Butterflies of
Britain & Europe
Small Tortoiseshell
Aglais urticae
LINNAEUS, 1758
Family - NYMPHALIDAE
subfamily -
NYMPHALINAE
Tribe - NYMPHALINI
introduction
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habitats |
lifecycle |
adult behaviour
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Aglais
urticae, Noar Hill, Hampshire |
Introduction
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The Small Tortoiseshell
may have been present on Earth longer than any other
butterfly species - a mid-Miocene fossil of
Aglais karaganica estimated to be
15
million years old,
is so similar to the present day Aglais urticae
as to be virtually indistinguishable.
( ref: Nekrutenko, 1965 )
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Small
Tortoiseshells vary considerably in appearance -
some
examples being a unicolorous
dull orange, while others are much redder with contrasting yellow patches. In
summers when weather conditions are cool, a high percentage of the butterflies
tend to have the black markings enlarged, or merging together. In hot summers,
and in warmer parts of the butterfly's range these markings often become reduced
in size.
The
race ichnusa,
found in mountainous areas of Corsica and Sardinia has a fiery orange ground
colour, and lacks the colon of black spots on the forewing. Some authors regard
it as a separate taxon. Captive reared adults however closely resemble the normal
form, so it seems likely that
ichnusa
is merely a habitat / climate generated form of
urticae.
The
Small Tortoiseshell occurs across the whole of Europe and temperate Asia. A closely related and visually inseparable species Aglais
kashmirensis is found in
mountainous regions of Kashmir, Tibet, Sikkim and Bhutan. In the western
Himalayas a third species occurs,
Aglais ladakensis,
which looks like a very heavily marked version of
urticae.
The only other member of the genus Aglais milberti
is a north American species in which the basal half of the
forewings is black, and in which the 2 small black spots in the median area are
absent.
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Aglais
urticae, Noar Hill, Hampshire |
The butterfly has always been subject to dramatic fluctuations in abundance from year
to year and in some seasons it can be very scarce.
Historically it has always had the ability to bounce back, and was until
recently regarded as one of Britain's most familiar and common butterflies. However
after about 1990 the species declined sharply in southern England, until the low
point of 2007 when most recorders in Hampshire, Surrey and Sussex failed to see
even a single specimen. There was some evidence of a minor come-back in 2008,
when most observers saw at least one or two specimens of the late summer brood.
Survival of hibernating adults in the cold and prolonged winter of 2008-2009 was
however exceptionally good, so the species got off to a good start in spring
2009. Current signs are that the Small Tortoiseshell has had a fairly good
summer season, with most observers seeing at least a dozen adults in June 2009.
Despite wet and generally cloudy mid-summer weather the resurgence in numbers
continued, with at least 2 or 3 adults recorded at almost every site visited by
recorders in southern Britain in late July and early August 2009. These figures
however still fall far short of the numbers seen in the 1960's and
70's.
The cycles of
abundance are linked at least in part to
climatic variations, which affect the population
dynamics of the butterfly and it's parasitoids. Luckily the population crashes are not synchronous -
at times when the species is
scarce in one part of the country it can be relatively abundant in another.
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Recent research suggests that a tiny
parasitic fly may have been responsible for the recent decline of the
Small Tortoiseshell. The fly Sturmia bella
( Tachinidae ) occurs mainly in
warmer climates, and first arrived in Britain in 1998. It lays it's
eggs on nettle plants. When the Small Tortoiseshell caterpillars
eat the nettle leaves, the microscopic eggs are ingested undamaged
and pass into the caterpillar's gut. There they hatch, and the
resulting grubs bore their
way through the soft flesh, consuming non-vital body tissues. When
the grubs are almost full grown they eat the
vital organs, and then break
out through the skin of the dying caterpillar to pupate.
The fly is widespread throughout southern Europe, north Africa and
Asia. In Japan it parasitises the caterpillars of a Danaine,
Parantica sita.
Interesting research by Hirai & Ishii has revealed that
P. sita can often survive attack by
Sturmia bella - in experiments over
70 percent of infested sita
larvae
survived and went on to produce perfect adult butterflies. Sadly
the Small Tortoiseshell lacks this immunity.
Although current research appears to suggest
that Sturmia bella
is at least partially responsible for the decline of the Small Tortoiseshell,
experiments have shown that only about 18% of Small Tortoiseshells
are affected by parasitoids, and
Sturmia bella
is just one of several that attack the butterfly.
It is also
notable that other nettle-feeding Vanessids e.g. Peacock,
Red Admiral & Comma are apparently less affected, even though
their larvae also ingest the
Sturmia eggs.
The Comma and Peacock in fact increased their numbers substantially during
the 2005-2009 period. All 3 species have very
similar body chemistry to that of the Small Tortoiseshell, so it
is unlikely that they possess a natural immunity.
Sturmia bella
is not host-specific, it parasitises a wide range of species in
Europe and Asia. One cannot help wondering therefore why the Small
Tortoiseshell seems to have been singled out, while Red Admiral,
Comma and Peacock continue to thrive.
It is feasible that the decline of
urticae could
be linked to pesticides. Small Tortoiseshells usually oviposit on
nettles growing in open sunlit places such as farmyards and field
edges - areas often exposed to high levels of pesticide and other
chemical sprays. The other Vanessids - Commas, Peacocks and Red Admirals all
tend to breed on nettles in woodland glades, gardens and lanes
where they are not exposed to pesticides and would thus be
relatively unaffected.
Climate change in itself could be a major
factor. The viability of each butterfly species is limited by
temperature and humidity. This defines it's geographical and
altitudinal distribution and is part of the reason why some
species are found only in the Arctic while others only in
particular elevations in cloudforest in the tropics. It's quite
possible that the Small Tortoiseshell simply can't cope with the
mild damp winters Britain has recently experienced, but that these may
be beneficial to the Comma, and well tolerated by the Red Admiral
and Peacock.
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Aglais urticae, Noar Hill, Hampshire |
Habitats
The
Small Tortoiseshell is
a highly mobile species which can occur in almost any habitat,
including woodlands,
grasslands, heaths, gardens, country lanes, and even in city
centres.
It is a strong flyer, with high powers of dispersal, reaching the most remote
islands, and the peaks of mountains. It has been recorded at altitudes as high
as 1200m in Scotland, and larval nests have been found at 335m. I have
watched adults flying near the peak of Ben Lawers in Tayside,
and high in the mountains at Langdale Pikes and various other sites in the Lake District.
The butterfly is
perhaps commonest on dairy and cattle farmland, where
the application of fertilisers and manure enriches the soil,
encouraging the growth of stinging nettles
Urtica dioica and U. urens.
Lifecycle
In northern Scotland there is usually only a single
generation, emerging in July, entering hibernation in August, and reappearing in
March or April. Elsewhere in Britain there are normally 2 broods per year,
one emerging in June, and the other in August. Adults of the latter brood enter
hibernation in September or October, and reawaken the following March.
The
resident population is supplemented by the occasional arrival of migrants from
the continent. The numbers arriving however are normally very low, and probably
have no significant effect on population levels.
In spring,
female Small Tortoiseshells are often seen flying around young nettle patches,
where they lay their greenish eggs in large
untidy heaps
of 80-100 on the underside of the
upper leaves. Often more than one female will lay eggs on the same leaf,
sometimes simultaneously. The females usually choose young plants growing near
the edge of the nettle bed, and always growing in warm, sunny and sheltered
conditions. Typical breeding sites include field edges at the bottom of south-facing
hillsides, dykes, riverbanks,
disused railway cuttings, and nettle patches growing on the south side of
hedgerows or dry stone walls.
The summer brood is less
fussy about oviposition sites, and will lay on nettle patches in gardens, in
woodland glades and rides, and on exposed hilltops.
In June 2009, I watched a female searching for
oviposition sites at Martin Down in Hampshire. She spent about half an hour
flitting back and forth, settling for a few seconds here and there on various
terminal leaves, and took a great deal of time before she eventually found one
which suited her. To my eyes it looked no different from any of the other leaves
on which she landed, so I assume that she just found that particular leaf easier
to grip. Once she had decided where to lay her eggs she clung tenaciously to the
leaf, which was constantly battered by strong gusts of wind, until she had laid
a batch of about 80 eggs.
The eggs hatch after about 12 days. Immediately after hatching, the
larvae devour their empty egg shells, and then spin a communal silk web around the
terminal leaves of the nettles. They shelter within the web at night, or in
adverse weather conditions, and feed avidly whenever the sun shines.
If disturbed the larvae
react in unison, wriggling and jerking as a defence against
parasitoid wasps or flies. Nevertheless
a high proportion of larvae turn out to be parasitised,
usually by the Tachinid fly Phryxe vulgaris.
When young, they can easily be mistaken for the caterpillars of the Peacock, but
Small Tortoiseshell larvae are paler, and even when quite small it is usually
possible to discern pale lines running along their backs.
Aglais urticae, 2nd instar larvae on stinging
nettle, Dorset
As
they grow, they split up into smaller groups,
spinning a new web
after each moult. The final instar sees a change in
behaviour, with the larvae abandoning their webs entirely and living solitarily. By this
time they are a dull blackish colour, spiky, with broad yellow lines running along their backs
and sides. These lines are usually very prominent, but
in some batches of larvae they can be pale and obscure. The
fully grown larvae can often be seen curled in a J-shaped
posture, resting on nettle leaves, and if disturbed will coil
into a tight circle and drop to the ground.
The
chrysalis is variable in colour, ranging from grey to olive
or buff,
often with a pinkish or golden
metallic sheen. It can be found suspended by the
cremaster, on woody stems, fence posts, walls, or beneath the
stems or leaves of nettles.
The adults emerge at dawn, about 12 days after pupation.
Adult behaviour
Over-wintered
adults
typically
awaken in late March or early April, and remain on the wing until
early May.
They are usually seen as
singletons, nectaring at dandelion,
daisies or
sallow catkins,
often in the vicinity of nettle patches.
On cool days in
spring they do not bask, but instead warm themselves up by
shivering their wings prior to flight. The rapid shivering
generates warmth by friction, and enables the butterfly to fly
even in quite cool conditions. On warm days they frequently bask
on bare soil or low foliage, but hot sunny conditions cause them to
settle for long periods on bare soil with their wings closed, at which time the sombre
"tortoiseshell" pattern on the underside affords them excellent
camouflage. When settled the abdomen is usually raised so that
minimum contact is made with the hot substrate.
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Aglais
urticae, female ovipositing on stinging nettle, Martin
Down, Hampshire |
Males establish
vantage points in
the
early
afternoon,
from which they await passing females. When a female flies by she is
intercepted and the courtship ritual begins. The male chases her until she settles on the ground. If she is receptive she
opens her wings and the male approaches her from behind, with his
wings also open. He then
steps onto
her
hindwings,
which he
vigorously drums with with
his antennae. The pair then fly a short distance and repeat the
process.
Any other males which attempt to interfere are briskly chased away
by the resident male, who then returns to his female to continue
wing-drumming.
This
bonding process continues for several hours, until just before
dusk, when
the female accepts the male's advances.
At this
point she leads him to a sheltered and shady spot, typically
beneath
a bush
or hedge. Both sexes then hold their wings erect,
and the male walks alongside the female, and curves his abdomen to
copulate. After about 20 minutes the pair straighten out to face
opposite directions. They remain
copulated
in this position
until the following morning.
The
summer brood butterflies,
which emerge from mid June to early July,
will nectar at almost any available wild or cultivated flowers.
Their favourite however is undoubtedly creeping thistle. At Noar
Hill in July 2009 I watched a female nectaring for over 2 hours at
a clump of these flowers. When disturbed by a bee or hoverfly she
would leave the thistles and briefly investigate other nearby
flowers including marjoram, knapweeds, bramble, scabious and hemp agrimony, but after
a quick taste always returned to the thistles.
The second generation is always more abundant
than the first, and emerges
at the end of
August.
In late summer
Small
Tortoiseshells
often visit
flowery areas in
the countryside, where they congregate to nectar at
thistles, ragwort
and other Compositae. They also commonly visit gardens,
where they are
strongly attracted to
Sedum,
Buddleia and michaelmas daisies.
Their mission at this time of year is to use every opportunity to
gorge on nectar, building up sufficient protein and fat reserves
in their bodies to enable them to survive hibernation.
In
southern Britain the butterflies fly until late September, when they enter houses,
sheds, churches and unheated farm buildings
to hibernate. In the north they enter hibernation earlier,
typically in late July or
early August, after just a few days on the wing. They often hibernate communally -
on many occasions I have
found groups of 3 or 4 nestling together in an attic or
outbuilding, and I once found
17 hibernating adults clustered
tightly together on
the ceiling in a hotel in
Tayside,
Scotland.
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