Information and Advisory Note Number 114 Back to menu
The distribution, ecology and conservation of the pearl-bordered fritiilary
butterfly Boloria euphrosyne in Scotland
1. Introduction
1.1 The pearl-bordered fritiilary is one of five fritiilary butterflies in the
UK, whose larvae feed on violets Viola spp. As adults all five are brightly
coloured orange and black on the upper-side. The pearl-bordered fritiilary is
medium-sized and most closely resembles the small pearl-bordered fritiilary
Boloria Selene which also flies between April and July.
The pearl-bordered fritiilary tends to fly earlier (late April to June) than the
small pearl-bordered (June and July) but is best
distinguished by careful examination of the underside of the hind-wing.
The pearl-bordered fritiilary has a small black spot between the central white
pearl spot and the body, and the wing margin has chestnut edges to the pearls.
In contrast the small pearl-bordered fritillary has a series of off-white pearls
around the middle of the wing, a black spot which is two to four times bigger,
and black edges to the pearls on the wing margin.
1.2 The pearl-bordered fritillary occurs throughout the Western Palearctic from
Western Europe across most of Asia. It is widespread throughout Central-Northern
Europe occurring in 38 countries, but its status is a cause of concern. It has
become extinct from the Netherlands and is in decline in most countries that are
well recorded.
1.3 In the UK it is absent from Northern Ireland, rare in Wales, but remains
locally common in parts of England and Scotland. The butterfly's range (by 10-km
squares) has declined by approximately 80% during the last 150 years, but of
greater concern by a rapid 55% in the last 15 years. The most severe declines
have been in Wales (78% in the last 15 years) and in southern England where the
current rate of loss of colonies has been estimated at 39% per decade.
A national survey in 1997/1998 by Butterfly Conservation (supported by SNH and
others) identified approximately 440 colonies at 320 sites in the UK, with 150
colonies at 120 sites in Scotland. The exact status of the pearl-bordered
fritillary in Scotland is not as clear as elsewhere in the UK due to
under-recording. The butterfly has declined in the lowlands but is locally
common in the glens of central-northern Scotland (Highland, Argyll, Perthshire,
Aberdeenshire and Moray). This region is an important UK stronghold for the
butterfly.
2. Ecology
2.1 In the UK, the butterfly breeds in three
main habitat types:
i. woodland clearings - including young conifer plantations, coppiced woodland
and wide rides in southern England, and along pylon lines in Scotland;
ii. well-drained grassland habitats with bracken and/or scattered scrub in
western Britain; and
2.2 open (mostly deciduous) woodland with
bracken in Scotland.
In each biotope the main larval hostplant is common dog-violet Viola riviniana,
although in Scotland, marsh violet Viola palustris, and in southwest England,
heath dog-violet Viola canina and pale dog-violet Viola lactea are used
occasionally.
2.3 In Scotland the pearl-bordered fritillary is
chiefly found in open woodlands on south-
acing hillsides. A variety of woodland types
are used including:
i. open birch woodland throughout central-northern Scotland - corresponding to
National Vegetation Classification (NVC) community W11 (Quercus petraea-Betula
pubescens-Oxalis acetosella woodland);
ii. open oak woodland - NVC W11 (Quercus petraea-Betula pubescens-Oxalis
acetosella woodland) in western Scotland (Argyll and south Highland);
iii. open pine woodland - NVC W18 (Pinus sylvestris-Hylocomium splendens
woodland) in a few localities in Aberdeenshire, Highland and Moray;
iv. hazel scrub with ash - NVC W9 {Fraxinus excelsior-Sorbus aucuparia-Mercurialis
perennis woodland) in Argyll.
2.4 In each of these woodlands pearl-
bordered fritillaries are found in open areas,
either in clearings between tree cover or on
the woodland edge. These open patches of
ground are characterised by being;
i. dry, sunny, sheltered and usually south-facing;
ii. composed of fairly short vegetation, which lacks significant natural tree
regeneration (presumably as a result of grazing pressure);
iii. comprised of a mosaic of bracken growing amongst mainly acidic grasses, and
spring woodland herbs such as the butterfly's main nectar source bugle Ajuga
reptans; either lightly grazed or ungrazed (with no heavy spring stock grazing).
2.5 Open areas used by the butterfly usually
conform to one or more of the following NVC
vegetation communities: W25 (Pteridium
aquilinium-Rubus fruticosus underscrub),
CG10 (Festuca ovina-Agrostis capillaris-
Thymus praecox grassland), and more
occasionally U20 (Pteridium aquilinium-
Galium saxatile community). Habitat patches
occupied in Scotland reflect the breeding
requirements of females. Eggs are typically
laid in low vegetation on or near young violet
leaves, which grow through flattened stands
of shallow bracken litter on drier ground. In
these micro-habitats grass cover is low, and
bracken litter cover is moderate to high
(>30%) although patchy. At the time of
breeding, bracken frond density is low with the
majority of eggs being laid amongst exposed
litter beds where the fronds have yet to unfurl
fully. In England micro-habitats used for egg-laying are essentially the same as
those used in Scotland, although in some situations eggs are laid amongst leaf
litter rather than dead bracken.
2.6 Bracken and other litter/dead brown plant material are critically important
to the pearl-bordered fritillary, because they provide a place for larvae to
hibernate in the winter, and because of the extra warm microclimate they provide
for development of the egg and larval stages.
2.7 Eggs are laid singly and hatch after about two weeks. The emergent
caterpillars feed intermittently on violet leaves over the late spring and
summer months passing through four moults. With the onset of colder weather in
September larvae select a place to hibernate. This is usually amongst litter
such as in a curled up leaf or crinkly bracken frond, located close to the
ground surface. The caterpillars emerge again in early spring, and undergo a
final development phase lasting about a month. In this final rapid growth phase
the caterpillars divide their time between feeding on violets and basking out in
the open on the surface of litter. After completing development they enter the
pupal stage, which lasts about three weeks.
2.8 The pearl-bordered fritillary has a protracted flight period in the UK that
can span from mid-April in southwest England to mid-July in the Scottish
Highlands. In Scotland the main flight period is generally from mid-May to
mid-June, and in the Grampian Mountains from early June to early July. Marking
studies have confirmed the insects low mobility, with the majority of
individuals moving less than three quarters of a kilometre over their lives,
even when the colony area is much larger. In addition to being sedentary the
pearl-bordered fritillary is colonial - that is the majority of butterflies
spend their lives in a discrete area, rather than move elsewhere.
Adults occupy separate colonies in islands of suitable habitat separated by
farmland, dense woodland or moorland, which form barriers to movement. In the
extensive landscape of the Scottish Highlands the butterfly may occur at low
density over large areas of open broadleaved woodland, but again the natural
limits of these expansive colonies are generally readily identifiable.
2.9 Although colonies are essentially 'closed,
there is occasional dispersal including
movement to nearby colonies (up to 5km
away) leading to a metapopulation structure (a
metapopulation is a network of colonies
connected by occasional dispersal where
there are local extinctions and colonisations).
2.10 There is great variation in the UK in the
size of pearl-bordered fritillary colonies. The
majority of colonies are small (containing less
than 100 adults) or medium (less than 1000),
although even in Scotland colonies may
contain thousands of individuals, and may
extend continuously for 5km or more.
Habitat patches that pearl-bordered fritillaries
occupy are at a precise and ephemeral early
successional stage, and the distribution of
individual colonies within metapopulations will
inevitably change considerably over time.
Consequently, timed counts rather than
transects are a more appropriate method of
monitoring the changing abundance of adults
over time. Timed counts record the number
of adults seen per minutes of search effort
across the whole colony area, which is
measured in hectares; whilst transects count
the number of butterflies on a fixed route of a
sample of the colony.
Monitoring immature stages might be an alternative to adult monitoring. However,
eggs, larvae and pupae are difficult to find reliably and techniques have not
yet been developed to monitor these stages.
2.11 The species is currently monitored
annually by a Butterfly Monitoring Scheme
transect at Morrone Birkwood National Nature
Reserve (NNR). Baseline transects were
established on four other sites in Scotland in
1995 through the Scottish Diurnal Lepidoptera Project, funded by SNH.
3. Protective and conservation status
3.1 The pearl-bordered fritillary is protected
under the 1981 Wildlife and Countryside Act
from Sale Only. This makes it illegal to trade
wild caught material.
It is identified as a priority species in the
Government's Biodiversity Steering Group
report Anon (1995).
In Scotland there are records from 37 Sites of
Special Scientific Interest (SSSI).
4. Threats and conservation aims
4.1 The main threats are the continued shift away from traditional forms of
farming (low intensity grazing) and forestry (coppicing) to
more intensive systems that either destroy or render habitats unsuitable.
4.2 Woodland clearing sites are threatened by continuing changes in wood
management, especially loss of open clearings and reduction in frequency of
canopy gaps within modern high forest systems, resulting in an increased
isolation of new clearings.
4.3 Open woodland sites that have been targeted for woodland improvement schemes
may be threatened in the long-term. Many open woodlands in Scotland have little
regeneration. WGS and other schemes for such woodlands may involve deer culling,
fencing to exclude stock and deer, or bracken spraying. These measures are a
cause of concern because they may lead to the loss of the open areas in woodland
on which the pearl-bordered fritillary and other invertebrates depend. Even
young trees may provide too much shade for the invertebrate communities.
4.4 Bracken hillside sites are threatened by wholesale habitat destruction
caused by agricultural improvements such as conversion to improved pasture; by
inappropriate management of sites, including abandonment of grazing which leads
to bracken becoming overgrown (too dense), or overgrazing by sheep which makes
swards too grassy and eliminates nectar plants. Tree planting and bracken
spraying pose further threats.
4.5 The conservation aim for this species will vary according to the broad
habitat type in which the butterfly occurs. Colonies in rotational coppice, and
young conifer plantations tend to become extinct within a few years once woody
growth has shaded out the field layer. If the species is to persist in managed
woodlands, new clearings need to be created near to existing colonies every 1-2
years to allow natural colonisation. Colonies on bracken hillsides require the
maintenance of a light bracken cover amongst patches of grassland and scrub. In
open woodlands the maintenance of a succession of gaps and edges, which have a
light bracken cover, is required, and where natural regeneration is suppressed.
4.6 The spatially dynamic nature of both the pearl-bordered fritillary and many
of its habitats, means that small habitat patches with isolated populations are
unlikely to survive in the long-term even with positive management, and there is
a need for the
preservation of networks of colonies at a landscape scale. Wider countryside
measures are therefore required in addition to the current SSSI network.
5. Basic site management requirements for the pearl-bordered fritiliary
5.1 Coppice and pylon lines: ensure a
succession of flower-rich, sunny clearings in
(woodland) herb-rich vegetation with bracken
or leaf litter. Continuity of habitat is essential,
and as the trees re-grow there must be wide
corridors for the species to move along to
locate new, freshly cleared areas where
conditions are suitable for breeding. The ideal
management is coppicing where adjacent
woodland plots (or pylon line sections) are cut
in succession with open, sunny rides
interlinking plots. This allows the whole
colony to move into each new patch of habitat
as it arises. The length of rotation needs to
be short (every 5-10 years), but could be
longer in Scotland if regeneration is slow or
sparse.
As a rough rule of thumb only one in every three clearings may be suitable for
the butterfly. New clearings may be unsuitable because they are on damp ground;
or because the ground flora is dominated by plants such as dog's mercury
Mercurialis perennis, bluebell Endymion non-scriptus and vigorous grasses such
as tufted hair Deschampsia caespitosa, which suppress violets.
When considering the maintenance and re-establishment of coppicing, deer control
should aim at reducing deer populations to levels where habitat damage is not
occurring or at worst is only very light.
5.2 Plantation woodlands: management is
needed to ensure that there is a regular
sequence of felling and replanting preferably
with deciduous trees, and a network of broad
sunny rides and glades. For woodland blocks
that are dominated by maturing conifers,
'crisis management', in the form of
uneconomic felling of crop trees, may be
required to ensure the butterfly's survival. It is
important that plantation compartments are
periodically well thinned to ensure that a
woodland ground flora develops following
harvesting. Wide rides, boundary banks and
ditch edges with bracken may be especially
important to enable pearl-bordered fritillaries
to persist in otherwise shady conifer
plantations; and it is important that any
spraying programmes do not eliminate bracken altogether. x
5.3 Bracken hillsides: preliminary studies
indicate that bracken/grass/scrub mosaics are
best maintained by extensive cattle and/or
pony grazing. High stocking rates should be
avoided during the spring as they may
eliminate nectar plants. Sheep are the least
appropriate grazing animal as they do not
trample bracken beds sufficiently, and may
also eliminate nectar plants.
Rotational cutting and burning regimes have maintained suitable conditions at
some sites. Stands of scrub and bracken should be cut on a 5-8 year rotation.
The long-term suitability of these methods is unclear, and they should only be
seen as an interim measure to the eventual reinstatement of grazing.
5.4 Open woodland: early studies indicate
that suitable conditions in open woodlands,
are generally maintained by extensive stock
and deer grazing; where stocking densities
are high enough to restrict natural
regeneration and allow the spread of a light
bracken cover. It is unclear how dynamic this
vegetation is and more studies are needed.
Woodland improvement schemes (supported by Forest Authority Woodland Grant
Schemes (WGS)) involving bracken spraying and other measures may lead to
increased Miliary populations in the short-term, by providing increases in both
adult nectar sources, and (in certain years) the extent of vegetation in a
condition preferred for egg-laying: both foodplants and bracken litter cover.
However
in the longer-term suitable areas may be shaded out.
Woodland improvement schemes at sites containing pearl-bordered fritillaries
need to be planned so that they maintain grazed open areas with a
bracken/grass/woodland herb cover. Fencing or otherwise managing only parts of
the wood rather than the entire woodland area might provide some refuges of open
woodland in such schemes. However, one would need to be sure that the excluded
areas were suitable for the pearl-bordered fritillary and continued to be
grazed.
Bracken spraying, as part of WGS needs to be sensitive to the needs of
pearl-bordered fritillaries, and guidelines may be needed -there are none at
present. Where pearl-bordered fritillaries occur, possible guidelines might
include:
i. only spraying areas of dense bracken and not areas with a light bracken
cover.
ii. by spraying more patchily rather than trying to completely eradicate
bracken.
iii. by spraying further away from the woodland edge to encourage the
development of open woodland, rather than a new woodland edge front;
iv. in large woodland blocks, it may be better to have a series of small WGS
schemes spread over time, rather than a single large one; a succession of
smaller schemes should ensure that natural regeneration or newly planted trees
occur as uneven-aged patches, rather than as dense even-aged stands.
6. Further reading
Anon (1995) Biodiversity: The UK Steering Group Report. Volume 2 Action Plans.
HMSO, London.
Barnett, L. & Warren, M.S. (1995). Species Action Plan: Pearl-bordered
Fritillary Boloria euphrosyne. Butterfly Conservation, Dorset.
Brereton, T. M. (1998). Pearl-bordered fritillary national survey 1997.
Butterfly Conservation News, 68,11-13.
Brereton, T. M. & Davis, J. (1998). Status of bracken-violet fritillaries at
Great Torrington Common, north Devon and recommendations for bracken management.
Butterfly Conservation, Wareham.
Brereton, T. M. & Warren, M.S. (1998). Ecology of the pearl-bordered fritillary
butterfly in Scotland and possible threats from bracken eradication measures In
Woodland Grant Schemes. A paper (in press) presented at the International
Bracken Group Meeting, University of Lancaster, September 10-11,1998: Bracken
Control in the Uplands.
Ravenscroft N. O M. (1998). Ecology of the Pearl-bordered Fritillary (Boloria
euphrosyne) in the open woodlands of Western Scotland. Unpublished report to
SNH.
Ravenscroft N. O M. (1999). Ecology of the pearl-bordered fritillary (Boloria
euphrosyne) in Scotland. II: woodland grant schemes and birch woods. Unpublished
report to SNH.
Warren, M.S. (1992). Britain's Vanishing Fritillaries. British Wildlife 3(5),
282-296.
Warren M.S. and Oates, M.R. (1995). The importance of bracken habitats for
butterfly populations. In: Bracken: an environmental issue (eds: R.T. Smith &
J.A. Taylor), pp. 178-181. International Bracken Group Special Publication No.
2.
7. Contacts for advice and information
Dr David Phillips
Species Group, Advisory Services
Scottish Natural Heritage
2 Anderson Place
EDINBURGH
EH6 5NP
Tel: 0131-4476 4784
Paul Kirkland
Conservation Officer
Butterfly Conservation - UK North
PO Box 12477
EDINBURGH
EH1 2YE
Tel: 0131-226 3385
8. Author
Dr Tom Brereton (Butterfly Conservation)
(with acknowledgement to the Butterfly Conservation action plan, Barnett and
Warren (1995), and data collected as part of Butterfly Conservation's Action
Programme to conserve the pearl-bordered fritillary 1997/1998).
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