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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 or moth may be capable of laying up to 500 eggs, but in practice only about 50 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 will be eaten by birds, be killed by parasitoids, or die from disease, leaving just 4 or 5 that reach pupation.
Over half of all wild pupae will be eaten, be killed by parasitoids, or die from desiccation, fungal attack, or other causes, leaving only 2 butterflies that will emerge.
 
When the butterflies emerge from the surviving pupae, as many as half will be killed before they mate or are able to lay eggs. The net result is that on average, despite the ability to produce those 500 eggs, and the potential of 500 butterflies, just a single butterfly will survive to produce another generation.
 

potential number of eggs per female

500

actual number of eggs laid, on average

50

number of eggs that hatch successfully

47

number of caterpillars that reach pupation

5

number of butterflies that emerge successfully

2

number of butterflies that survive long enough to mate

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 !
 
Caterpillar parasitoids
 

Common Awl Skipper Hasora badra ( Singapore ) surrounded by parasitoid wasp cocoons
Photo courtesy SoonChye
 
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.

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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