Butterflies of the World - Lifecycle, Ecology, Taxonomy, Conservation, Photography, Butterfly Holidays, Photo Galleries, Book Reviews and more.........
Text and photographs protected by Copyright © Adrian Hoskins 2007, and must not be published in part or in whole elsewhere without prior written permission from the author.
 
Parasites and Parasitoids

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.

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 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 directly 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.

Parasitoid flies on the other hand usually lay a single egg on the caterpillar's back, and 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 and grow within the caterpillar, slowly consuming it's flesh while being sure to leave 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 tiny 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 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 and 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 the thorax.

Studies have demonstrated that mites infesting Meadow Browns have no detectable effect on the flight performance, orientation ability or lifespan of the butterflies.

Photograph courtesy Pauline Richards © 2007.

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 Balance of Nature

Despite the enormous losses ascribable to predators, parasitoids, diseases and other natural causes ( rain washing away eggs, caterpillars falling from their foodplants etc ), enough offspring generally survive to maintain reasonably stable populations.

Butterflies, like all fauna and flora, are however caught up in a delicate balancing act - the so called "balance of nature" between predator ( or parasitoid ) and prey.

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 cold and sunny, larvae of the Marsh Fritillary Euphydryas aurinia can warm themselves up by basking in the sunshine, and are able to 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.

If the weather in early spring is generally 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 when the wasps emerge they find large numbers of half-grown caterpillars to attack, their numbers are depleted, and the number of adult butterflies which emerge is much lower.

Metapopulations

The 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 a sizeable area of contiguous habitat exists, 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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