The Enemies of
Butterflies
PAGE 1
PAGE 1 - PARASITES, PARASITOIDS & PATHOGENS
PAGE 2 - PREDATORS
PAGE 3 - HUMANS
Parasites, Parasitoids & Pathogens
Predation,
parasitism, disease and fungal attacks cause very heavy population losses. A butterfly may be capable of laying
up to 500 eggs, but in practice less than 100 are laid per butterfly, as most
females die before they are able to lay all their eggs.
Perhaps 95
percent of those eggs will hatch, but at least
90 percent of the resulting caterpillars are likely to be eaten by birds
or wasps, be killed by
parasitoids, or die from disease, leaving just 4 or 5 that 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
female will ultimately result in no more than one or two adult butterflies.
As many as half of emerged adults 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, on average just a
single butterfly will survive to produce another generation.
There
will obviously be many other adults that emerge - the progeny of
numerous females of the previous generation - to ensure the survival
of the species, but for the population to maintain a consistent
level, on average each pair of adults will produce just one new pair. In practice
of course the numbers surviving will vary according
to climatic conditions, foodplant availability, predation levels
etc, so in "good" years populations expand, and in "bad" years they
contract.
|
potential number
of eggs per female |
500 |
|
actual number of
eggs laid, on average |
100 |
|
number of eggs
that hatch successfully |
95 |
|
number of
caterpillars that reach pupation |
5 |
|
number of
butterflies that emerge successfully |
2 |
|
number of
females that
survive long enough to mate and lay eggs |
1 |
Parasitoids
All stages of
the lifecycle are threatened by parasitoids - creatures which feed
on other organisms, ultimately bringing about their death.
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 !
This
unidentified hawkmoth larva from Peru has been parasitised by a
wasp which injected it's eggs into the larva. When fully grown the
wasp grubs break out through the skin of the larva, and form their
papery white cocoons, as seen in this photo.
Caterpillar parasitoids
Throughout their lives caterpillars are very vulnerable to attack
from parasitoids. Studies in Britain have revealed that with most
butterfly and moth species, about 80
percent of their larvae are attacked by parasitoid wasps or flies.
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
wasps inject their eggs into the caterpillar's body by
means of a long sting-like ovipositor. Some wasps
inject just a single egg, which divides inside the host, producing
many wasp grubs.
The caterpillar of
Hasora badra illustrated above has been
parasitised by an Apanteles species - a
total of 67 wasps emerged from the cocoons which surround it.
The wasp grubs feed on the body tissues, taking care to avoid the
vital organs as it is in their interest to keep the caterpillar
alive for a while. Eventually when the grubs are fully grown and
ready to pupate they eat the caterpillar's vital organs and then
bore their way out through its skin to pupate in a mass of little
cocoons beside the host's shrivelled body.
Parasitoid flies
on the other hand
usually
lay a single egg on the
caterpillar's back. Upon hatching the
tiny grub bores
it's way into the caterpillar's body. Some
flies such as
Sturmia bella
lay their eggs on the caterpillar's foodplants instead. These are
ingested undamaged
and pass into the caterpillar's gut.
In the case of both wasps and flies the grubs feed within the caterpillar, consuming
it's
flesh but leaving the vital organs alone so the caterpillar
can 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 caterpillar, or eat their
way out of it's skin and pupate externally.
Common
Awl Skipper Hasora badra (
Singapore ) surrounded by parasitoid wasp cocoons
Photo courtesy
©
SoonChye
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
Pteromalus
puparum, which attacks pre-pupal larvae and newly formed
pupae of the Large White
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.
Parasites
Parasites are defined as creatures which feed on other organisms, but
unlike parasitoids they do NOT bring about the death of their hosts. In the case of
Lepidoptera, these generally affect the adult butterflies and moths,
rather than the early stages.
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".
Fungal and viral pathogens
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.
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 huge 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"
Vertebrate predators possess varying degrees of intelligence and
quickly learn how to overcome the huge variety of
defence mechanisms employed by
their prey. In many cases it is known that birds can pass on their
learnt behaviour to their offspring, so their ability to locate and
consume butterflies and their larvae increases with each passing
generation.
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 enables them
to recover their numbers quickly even after major losses of
population. Secondly, butterflies and moths intrinsically produce
high genetic diversity. These 2 factors result in an ability to
evolve rapidly, the consequence being that the battle between birds
and butterflies has become a closely run race - the 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" - a long term evolutionary
battle for survival 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 reverse is the case - cloudy weather
stops night time temperatures from dropping, and increases the
caterpillar's growth rate. So in poor weather they develop quicker
and less get attacked by parasitoids.
It can be seen from the above examples that climatic conditions have a major impact on
population dynamics. Predators, parasitoids and their host
species each have their own "ideal" climate, as illustrated in
the following example :
Marsh Fritillaries v
Braconid wasps
If the weather
in early spring is dominated by clear skies, larvae of the Marsh Fritillary
Euphydryas aurinia
can warm themselves up by
basking in the sunshine, and can feed and develop quickly.
Their parasitoid however, the Braconid wasp
Apanteles bignellii spends the early spring as
a pupa, hidden in the
shade amongst grasses where the temperature is lower. Consequently the
aurinia
larvae develop more rapidly than the
Apanteles pupae, and have pupated themselves before
the adult wasps emerge. A few weeks later large numbers of Marsh
Fritillaries emerge and breed.
On the other hand, if
the spring weather is predominantly cloudy, the wasps have the
advantage. Their pupae develop at much the same speed as in a sunny
spring because in both situations they metamorphose in cool shady situations. The
aurinia
larvae however feed and grow more slowly, as they need sunshine to
make them active. Consequently the wasps
emerge early in relation to their hosts, and find large numbers of half-grown caterpillars to
attack, so the number of adult
butterflies which ultimately emerge is much lower.
Metapopulations
These
differing requirements of hosts, parasitoids and predators, in
combination with the variable climate, mean that butterfly
populations are in a state of constant flux. Often, at marginal
sites, they suffer local extinctions. This matters little if sizeable
areas of contiguous habitat exist, because the butterflies can
recolonise from other nearby sites.
Many
species exist as "metapopulations", where a strong core population
is surrounded by smaller marginal "satellite" populations. Part of
the metapopulation may be temporarily lost due to changing habitat
conditions or localised predation, but recolonisations from the
"core" ensure that the overall population survives.
Unfortunately, human activities have reduced previously vast
expanses of wild habitat to a patchwork of tiny fragmented areas.
Butterflies are extremely reluctant to leave the sanctuary of their
habitats to cross hostile landscapes in search of new breeding
sites, and so are unable to recolonise areas from which they have
previously disappeared.
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