer to the w-shaped white hair-line
streak on the underside hindwings.
The
butterfly is widely distributed in central and eastern Europe, but is absent
from Scotland, Ireland, the Iberian peninsula, and northern Scandinavia. It's
range extends eastwards across temperate Asia to Japan.
Habitats
Prior to 1970 this was a common and widespread butterfly in southern
Britain, though it has always been considered elusive, as it spends almost all
of it's life at the top of elm trees.
Elms dominated the British landscape for centuries,
but have now almost entirely disappeared following the arrival in Britain in
1976 of a particularly virulent strain of Dutch elm disease, imported on elm
logs from North America. The disease quickly spread, transported by elm bark
beetles Scolytus multistriatus S.destructor and
S.scolytus. The beetles live under the bark
and propagate an ascomycete microfungus Ophiostoma
novo-ulmi which is responsible for the disease.
Within the space of 5 years the disease had ravaged Britain, destroying over 99%
of the nation's elms. White-letter
Hairstreak caterpillars feed exclusively on the flowers and leaves of elms, so
the loss of literally millions of trees spelt devastation for the butterfly.
In
the case of English elms Ulmus
procera, mature
trees were killed, but the roots were unaffected, so sucker growth quickly
returned. The tiny larvae of the butterfly hatch very early in the year, and need
to feed at first on elm flowers, which appear about a month before the leaves.
Unfortunately the age at which elms first flower coincides with the time they
become attractive to the bark beetles, and prone to further disease, so White-letter
Hairstreak colonies are often short-lived.
The
caterpillars also feed on wych elms
Ulmus glabra, but
these are now very rare. Most of the surviving trees are within fairly dense
forest, where they have escaped the notice of the bark beetles.
Luckily the butterfly is a fairly mobile species, females often being seen a
considerable distance from their emergence sites wandering in search of healthy
elms on which to lay their eggs. In exceptionally warm summers when butterfly dispersal is
always highest, they are sometimes able to establish new colonies to replace those
that have been lost.
The trend
however is one of continuing decline, and the species will
remain very scarce until elms are able to develop a natural immunity to the
fungal disease, or until very substantial numbers of disease-resistant strains
of elm are established in Britain.
A program of research into disease-resistant
elms has been under way for several years, and the best hope for the butterfly
seems to centre around the planting and long term establishment of varieties
including Lutece ®, Vada ®, and White Elm
Ulmus laevis.
Lifecycle
The
butterflies are single-brooded, emerging in late June and early July.
The
dark brown eggs are
shaped like an inverted saucer. They are laid singly in July and
early August on elm twigs, always at the point where the current year's growth
and the older growth meet. They are normally laid near the top of the tree, or
on the sunlit southern side.
The larvae develop within
the eggs in the autumn, but do not hatch until late February or early March of
the following year. In March 1980, a remarkably lucky bit of timing enabled me
to observe a larva in the process of hatching. It nibbled it's way out of the
eggshell, and then immediately sprinted along the twig until it found an elm
flower, into which it bored and disappeared from view. The whole process from
hatching to burial within the flower took no more than 15 seconds.
After feeding within the
flowers for about 3 or 4 weeks, the larvae emerge and start to feed on the newly
appearing leaves. When fully grown in early June, they can be found resting on
the underside of the leaves.
I have found pupae,
attached by a silk girdle to pads of silk spun on the
under-surface of elm leaves, and have heard reports that they can also be found
attached to small twigs.
Adult behaviour
The
males emerge in late June or early July, and spend almost all of
their lives sitting motionless on foliage at the top of wych elms
or high on English elm
sucker growth. They will sometimes fly up to chase and investigate
other butterflies, and on such occasions up to 3 or 4
w-album males can be seen
spiralling around each other above the tree tops.
Females emerge about a week
after the males.
I have never observed courtship or copulation, but it is probable
that both take place hidden from view in the tree tops.
Both sexes are highly elusive,
but occasionally descend from the trees to take nectar from
bramble blossom, thistles or hemp agrimony. Sometimes it is
possible to find up to half a dozen adults feeding together on
flowers, usually within a few metres of the parent elm.
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White-letter Hairstreak
Satyrium w-album, male, nectaring
at bramble |
When nectaring,
the butterflies walk slowly over the flowers, repeatedly turning
and changing position. The wing markings tend to direct the eye of
human observers, and doubtless also of birds, away from the head (
which is usually half-buried within the flower ), and towards the
tornus on the hindwings. These have short tails that probably
function as "false antennae", diverting bird attacks away from the
vulnerable head.
As with other hairstreak
species, the flight is fast and erratic, and the males tend to
return to the precise leaf from which they originally departed.
Females have a more direct flight pattern, and tend to cover longer
distances during the course of their lives. They can be encountered
at least a kilometre away from the nearest elms when dispersing in
the latter part of the flight season.
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