Butterflies of Britain
Family - LYCAENIDAE
subfamily - THECLINAE
Tribe - EUMAEINI
Satyrium w-album, male, Bentley
Wood, Wiltshire ©
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 and northern Scandinavia. It's range extends
eastwards across temperate Asia to Japan.
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.
Tall elm trees
dominated the British countryside for centuries,
but almost entirely disappeared due to the arrival in Britain in
1976 of a 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 the ascomycete
micro-fungus 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 larvae 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 in
particular 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.
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.
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
The butterflies are single-brooded, emerging in late June and early
dark brown eggs are shaped like an inverted saucer. They are laid
singly in July 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.
develop within the eggs in the autumn, but don't hatch until late
February or early March of the following year. In March 1980, a
remarkably lucky bit of timing enabled me to watch a larva in the
process of hatching. It nibbled its 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
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
I have found
pupae, attached by a silk girdle to pads of silk spun on the
undersurface of elm leaves, and have heard reports that they can
also be found attached to small twigs.
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
growth. They will sometimes fly up to chase and investigate other
butterflies, and on such occasions up to 3 or 4 territorial
w-album males can be seen spiralling
rapidly around each other above the tree tops.
Females emerge about a week later
than males. They probably mate at the emergence site and lay a high
percentage of their eggs locally, but later disperse covering
distances of at least 1km to lay the remainder of their eggs.
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.
Satyrium w-album, male, nectaring at
nectaring, the butterflies walk about 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.