Parasitoid

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ovipositing into the body of a spotted alfalfa aphid (Therioaphis maculata, Calaphidinae), a behaviour that is used in biological pest control[a][2]

In evolutionary ecology, a parasitoid is an organism that lives in close association with its host at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six major evolutionary strategies within parasitism, distinguished by the fatal prognosis for the host, which makes the strategy close to predation.

Among parasitoids, strategies range from living inside the host (endoparasitism), allowing it to continue growing before emerging as an adult, to

oak galls, up to five levels of parasitism are possible. Some parasitoids influence their host's behaviour
in ways that favour the propagation of the parasitoid.

Parasitoids are found in a variety of

endopterygote insects. Some of these, usually but not only wasps, are used in biological pest control
.

The 17th-century zoological artist

alien species in Ridley Scott's 1979 film Alien
.

Etymology

The term "parasitoid" was coined in 1913 by the Swedo-Finnish writer Odo Reuter,[3] and adopted in English by his reviewer,[4] the entomologist William Morton Wheeler.[5] Reuter used it to describe the strategy where the parasite develops in or on the body of a single host individual, eventually killing that host, while the adult is free-living. Since that time, the concept has been generalised and widely applied.[6]

Strategies

Evolutionary options

A perspective on the evolutionary options can be gained by considering four questions: the effect on the reproductive fitness of a parasite's hosts; the number of hosts they have per life stage; whether the host is prevented from reproducing; and whether the effect depends on intensity (number of parasites per host). From this analysis, proposed by K. D. Lafferty and A. M. Kunis, the major evolutionary strategies of parasitism emerge, alongside predation.[7]

Evolutionary strategies in parasitism and predation[7]
(intensity-dependent: green, roman;
       intensity-independent: purple, italics)
Host fitness Single host, stays alive Single host, dies Multiple hosts
Able to
reproduce
(fitness > 0)		 Conventional parasite
   Pathogen		 Trophically transmitted parasite[b]
   Trophically transmitted pathogen		 Micropredator
   Micropredator
Unable to
reproduce
(fitness = 0)		 —
   Parasitic castrator		 Trophically transmitted parasitic castrator
   Parasitoid		 Social predator[c]
   Solitary predator

Parasitoidism, in the view of

adaptive peaks, with many possible intermediate strategies, but organisms in many different groups have consistently converged on these six.[8][9]

Parasitoids feed on a living host which they eventually kill, typically before it can produce offspring, whereas conventional parasites usually do not kill their hosts, and predators typically kill their prey immediately.[10][11]

Basic concepts

braconid wasp, itself a koinobiont parasitoid of Lepidoptera

Parasitoids can be classified as either endo- or ectoparasitoids with idiobiont or koinobiont developmental strategies. Endoparasitoids live within their host's body, while ectoparasitoids feed on the host from outside. Idiobiont parasitoids prevent further development of the host after initially immobilising it, whereas koinobiont parasitoids allow the host to continue its development while feeding upon it. Most ectoparasitoids are idiobiont, as the host could damage or dislodge the external parasitoid if allowed to move and

moult. Most endoparasitoids are koinobionts, giving them the advantage of a host that continues to grow larger and avoid predators.[12]

Primary parasitoids have the simplest parasitic relationship, involving two organisms, the host and the parasitoid.

oak gall systems, there can be up to five levels of parasitism.[13] Cases in which two or more species of parasitoids simultaneously attack the same host without parasitizing each other are called multi- or multiple parasitism. In many cases, multiple parasitism still leads to the death of one or more of the parasitoids involved. If multiple parasitoids of the same species coexist in a single host, it is called superparasitism. Gregarious species lay multiple eggs or polyembryonic eggs which lead to multiple larvae in a single host. The end result of gregarious superparasitism can be a single surviving parasitoid individual or multiple surviving individuals, depending on the species. If superparasitism occurs accidentally in normally solitary species the larvae often fight among themselves until only one is left.[14][15]

Influencing host behaviour

altering its behaviour
Further information: Behavior-altering parasite

In another strategy, some parasitoids

phorid fly Apocephalus borealis oviposits into the abdomen of its hosts, including honey bees, causing them to abandon their nest, flying from it at night and soon dying, allowing the next generation of flies to emerge outside the hive.[19]

Taxonomic range

About 10% of described insects are parasitoids, in the orders

Endopterygota, which form a single clade), and it is always the larvae that are parasitoidal.[22] The metamorphosis from active larva to an adult with a different body structure permits the dual lifestyle of parasitic larva, freeliving adult in this group.[23] These relationships are shown on the phylogenetic tree;[24][25] groups containing parasitoids are shown in boldface, e.g. Coleoptera, with the number of times parasitoidism evolved in the group in parentheses, e.g. (10 clades). The approximate number (estimates can vary widely) of parasitoid species[26]
out of the total is shown in square brackets, e.g. [2,500 of 400,000].

Endopterygota
Neuropterida

Raphidioptera

Megaloptera

Neuroptera (net-winged insects) (1 clade) [c. 15 of 6,000]

Coleopterida

Coleoptera (beetles) (10 clades) [c. 2,500 of 400,000] Ripiphorid beetle triungulin larva on the wing of a braconid wasp

(1 clade)

Strepsiptera (twisted-wing parasites) [600 of 600]

Hymenoptera

Symphyta

(1 clade)

Orussoidea (parasitic wood wasps) [75 of 75] Parasitic wood wasp

Apocrita (wasp-waisted insects) [c. 50,000 of 100,000] The parasitoid wasp Megarhyssa macrurus ovipositing into host through wood. Her body is c. 50 mm long, her ovipositor c. 100 mm.

Panorpida

Diptera (true flies) (21 clades) [c. 17,000 of 125,000] Stylogaster, a conopid fly, showing the long ovipositor

Mecoptera

Siphonaptera

Trichoptera
(caddis flies) (1 clade) [c. 10 of 14,500]

Lepidoptera (butterflies, moths) (2 clades) [c. 40 of 180,000] Moth Epiricania hagoromo (Epipyropidae) feeding on planthopper Euricania facialis

Hymenoptera

Main article: Parasitoid wasp
Potter wasp, an idiobiont, building a mud nest; she will provision it with paralysed insects, on which she will lay her eggs; she will then seal the nest and provide no further care for her young

Within the Hymenoptera, parasitoidism evolved just once, and the many described

Chalcidoidea, 5,500 Vespoidea, 4,000 Platygastroidea, 3,000 Chrysidoidea, 2,300 Cynipoidea, and many smaller families.[26] These often have remarkable life cycles.[28]
They can be classified as either endoparasitic or ectoparasitic according to where they lay their eggs.
Oviposition depends on finding the host and on evading host defences; the ovipositor is a tube-like organ used to inject eggs into hosts, sometimes much longer than the wasp's body.[30][31][32] Hosts such as ants often behave as if aware of the wasps' presence, making violent movements to prevent oviposition. Wasps may wait for the host to stop moving, and then attack suddenly.[33]

Parasitoid wasps face a range of obstacles to oviposition,

virus which interferes with the host's immune system.[36]
Some parasitoid wasps locate hosts by detecting the chemicals that plants release to defend against insect herbivores.[37]

Other orders

The head of a sessile female strepsipteran protruding (lower right) from the abdomen of its wasp host; the male (not shown) has wings

The true flies (

flesh flies are parasitoids: for instance Emblemasoma auditrix is parasitoidal on cicadas, locating its host by sound.[41]

The Strepsiptera (twisted-wing parasites) consist entirely of parasitoids; they usually sterilise their hosts.[42]

Two

Carabidae are parasitoids.[26]

A few Neuroptera are parasitoidal; they have larvae that actively search for hosts.[46] The larvae of some Mantispidae, subfamily Symphrasinae, are parasitoids of other arthropods including bees and wasps.[26]

Although nearly all Lepidoptera (butterflies and moths) are herbivorous, a few species are parasitic. The larvae of

Galapagos Islands.[22]

Parasitism is rare in the Trichoptera (caddisflies), but it is found among the Hydroptilidae (purse-case caddisflies), probably including all 10 species in the Orthotrichia aberrans group; they parasitise the pupae of other trichopterans.[48]

Entomopathogenic fungi

All known fungi in the genera Cordyceps and Ophiocordyceps are endoparasitic.[49] One of the most notable fungal parasitoids is O. unilateralis which infects carpenter ants by breaching the ant's exoskeletons via their spores and growing in the ant's hemocoel as free living yeast cells. Eventually the yeast cells progress to producing nerve toxins to alter the behaviour of the ant causing it to climb and bite onto vegetation, known as the 'death bite'.[50] This approach is so fine-tuned it causes the ant to bite down on the part of the leaf most optimal for the fungus to fruit; the adaxial leaf midrib. In fact, it has been found that in specific circumstances, the time of the death bite is synchronised to solar noon.[51] As much as 40% of the ant's biomass is fungal hyphae at the moment of the death bite.[52] After the ant dies, the fungus produces a large stalk, growing from the back of the ant's head[53] which subsequently releases ascospores. These spores are too large to be wind dispersed and instead fall directly to the ground where they produce secondary spores that infect ants as they walk over them.[54] O. sinesis, is a parasitoid as well, parasitising ghost moth larvae, killing them within 15-25 days, a similar process to that of O. unilateralis.[55]

Interactions with humans

In biological pest control

Main article: Biological pest control
Encarsia formosa, an endoparasitic aphelinid wasp, bred commercially to control whitefly in greenhouses

Parasitoids are among the most widely used biological control agents. Classic biological pest control using natural enemies of pests (parasitoids or predators) is extremely cost effective, the cost/benefit ratio for classic control being 1:250, but the technique is more variable in its effects than pesticides; it reduces rather than eliminates pests. The cost/benefit ratio for screening natural enemies is similarly far higher than for screening chemicals: 1:30 against 1:5 respectively, since the search for suitable natural enemies can be guided accurately with ecological knowledge. Natural enemies are more difficult to produce and to distribute than chemicals, as they have a shelf life of weeks at most; and they face a commercial obstacle, namely that they cannot be patented.[56][57]

From the point of view of the farmer or horticulturalist, the most important groups are the

true bugs.[58] Commercially, there are two types of rearing systems: short-term seasonal daily output with high production of parasitoids per day, and long-term year-round low daily output with a range in production of 4–1000 million female parasitoids per week, to meet demand for suitable biological control agents for different crops.[59][60]

Maria Sibylla Merian

Parasitic wasps (centre right) with their garden tiger moth host, by Maria Sibylla Merian

Maria Sibylla Merian (1647–1717) was one of the first naturalists to study and depict parasitoids and their insect hosts in her closely-observed paintings.[61]

Charles Darwin

Parasitoids influenced the religious thinking of Charles Darwin,[e] who wrote in an 1860 letter to the American naturalist Asa Gray: "I cannot persuade myself that a beneficent and omnipotent God would have designedly created parasitic wasps with the express intention of their feeding within the living bodies of Caterpillars."[63] The palaeontologist Donald Prothero notes that religiously minded people of the Victorian era, including Darwin, were horrified by this instance of evident cruelty in nature, particularly noticeable in the ichneumonid wasps.[64]

In science fiction

Further information:
Alien (creature in Alien franchise)
Xenomorph[65] parasitoid from the film Alien[66]

Parasitoids have inspired

Xenomorph in Ridley Scott's 1979 film Alien, which runs rapidly through its lifecycle from violently entering a human host's mouth to bursting fatally from the host's chest.[68][69][70] The molecular biologist Alex Sercel, writing in Signal to Noise Magazine, compares "the biology of the [Alien] Xenomorphs to parasitoid wasps and nematomorph worms from Earth to illustrate how close to reality the biology of these aliens is and to discuss this exceptional instance of science inspiring artists".[71] Sercel notes that the way the Xenomorph grasps a human's face to implant its embryo is comparable to the way a parasitoid wasp lays its eggs in a living host. He further compares the Xenomorph life cycle to that of the nematomorph Paragordius tricuspidatus which grows to fill its host's body cavity before bursting out and killing it.[71] Alistair Dove, on the science website Deep Sea News, writes that there are multiple parallels with parasitoids, though there are in his view more disturbing life cycles in real biology. In his view, the parallels include the placing of an embryo in the host; its growth in the host; the resulting death of the host; and alternating generations, as in the Digenea (trematodes).[72] The social anthropologist Marika Moisseeff argues that "The parasitical and swarming aspects of insect reproduction make these animals favoured villains in Hollywood science fiction. The battle of culture against nature is depicted as an unending combat between humanity and insect-like extraterrestrial species that tend to parasitise human beings in order to reproduce."[67] The Encyclopedia of Science Fiction lists many instances of "parasitism", often causing the host's death.[73]

Notes

  1. ^ The species has been introduced to Australia to control the spotted alfalfa aphid.[1]
  2. ^ Trophically transmitted parasites are transmitted to their definitive host, a predator, when their intermediate host is eaten. These parasites often modify the behaviour of their intermediate hosts, causing them to behave in a way that makes them likely to be eaten, such as by climbing to a conspicuous point: this gets the parasites transmitted at the cost of the intermediate host's life.
  3. ^ The wolf is a social predator, hunting in packs; the cheetah is a solitary predator, hunting alone. Neither strategy is conventionally considered parasitic.
  4. ^ There may be far more species of parasitoid wasp not yet described.
  5. ^ Darwin mentions "parasitic" wasps in On the Origin of Species, Chapter 7, page 218.[62]

References

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