The Enemies of
Butterflies
1
- Predators
2
- Parasites, Parasitoids & Pathogens
3 - Human impact - habitat destruction
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.
Broad-bodied Chaser Dragonfly
Libellula depressa commonly preys on
butterflies �
Adrian Hoskins
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.
|
Theridiid spider
Enoplognatha ovata, devouring
Chalkhill Blue Lysandra coridon �
Adrian Hoskins |
|
Hunting
spider
Pisaura mirabilis �
Adrian Hoskins
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.
|
Coenonympha glycerion, ambushed by
crab spider Thomisus onustus
� Peter
Bruce-Jones |
Altinote
dicaeus attacked and eaten by a cricket in Manu cloudforest,
Peru �
Adrian Hoskins
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 species from Peru often attack and eat
larvae �
Adrian Hoskins
|
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 �
Adrian Hoskins |
|
Common Toads are major
predators of European caterpillars such as Ringlet and
Speckled Wood.
Click
here for more about the
Common Toad �
Adrian Hoskins
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