Inevolutionary ecology, aparasitoid is anorganism that lives in close association with itshost at the host's expense, eventually resulting in the death of the host. Parasitoidism is one of six majorevolutionary strategies withinparasitism, distinguished by the fatalprognosis for the host, which makes the strategy close topredation.
Among parasitoids, strategies range from living inside the host (endoparasitism), allowing it to continue growing before emerging as an adult, toparalysing the host and living outside it (ectoparasitism). Hosts can include other parasitoids, resulting inhyperparasitism; in the case ofoak galls, up to five levels of parasitism are possible. Some parasitoidsinfluence their host's behaviour in ways that favour the propagation of the parasitoid.
Parasitoids are found in a variety oftaxa across theinsect superorderEndopterygota, whose completemetamorphosis may havepre-adapted them for a split lifestyle, with parasitoidlarvae and free-living adults. Most are in theHymenoptera, where theichneumons and many otherparasitoid wasps are highly specialised for a parasitoidal way of life. There are parasitoids, too, in theDiptera,Coleoptera and otherorders ofendopterygote insects. Some of these, usually but not only wasps, are used inbiological pest control.
The 17th-century zoological artistMaria Sibylla Merian closely observed parasitoids and their hosts in her paintings. The biology of parasitoidism influencedCharles Darwin's beliefs and has inspiredscience fiction authors andscriptwriters to create numerous parasitoidal aliens that kill their human hosts, such as thealien species inRidley Scott's 1979 filmAlien.
The term "parasitoid" was coined in 1913 by theSwedo-Finnish writerOdo Reuter,[3] and adopted in English by his reviewer,[4] the entomologistWilliam 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]
A perspective on the evolutionary options can be gained by considering four questions: the effect on thereproductive 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]
| 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 ofR. Poulin and H. S. Randhawa, is one of six mainevolutionary strategies withinparasitism, the others beingparasitic castrator, directly transmitted parasite,trophically transmitted parasite,vector-transmitted parasite, and micropredator. These areadaptive peaks, with many possible intermediate strategies, but organisms in many different groups have consistentlyconverged 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]
Parasitoids can be classified as either endo- or ectoparasitoids withidiobiont orkoinobiont 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 andmoult. 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.Hyperparasitoids are parasitoids of parasitoids; secondary parasitoids have a primary parasitoid as their host, so there are three organisms involved. Hyperparasitoids are either facultative (can be a primary parasitoid or a hyperparasitoid depending on the situation) or obligate (always develop as a hyperparasitoid). Levels of parasitoids beyond secondary also occur, especially among facultative parasitoids. Inoak 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 calledsuperparasitism. 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]
In another strategy, some parasitoidsinfluence the host's behaviour in ways that favour the propagation of the parasitoid, often at the cost of the host's life. A spectacular example is thelancet liver fluke, which causes host ants to die clinging to grass stalks, where grazers or birds may be expected to eat them and complete the parasitoidal fluke's life cycle in itsdefinitive host. Similarly, asstrepsipteran parasitoids of ants mature, they cause the hosts to climb high on grass stalks, positions that are risky, but favour the emergence of the strepsipterans.[16] Among pathogens of mammals, therabies virus affects the host'scentral nervous system, eventually killing it, but perhaps helping to disseminate the virus by modifying the host's behaviour.[17] Among the parasitic wasps,Glyptapanteles modifies the behaviour of its host caterpillar to defend the pupae of the wasps after they emerge from the caterpillar's body.[18] Thephorid flyApocephalus borealis oviposits into the abdomen of its hosts, includinghoney 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]
About 10% of described insects are parasitoids, in the ordersHymenoptera,Diptera,Coleoptera,Neuroptera,Lepidoptera,Strepsiptera, andTrichoptera. The majority are wasps within the Hymenoptera; most of the others are Dipteran flies.[6][20][21] Parasitoidism hasevolved independently many times: once each in Hymenoptera, Strepsiptera, Neuroptera, and Trichoptera, twice in the Lepidoptera, 10 times or more in Coleoptera, and no less than 21 times among the Diptera. These are allholometabolous insects (Endopterygota, which form a singleclade), and it is always the larvae that are parasitoidal.[22] The metamorphosis from active larva to an adult with a different body structurepermits the dual lifestyle of parasitic larva, freeliving adult in this group.[23] These relationships are shown on thephylogenetic 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 |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Within the Hymenoptera, parasitoidism evolved just once, and the many described[d] species ofparasitoid wasps[27] represent the great majority of species in the order, barring those like theants,bees, andVespidae wasps that have secondarily lost the parasitoid habit. The parasitoid wasps include some 25,000Ichneumonoidea, 22,000Chalcidoidea, 5,500Vespoidea, 4,000Platygastroidea, 3,000Chrysidoidea, 2,300Cynipoidea, 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.[29] Endoparasitic wasps insert their eggs inside their host, usually as koinobionts, allowing the host to continue to grow (thus providing more food to the wasp larvae), moult, and evade predators. Ectoparasitic wasps deposit theirs outside the host's body, usually as idiobionts, immediately paralysing the host to prevent it from escaping or throwing off the parasite. They sometimes carry the host to a nest where it will remain undisturbed for the wasp larva to feed on.[6] Most species of wasps attack the eggs or larvae of their host, but some attack adults.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,[6] including behavioural, morphological, physiological and immunological defences of their hosts.[29][34] To thwart this, some wasps inundate their host with their eggs so as to overload its immune system's ability to encapsulate foreign bodies;[35] others introduce avirus 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]
The true flies (Diptera) include several families of parasitoids, the largest of which is theTachinidae (some 9,200 species[26]), followed by theBombyliidae (some 4,500 species[26]), along with theAcroceridae,Pipunculidae, andConopidae, which are exclusively parasitoidal genera (such asStylogaster). Other families of flies include someprotelean species.[38] SomePhoridae are parasitoids of ants.[39][40] Someflesh flies are parasitoids: for instanceEmblemasoma auditrix is parasitoidal on cicadas, locating its host by sound.[41]
TheStrepsiptera (twisted-wing parasites) consist entirely of parasitoids; they usually sterilise their hosts.[42]
Threebeetle families,Ripiphoridae (450 species[26]),[43][44]Passandridae, andRhipiceridae, are largely parasitoids, as areAleocharaStaphylinidae; in all, some 400 staphylinids are parasitoidal.[26][38][45] Some 1,600 species of the large and mainly freeliving familyCarabidae (mostly in the subfamiliesBrachininae andLebiinae) are parasitoids.[26]
A few Neuroptera are parasitoidal; they have larvae that actively search for hosts.[46] The larvae of someMantispidae, 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 ofEpipyropidae feed onHemiptera such as leafhoppers and cicadas, and sometimes on other Lepidoptera. The larvae ofCyclotornidae parasitise first Hemiptera and laterant brood.[47]
Parasitism is rare in the Trichoptera (caddisflies), but it is found among theHydroptilidae (purse-case caddisflies), probably including all 10 species in theOrthotrichia aberrans group; they parasitise the pupae of other trichopterans.[48]
Mites of the familyAcarophenacidae are ectoparasitoids of insect eggs. Unlike the insect parasitoids, it is the adult stage in Acarophenacidae that acts as a parasitoid. Specifically, adult female mites feed on insect eggs and their body swells up with offspring, which eventually emerge as adults.[49]
All known fungi in the generaCordyceps andOphiocordyceps are endoparasitic.[50] One of the most notable fungal parasitoids isO. 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, an act known as the 'death bite'.[51] This approach is so fine-tuned, it causes the ant to bite down on the adaxial leaf midrib, which is the part of the leaf most optimal for the fungus to fruit. In fact, it has been found that in specific circumstances, the time of the death bite is synchronised to solar noon.[52] As much as 40% of the ant's biomass is fungal hyphae at the moment of the death bite.[53] After the ant dies, the fungus produces a large stalk, growing from the back of the ant's head,[54] 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.[55]O. sinesis is a parasitoid as well, parasitising ghost moth larvae, killing them within 15–25 days, a similar process to that ofO. unilateralis.[56]
Host location has been studied inOrmia ochracea, a parasitoid tachinid fly that locates their field cricket host acoustically (phonotaxis).[57] Preference for the dominant local host species was not explained by DNA analysis. In fact, populations across the southern U.S. were inexplicably closely related, considering rate of range expansion from a presumed Central American origin.[58] A captive population of lab-reared flies were raised on two different host songs (Gryllus integer orG. lineaticeps). Responsive adult females overwhelmingly chose their familiar song, indicating the use of a learned, auditory search image. Thisphenotypic plasticity allows such a highly specialized parasitoid to avoid overspecialization disasters. Interestingly, when receptive females only heard silence the night before testing for preference, they chose the host songs equally, 50/50.[59] This capacity for learning and use of search images paired with a highly specialized morphology and lifestyle (eg. tympana tuned to host sound cues, larviparous) supports the extraordinarily fast range expansion ofO. ochracea, as well as the presence and power of learning in parasitoids.
Parasitoids are among the most widely used biological control agents. Classicbiological 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.[60][61]
From the point of view of the farmer or horticulturalist, the most important groups are theichneumonid wasps, which prey mainly oncaterpillars ofbutterflies andmoths;braconid wasps, which attack caterpillars and a wide range of other insects includinggreenfly;chalcidoid wasps, which parasitise eggs and larvae of greenfly,whitefly,cabbage caterpillars, andscale insects; andtachinid flies, which parasitise a wide range of insects including caterpillars, adult and larvalbeetles, andtrue bugs.[62] 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.[63][64]
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.[65]
Parasitoids influenced the religious thinking ofCharles Darwin,[e] who wrote in an 1860 letter to the American naturalistAsa 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."[67] The palaeontologistDonald Prothero notes that religiously minded people of theVictorian era, including Darwin, were horrified by this instance of evident cruelty in nature, particularly noticeable in theichneumonid wasps.[68]
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Parasitoids have inspiredscience fiction authors and screenwriters to create terrifyingparasitic alien species that kill their human hosts.[71] One of the best-known is theXenomorph inRidley Scott's 1979 filmAlien, which runs rapidly through its life cycle from violently entering a human host's mouth to bursting fatally from the host's chest.[72][73][74] The molecular biologist Alex Sercel, writing inSignal to Noise Magazine, compares "the biology of the [Alien] Xenomorphs to parasitoid wasps andnematomorph 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".[75] 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 nematomorphParagordius tricuspidatus which grows to fill its host's body cavity before bursting out and killing it.[75]Alistair Dove, on the science websiteDeep Sea News, writes that there are multiple parallels with parasitoids, although in his view, there are more disturbing life cycles in real biology. Dove stated that the parallels include the placing of an embryo in the host; its growth in the host; the resulting death of the host; andalternating generations, as in theDigenea (trematodes).[76] The social anthropologistMarika Moisseeff argues that "The parasitical and swarming aspects of insect reproduction make these animals favouredvillains inHollywood 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."[71]The Encyclopedia of Science Fiction lists many instances of "parasitism", often causing the host's death.[77]
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{{cite book}}:|journal= ignored (help)Predation, herbivory, and parasitism exist along a continuum of severity in terms of the extent to which they negatively affect an organism's fitness. ... In most situations, parasites do not kill their hosts. An exception, however, occurs with parasitoids, which blur the line between parasitism and predation.
The length of the ovipositor compared with the body of the parasitic wasp varies enormously between taxa, from being a fraction of the length of the metasoma to more than 14 times longer than the head and body. (Townes 1975; Achterberg 1986; Compton & Nefdt 1988).
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