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The Enemies of
Butterflies
Page 1
1
- Predators
2
- Parasites, Parasitoids & Pathogens
3 - Human
impact - habitat destruction & climate change
Predators
Population control in nature
Strictly speaking, predators and parasitoids should not be
considered as enemies of butterflies. They could perhaps instead be
thought of as Nature's way of preventing butterfly populations from
getting out of control - if they were not kept in check, the
populations would rapidly expand and would quickly deplete all
available food resources, ultimately leading to their own demise.
A butterfly may be capable of laying up to 500 eggs. On average
however only about 100 will be laid, as many females die before they
are able to lay all their eggs.
Perhaps 95 of those eggs will hatch. 85 of the resulting caterpillars are likely to be
killed by birds, wasps, spiders or parasitoids, leaving just 10 to reach pupation. Studies have found that over half of all
wild pupae will be eaten, be killed by parasitoids, or die from
desiccation, fungal attack, or other causes.
The net result is that the eggs laid by a single butterfly will,
averaged over several years, result in only about 4 adults per
generation. As
many as half of the adult butterflies will be killed before they mate or
are able to lay eggs. So, despite the ability to produce those 500
eggs and the potential of 500 butterflies, just 2
butterflies will result from each brood of eggs. With luck one of
these will be a male and the other a female, and another batch of
eggs will be produced.
Avian
predators
Throughout the world, adult
butterflies are killed in vast numbers by birds including sparrows,
tits, thrushes, robins, orioles, jays, grosbeaks, crossbills,
flycatchers, jays, tanagers and jacamars.
Various studies
have provided statistical data on avian predation. One study for
example revealed that 160 out of 697 examined specimens ( 23% ) of
Ascia monuste bore beak marks on their
wings indicating that they had been attacked by birds, but had
escaped or been rejected. This figure does not of course include the
specimens that were actually eaten.
Another study of
the feeding behaviour of ( captive ) rufous-tailed jacamars
Galbula ruficauda in Costa Rica found
that when 1679 butterflies of 133 species were offered to the birds
approximately 5% were classed as failed attacks. 35% were ignored or
sight-rejected. About 20% were attacked, captured and then
taste-rejected. The remaining 40% were attacked, killed and eaten.
The species offered to the birds were categorised according to
colour. Those that were sight-rejected or taste-rejected generally
were aposematically coloured species, while those that were devoured
tended to be the less colourful or cryptically patterned
butterflies.
It is apparent from
the many studies carried out that at least
50% of wild butterflies are killed and eaten before they are able to mate and
reproduce. Some are attacked when they are emerging or drying their
wings prior to their first flight. Others fall victim when basking
on the ground or visiting flowers, although many are lucky to escape
with nothing more than a peck taken out of a wing.
Birds and other vertebrate predators rely
primarily on sight to locate prey, so butterflies and moths have
evolved numerous visually mediated means to avoid
attack. These include passive defence mechanisms e.g. camouflage, disguise, mimicry,
warning colouration and transparency. Sometimes passive mechanisms
fail, and a butterfly will find itself under direct attack. At this
stage secondary or active mechanisms come into play.
The flowchart below illustrates many of
the
survival strategies which
butterflies have evolved to defend themselves against attacks by insectivorous
birds :
Spiders
and predatory insects
Although birds are
probably the main predators, adult butterflies also have to
contend with spiders, wasps, dragonflies, robber flies and
crickets. In warm climates they are also attacked by
mantises and numerous other arthropods.
It is common
to see butterflies caught in the grip of crab spiders, which
lie in wait among flowers, ambushing any butterflies that
visit them for nectar. Butterflies also often wander into the webs of orb spiders.
The smaller weaker butterflies such as
Polyommatus, Lysandra
and Coenonympha invariably
become entangled and are quickly wrapped up in silk for later
consumption. Larger butterflies however such as
Vanessa and Argynnis are
often able to struggle free before being
pounced upon by the owner of the web.
Close examination of adult
butterflies often reveals a give-away reticulation pattern
across the wings, marking the places where loose wing
scales have become detached and left behind on the sticky
threads of the web. It is even possible that wing scales
have evolved to be easily detachable as a survival
mechanism.
Hornets and wasps are major predators of butterflies in
mid-late summer. By way of example, in July 2009 at Alice Holt forest in
England I watched a hornet Vespa
crabro chasing after Ringlets and Meadow
Browns. It failed to catch any, but moments later, when I was
trying to photograph a White Admiral nectaring at
bramble, another hornet suddenly shot down and snatched the
butterfly from the flower. In a split second it had grabbed
it, bitten off its
forewings, and used its hindwings to wrap the paralysed
butterfly up into a tight ball. Seconds later, carrying the
parcel in it's mandibles, it flew up to its nest at the top of a
small oak. Once there it would have chewed the butterfly
into a pulp and regurgitated it to feed its developing grubs
- adult hornets are strictly vegetarian, feeding on nectar
and fruit.
The Broad-bodied
Chaser Dragonfly Libellula depressa
commonly preys on butterflies
In southern Britain
one of the commonest butterfly predators is the spider
Enoplognatha ovata, a member of the
Theridiidae. This small species traps summer butterflies
which fly into the sticky strands of an untidy web which it spins on
grass-heads and wild flowers. I made a brief study of predation at
Magdalen Hill Down in Hampshire in July 2009 and estimated that
about 5% of the population of Chalkhill Blues
Lysandra coridon fell victim to this
spider.
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Theridiid spider
Enoplognatha ovata, devouring
Chalkhill Blue Lysandra coridon |
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Hunting spider
Pisaura mirabilis
Male
hunting spiders
Pisaura mirabilis
attack butterflies that settle on low herbage. They wrap their victims tightly
in silk and present them to the female spider as a courtship gift.
While the spiders are mating the female feeds on the butterfly. |
Crab spiders are a major predator
of small butterflies. An individual will sometimes spend several days
sitting motionless on a flower head, waiting it's next victim to fly
in. The peripheral vision of the spider is poor,
so much so that it is possible for
a butterfly to settle alongside it without
being noticed.
If on the other hand it is
unfortunate enough to walk across the spider's field of forward
vision, the arachnid will
move immediately and stealthily
towards the butterfly and seize it with its powerful pincer-like
forelegs. The spider then bites the butterfly on the neck, injecting
it with a paralysing venom which incorporates enzymes that liquefy
the victim's internal tissues.
The photo below illustrates a
Chestnut Heath
Coenonympha glycerion that has been ambushed by the crab
spider Thomisus onustus.
This remarkable spider has a chameleon-like ability to change colour
to match it's surroundings. It can be white, yellow, pink or
variegated in appearance. The change of colour takes about 2 or 3
days to complete however, so it's common to find a spider on the
"wrong" coloured flower.
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Coenonympha glycerion, ambushed
by crab spider Thomisus onustus
© Peter Bruce-Jones |
Altinote dicaeus attacked and eaten by a cricket in Manu
cloudforest, Peru
Predators
of caterpillars
Butterfly and moth larvae are taken in huge numbers by predators. A
study of predation on Pieris rapae
estimated that 52% and
63% of 1st / 2nd instar larvae were eaten in 2 consecutive years by
invertebrate predators including Carabid beetles, Hemipteran bugs,
wasps, mites and spiders. The same study estimated that
as many as 22% of older Pieris rapae
larvae were taken by birds, which of course
have to feed not only themselves but their offspring at the nest.
Birds, reptiles, amphibians and small mammals rely
primarily on sight to locate prey. Lepidopteran larvae have
correspondingly evolved a battery of visually
directed defence mechanisms to
reduce the likelihood of being eaten. These include camouflage, disguise, mimicry,
aposematic / diematic colouration and the use of
stinging spines to ward off attacks.
These strategies work quite effectively against vertebrates but they provide no protection against
invertebrates such as spiders, wasps,
bugs and ants, which rely primarily
on smell to locate their prey. The larvae of
many species have consequently evolved alternative solutions. The larva of the Puss moth Cerura vinula
for example is armed with long "tail whips". If attacked it arches
it's body into an aggressive posture and uses the whips to thrash
the attacker to
drive it away.
Swallowtail larvae are palatable to birds and
employ cryptic colours and patterns as a first line of defence. If
discovered however they activate secondary defences. Many of them
are marked with a pair of "false eyes" on the thoracic segments, so
they inhale air through the spiracles and puff up these segments to
emphasise their threatening appearance. This is often enough to
deter avian and reptilian predators. Molestation by insect predators
and parasitoids elicits a different response from the larvae. In
this instance they evert a fleshy structure behind
their heads called an osmaterium. This discharges airborne
isobutyric and 2-methylbutyric acids which has been shown to repel
ants and Homopteran predators. It also deters oviposition by parasitoid wasps and flies.
The larvae of species such as the Peacock
butterfly Inachis io, the Tiger moth
Arctia caja and the Fox moth
Macrothylacia rubi rely on escape
tricks. If molested they simply
roll into a ball and drop to the ground. Geometrid moth larvae use
disguise as their primary defence - they look just like tiny twigs,
and reinforce this similarity by stretching out their bodies in a
straight line so that they project twig-like from a sprig of their
foodplant. If they are molested they release grip on the sprig and
drop instantly from a bungee-cord of silk. They dangle at the end of
this
until the attacker has moved on. After a while they haul themselves back up, consuming the silk thread as
they does
so.
The larvae of many members of the family Lycaenidae
defend themselves by forming a beneficial association with ants. The
anal segment of the larva houses a dorsal honey gland which exudes
a sweet tasting fluid which ants love to drink. The larvae thus find
themselves constantly attended by dozens of ants, the presence of
which deters predators & parasitoids from attacking. Many species
take the association a stage further and are carried by the ants
into their nests where they feed on ant grubs, aphids or fungi.
The ants don't attack the larvae as the latter are able to
mollify them, either by using chemical deterrents or by stridulating
to produce an
"appeasement song". Research on several Hairstreaks and
Blues in Europe has
demonstrated that their larvae and pupae are able to generate an
audible
"chirp" which deters ants from attacking.
Caterpillar defence
mechanisms are discussed more fully in the
lifecycle section.
Heteropteran bugs
such as this unidentified species from Peru often attack
and eat caterpillars
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Adders and other juvenile snakes eat diurnal butterfly
and moth larvae, as well as beetles, spiders and nestling
rodents. Click
here for more about the Adder. |
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Common Toads are major predators of
European caterpillars such as
Ringlet and Speckled Wood.
Click
here for more about the Common Toad.
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