Butterflies of Britain
Family - LYCAENIDAE
subfamily - THECLINAE
Tribe - EUMAEINI
Gorse, Dorset ©
The Brown Hairstreak is
the last of the single-brooded butterflies to emerge in Britain, the
males usually appearing around 4th August, and the females a week
later. Females are distinguished by having a broad orange patch on
the upperside forewings, and a more richly coloured golden-orange
gets its common name from the white hair-line streaks on the
Its scientific name betulae seems to be
a complete misnomer - the word translates as "birch", but it is not
in any way associated with birch trees.
is widely distributed across most of Europe, but absent from
Portugal, southern Spain, Italy, the Mediterranean islands, Scotland
and northern Scandinavia. Beyond Europe it occurs eastward across
temperate Asia to Korea.
There are no other similar species in Europe, but
novices often confuse this butterfly with the male Vapourer moth
Orgyia antiqua which also flies around
the tops of trees on sunny days. The Vapourer has a wild and erratic
twisting flight over long distances, whereas the Brown Hairstreak
tends to undertake very short flights, usually settling high in ash
trees, but occasionally descending to visit flowers.
In Britain this beautiful butterfly is considered a rarity,
occurring in small discrete colonies scattered mainly across
Hampshire, West Sussex, Dorset, Devon, Somerset, Oxfordshire and the
western counties of Wales and Ireland. It is however a very elusive
species, and certainly under-recorded, so it is likely that its
distribution and abundance are both under-estimated.
This species breeds
mainly along unmanaged blackthorn hedgerows, usually in association
with ash trees which the adults use as assembly points when seeking
mates. There are nevertheless several colonies where ash is not
present, and at these sites the butterfly may use sycamores, oaks or
other trees for assemblage. Colonies also exist in blackthorn
thickets on sheltered scrubby downland; and along woodland edges and
at the edges of rides in mixed or deciduous forests.
Most colonies are small
and very localised, confined to a particular corner of a woodland or
thicket, and comprising no more than a dozen or so adults at peak,
and perhaps a total of about 60 adults over the entire flight
season. Many colonies are smaller - e.g. there is a colony based on
an isolated clump of blackthorn and ash at South Harting in Sussex,
which probably comprises less than 6 adults at the peak of the
flight period. At some sites the butterfly is much more widely
dispersed, occuring at low densities over several hectares of
unmanaged blackthorn / ash hedgerow. Sadly the current obsession
with tidying up the countryside means that most hedgerows are
trimmed mechanically, creating dense hedging that is unsuitable for
the butterfly. Unfortunately misguided management also takes place
at certain nature reserves, where obsessive scrub removal has
drastically reduced suitable breeding habitat.
The butterfly can
sometimes turn up in very odd situations, e.g. I once found an
immaculate female in a dense beech plantation in West Sussex, half a
kilometre from the nearest blackthorn bush, and 3 kilometres from
the nearest known breeding site.
Females disperse over a
wide area to lay their eggs, sometimes founding new colonies.
Conversely, within their distribution range there are many
apparently suitable sites from which the butterfly is entirely
female basking between bouts of egg-laying, West Sussex ©
The best way to locate colonies
of this species is to search for the dome-shaped white eggs, which
are laid singly or in pairs in the forks of blackthorn twigs.
They are laid
in August, but huge numbers are destroyed by hedge trimming
operations, or by misguided or over-zealous scrub clearance work on
Thecla betulae, female,
ovipositing on blackthorn ©
searching almost any blackthorn bush, although the females prefer to
lay on young shoots growing in sunny sheltered situations along
woodland edges, particularly in the vicinity of ash trees.
One day in September 2009 at Tidworth Ranges I found 28 Brown
Hairstreak eggs during a 2 hour search. All were laid on young
blackthorn shoots, while older lichen-encrusted growth was ignored
by the ovipositing females. The vast majority of eggs were laid on
east or south facing woodland edge blackthorns, often 100 metres or
more away from the ash trees where the adults meet and copulate.
There were definite "hot-spots" favoured by the females, where up to
half a dozen eggs could be found close together along a one metre
stretch of blackthorn.
overwinter, and hatch the following April, coinciding with the
appearance of the blackthorn buds. The larvae reportedly take up to
a whole day to nibble their way out of the egg, after which they
rapidly crawl into an unfurling leaf bud where they remain until
their first skin moult.
larvae are green with rows of short diagonal whitish dashes along
the sides, and very slightly hairy. They live solitarily, feed
nocturnally, and rest during the day on the underside of blackthorn
leaves, where there cryptic colouration and patterning renders them
virtually invisible. Despite the effectiveness of their camouflage
however at least 80% of larvae are killed and eaten by predators -
spiders, wasps and insectivorous birds.
When fully grown they descend
to pupate amongst leaf litter and broken twigs beneath blackthorn
bushes. Some authors state that the pupa can also be found attached
to the upper surface of leaves or twigs on the bushes. The pupa is
dark brown and shiny, with darker freckling on the abdomen. It is
reportedly attended by ants Lasius niger
which probably offer it a degree of protection against
carnivorous beetles. Studies by Jeremy Thomas however concluded that
mice and shrews found and consumed a high percentage of pupae, and
that at one particular site in Sussex, almost four-fifths of wild
pupae were killed by predators.
ovum in fork of
Thecla betulae, female,
Steyning, West Sussex ©
Hairstreaks begin to emerge at the beginning of August, when recently
emerged males can sometimes be seen resting on the terminal leaves of
ash saplings. Their first flight takes them to an assembly point -
usually at the top of the tallest ash tree in the vicinity. In the
absence of ash trees, sycamore, oak and other species are sometimes
spend most of their lives high up in these so called "master trees",
where they feed on aphid secretions which coat the buds and
leaves. They are rarely seen low down, but do occasionally descend to
nectar at hemp agrimony or ragwort in years when aphid populations are
low, resulting in a scarcity of honey-dew on the ash trees.
female, Noar Hill,
emerged virgin females can occasionally be found at rest on bushes in
the early morning, but as soon as their wings are fully dried and
fly up to join the males in the master trees,
of which there are usually several serving each colony. Courtship is
very brief and takes place in mid-morning on sunny days. Males have a
patch of androconia ( scent scales ) on their upper forewings, from
which they disseminate pheromones. As soon as the sexes meet the
female responds to this chemical attractant by leading the male to a
more sheltered spot lower down on the tree, where copulation takes
place immediately. The pair remain joined for about an hour, after
which the male returns to the tree tops, where he probably mates with
females are reputed to remain in the ash trees for about a week while
their eggs are maturing, but thereafter spend their lives
at lower levels. They are decidedly elusive, but with determination
and luck can be discovered crawling about purposefully over young
blackthorn twigs, probing with their abdomens to locate suitable
places to lay their eggs. On a sunny morning a female will begin
ovipositing at about 9.30am, laying perhaps 20 eggs in the course of
the next 3 or 4 hours.
bouts of egg-laying the butterflies rest for long periods, usually on
blackthorn bushes or ash saplings.
Hairstreaks are generally regarded as being sedentary in behaviour,
but my observations show that this is certainly not always the case.
In late August 2010 for example, I visited Dunch Hill in Wiltshire
where I saw 3 females, each on small isolated "islands" of blackthorn
scrub, surrounded by broad stretches of grassland. I watched one
female basking on blackthorn, and soon afterwards she took flight
across an open expanse of grassland, eventually resettling on another
blackthorn bush about 150 metres away.
isolated bushes were about 1km distant from the nearest known
site - a blackthorn thicket across the county border in Hampshire.
Hairstreaks were also reported at the same time from several
previously unrecorded sites in Hampshire and Sussex, indicating that
in favourable years females will disperse from their traditional
habitats, and can turn up at least 2kms away from known sites.
Thecla betulae, female, Tidworth
ranges, Hampshire ©
Nectaring behaviour varies considerably from site to site - at
Selborne for example both sexes are commonly seen at the flowers of
hemp agrimony, but at Copsale they nectar almost exclusively at
ragwort. Bramble flowers are visited at many sites, and less commonly
fleabane or creeping thistle. Once they have settled to feed they
often remain at the same clump of flowers for half an hour or more.
Both sexes also commonly imbibe the sugary secretions which coat the
black buds of ash; and can often be seen probing the surface of ash
leaves, feeding on honey dew.
sexes feed with their wings tightly closed, but in weak hazy sunlight
they can sometimes be found basking on blackthorn bushes or on low
foliage, with wings fully outspread.
Thecla betulae, female, Alner's
Gorse, Dorset ©