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The Enemies of
Butterflies
Page 2
1
- Predators
2
- Parasites, Parasitoids & Pathogens
3 - Humans
Parasites, Parasitoids & Pathogens
Parasitoids
All
stages of the lifecycle are threatened by parasitoids : creatures
that feed on other organisms, and ultimately bring about their
death.
Note that parasitoids shouldn't be confused with parasites -
which unlike parasitoids do NOT bring about the death of their
hosts.
Egg parasitoids
Many butterfly species fall victim to tiny wasps that inject their
eggs into the soft newly laid butterfly eggs. When the wasp grubs
hatch they feed on the organic matter within the egg, killing the
potential caterpillar. The adult wasps emerge a few days later and
use their mandibles to cut minute exit holes to make their escape
from the eggs. In some cases as many as 60 microscopic wasps can
emerge from a single butterfly egg.
South American
Caligo
Owl butterflies are parasitised by a tiny Trichogrammatid wasp
that rides from place to place on the hindwings of female
butterflies. When the butterfly settles momentarily to lay her
eggs the wasp hops off, injects it's own eggs into those of the
butterfly, and then hops back onto the butterfly's wing in time to
be transported to the next egg laying site !
Caterpillar parasitoids
Throughout their lives caterpillars are very vulnerable to attack
from parasitoids. Studies in Britain have revealed that with most
Lepidoptera species, about 80% of larvae are
attacked by parasitoid wasps or flies. In the case of the Marsh
Fritillary Euphydryas aurinia between
90-95% of larvae are parasitised by the wasp
Apanteles bignelli in certain years.
Wasps such as
Apanteles,
Amblyteles,
Netelia,
Ophion,
Protichneumon and
Ichneumon;
and flies such as
Tachina,
Gymnochaeta and
Gonia spend their larval stage within
the bodies of caterpillars.
The adult wasps and flies inject their eggs into the
caterpillar's body by means of a long sting-like ovipositor, or in
some cases lay their eggs on leaves which are ingested by the
caterpillar.
Hawkmoth larva Eumorpha fasciatus,
Peru
The
larva of Eumorpha fasciatus illustrated
above has been parasitised by a wasp that injected it's eggs
into it. When the wasp grubs are fully grown they break out through
the skin of the larva and form their papery white cocoons. The
larva shrivels and dies. A few days or weeks later the adult wasps
emerge from the cocoons and search for another larva to parasitise.
The larva of
Hasora badra shown below has been
parasitised by an Apanteles species - a
total of 67 wasps emerged from the cocoons which surround it.
Common Awl Skipper Hasora badra
surrounded by Apanteles cocoons
( image ©
SoonChye )
Defence strategies
( see also
Larval survival mechanisms )
Many larvae are equipped with thick coats of hair, or
fierce-looking spikes that make it difficult for parasitoids to
settle on them. Others, e.g. those of the Puss moth
Cerura vinula and the hawkmoth
Isognathus leachi are armed with long whip-like
structures which they use to drive off any wasp or fly that attempts to attack.
Isognathus leachi, Fazenda Rancho
Grande, Rondonia, Brazil
Unfortunately in practice it the whip is not very effective as a
defence, as Paul Bertner describes :
"We spotted the
Winthemia tachinid fly circling
around the Isognathus larva. The
extremely long tail of the larva appears to serve a defensive
function. As the fly landed close to the rear, the larva
would flick its tail, dislodging the fly to prevent it from
ovipositing. The fly undeterred walked up the body until it was
close to the head and out of range of the tail where it began to
lay eggs. It seemed to prefer the posterior end of the larva for
some reason as it kept on trying to move back there, perhaps
laying too close to the front might kill the larva faster and
thereby not leave enough time for the fly grubs to mature, whereas
the rear of the larva may not house such vital organs to be
destroyed by the grubs".
The long orange
ovipositor of the fly can be seen quite clearly in the photo below
:
Tachinid fly
Winthemia sp, injecting eggs into an Isognathus
larva
( image © Paul Bertner )
Some flies such as Sturmia
bella don't oviposit directly on the larva. Instead
they lay their eggs on its foodplants. These are
ingested and pass
into the larva's gut.
After the eggs hatch the resulting grubs consume the
larva's flesh, but leave the vital organs alone, allowing the
larva to continue to live and grow. Eventually when the
parasitoid grubs are mature and ready to pupate they eat the vital
organs. They then pupate either within the body of the dying
larva, or eat their way out of it's skin and pupate
externally.
Nematode worms
The larvae of many butterflies and moths are attacked by
entomopathogenic nematode worms. The tiny juvenile
worm is often unintentionally ingested by a caterpillar as it browses on
leaves. In some cases it may enter instead via the caterpillar's anus. The
Cyclosia papilionaris larva depicted above is a member of the Zygaenidae. It has been parasitised by a single
worm which has
fed and grown within it for several days. The fully grown worm
ultimately exits the larva, which shrivels and dies.
Cyclosia papilionaris with parasitic worm, Cambodia
( image © Dani Jump )
Parasitoids of pupae
Other wasps attack newly formed butterfly pupae while the skin is
still soft and easily punctured. An example is the beautiful
metallic green wasp
Pteromalus puparum, which attacks
pre-pupal larvae and newly formed pupae of
Pieris brassicae. The entire
lifecycle of
Pteromalus
takes place within the butterfly pupa, and the tiny wasps emerge in
dozens after making exit holes in the pupal skin.
The pupae of many Lycaenidae species are attended by ants ( see
myrmecophily ). The presence of
ants is beneficial
to the pupae because ants drive away predatory insects and
parasitoid wasps that would otherwise attack them. Experiments
with the Australian hairstreak Jalmenus
evagoras for example have shown that in cases where ants have
been denied access to the pupae the latter have suffered up to 95%
parasitism by the wasp Brachymeria reginia.
Conversely, pupae attended by the ants experienced zero parasitism.
Parasites
Parasites are creatures which feed upon other organisms,
but unlike parasitoids they do not cause the death of their
hosts. In the case of Lepidoptera, they generally affect the adult
butterflies and moths rather than caterpillars.
Certain butterflies, particularly males of Meadow Brown
Maniola jurtina, Marbled White
Melanargia galathea,
Common Blue
Polyommatus icarus
& Small Skipper
Thymelicus sylvestris
are commonly
parasitised by tiny
red mites
Trombidium breei, which normally
attach themselves to the thorax or legs of
the butterfly. They transfer from host to host when the butterflies
alight to nectar at flowers.
Marbled White
Melanargia galathea
with parasitic red mites
Trombidium breei
attached to thorax.
Studies have shown that the Trombidium
mites have no detectable effect on the flight performance,
orientation ability or lifespan of the butterflies. In New Zealand
however there is another species of mite
called Dicrocheles ,
which does have an injurious affect on adult Noctuid moths. They
infest the "ears" on the wings of the moths. Apparently, in areas
where there are no predatory bats, the mites attack both ears,
making the moths go completely deaf. But in areas where bats thrive,
the mites seem to only attack one of the ears, so the moth is still
able to detect the bat's acoustic pulses and take avoiding action.
The theory goes that "if the moth cannot hear the bat, both the moth
and the mites will almost certainly be eaten, so they make sure to
keep one ear functional".
In South American cloudforests, adult Glasswing butterflies
Greta andromica, are often attacked
by Ceratopogonid midges, which feed on the blood in the
butterfly's wing veins and eyes. The same midges also attack
larvae, sucking their haemolymph.
Pseudoscorpion hitch-hikers
Close examination of recently emerged
butterflies can sometimes reveal the presence of very tiny
scorpion-like creatures clinging by their pincers to the legs or
antennae. These "pseudoscorpions" are carnivorous, typically
feeding on mites, insect eggs and young larvae, but don't harm the
butterflies. They simply hitch a lift on butterflies and other
insects, using them as transportation to enable them to disperse
to new habitats.
One tactic they use is to ambush a fully grown
caterpillar, grabbing its spines or head horns with their powerful
pincers. When the pincers "bite", the pseudoscorpion becomes
quiescent. After a few hours the caterpillar pupates, and
the pseudoscorpion remains attached to the shed larval skin, which
itself remains attached to the base of the pupa. Eventually the
butterfly emerges from the pupa, and the pseudoscorpion then
scuttles on board, grabbing hold of its antennae or legs. This
causes the butterfly to take flight. Sometime later, when the
butterfly lands in a suitable place, the pseudoscorpion drops off,
and colonises it's new found habitat. Pseudoscorpions are related
to spiders, mites, scorpions and harvestmen. Their hitch-hiking
behaviour is known as "phoresy".
Pathogens etc
Fungal
and viral diseases are most prevalent in cool damp conditions and
cause the death of many hibernating larvae and pupae in temperate
regions. They also affect the early stages of tropical species,
particularly during the rainy season. Living caterpillars are
subject to infection by many fungi including
Cordyceps,
Metarrhizium and Beauvaria.
The caterpillar below may look as if it is dead and going mouldy but
in fact it was very much alive, crawling rapidly over the leaves and
stems of a small bush deep in the Amazonian rainforest. It is quite
possible that the fluff attached to the lateral setae is in some way
cultivated by the caterpillar, which may benefit from looking mouldy
and thus be ignored by birds and other predators. On the other hand
the caterpillar could be the innocent victim of an entomophagous
fungus, and its rapid movement might be an example of how behaviour
is altered in an infected host.
Caterpillars are often attacked
by nuclear polyhedrosis viruses, granulosis viruses and
cytoplasmic polyhedrosis viruses. Affected larvae become limp,
darken in colouration, and produce liquid faeces prior to death.
The condition is known as wilt disease, and is highly infectious.
unidentified moth larva ( ref 016 ), Rio Madre de Dios, 400m,
Peru
Despite the massive losses ascribable to predators, parasitoids, viral
/ fungal diseases and physical causes ( rain washing away eggs,
caterpillars falling from their foodplants etc ), enough offspring
generally survive to maintain reasonably stable populations.
The Balance of Nature
Invertebrate predators possess varying degrees of intelligence and
quickly learn how to overcome the huge variety of defence mechanisms employed by
their prey. It is known that some birds can pass on learnt behaviour to their offspring, so their ability to locate and
butterflies and their larvae increases with each passing
generation. Many birds also possess empathic learning abilities, so
an individual can learn to avoid toxic butterflies simply by
observing another bird vomiting or suffering muscular spasms after
regurgitating one.
Luckily butterflies have a couple of advantages over birds. Firstly,
a high percentage of species are polyvoltine ( producing several
generations per year ). This rapid rate of reproduction allows them
to recover their numbers rapidly even after major losses of
population. Secondly, butterflies and moths intrinsically produce
high genetic diversity. These two factors result in an ability to
evolve rapidly, the consequence being that the battle between birds
and butterflies has become a closely run race. Birds catch
enough caterpillars and butterflies to feed themselves and their
offspring, but at the same time enough butterflies survive to ensure
that both predator and prey can continue to co-exist. This is the
so-called "balance of nature", an ongoing evolutionary
battle between predator ( or parasitoid ) and prey.
Short term variations in this balance are mainly attributable to
seasonal fluctuations in climate :
Caterpillars are cold-blooded, and need warmth to induce feeding
activity. Thus in a cool spring they take longer to develop, so more
get eaten by birds, and more get attacked by parasitoids. In the
case of nocturnal larvae the opposite is the case - cloudy weather
stops night temperatures from dropping and increases the
caterpillar's growth rate. So in poor weather they develop more
rapidly
and fewer get attacked by parasitoids.
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