Parasitoid - Definitions and Distinctions

Definitions and Distinctions

The term parasitoid was coined in 1913 by the German writer O. M. Reuter (and adopted in English by his reviewer, William Morton Wheeler) to describe the strategy in which, during its development, the parasite lives in or on the body of a single host individual, eventually killing that host, the adult parasitoid being free-living. Since that time however, the concept has been variously generalised and widely applied.

In practice it is not always necessary to distinguish parasitoidy from parasitism, nor is it always even possible to do so cleanly. However, when it is appropriate to do so, a typically parasitic relationship is one in which parasite and host interact without lethal harm to the host, and without dramatically reducing the host's reproductive success. In most such relationships, the parasite arrogates enough nutrients or other resources to thrive without preventing the host from reproducing. In contrast, in a parasitoidal relationship the exploiting organism kills or sterilises the host, typically before it can produce offspring. A non-lethal parasite sometimes is termed a biotroph. In contrast, when a parasitoidal relationship is regarded as a form of parasitism, the parasitoid may be called a necrotroph.

When an organism sterilises its host without directly killing it, then whether to term it a parasitoid or a parasite, is a matter of context and preference. Often for a parasite to prevent reproduction of the host is incidental, but various forms of systematic parasitic castration occur among parasitoids, and many such biological strategies are highly sophisticated. Crustacean parasites or parasitoids include several impressive examples.

Protelean is a term that various authors use to denote organisms that live as parasites only during the early, growing, phases of their lives; typically they then begin by behaving as internal parasites; also typically they end that phase of their lives parasitoidally by killing or consuming the host. Finally they emerge as free-living adults, with or without an intervening phase of diapause or metamorphosis.

Protelean organisms are widely regarded as a special class of parasites, or more usually parasitoids. The most typical examples of proteleans are the parasitoidal Hymenoptera, Diptera, Strepsiptera, and some other insects. Usually such insects are holometabolous. It is reasonable to regard holometaboly as preadaptation for the protelean life history because it implies that their larval stage of life differs drastically from the adult stage, both functionally and morphologically.

Idiobiont parasitoids are those that prevent further development of the host after initially immobilizing it, and, almost without exception, develop outside the host. Koinobiont parasitoids allow the host to continue its development while feeding upon it, and may parasitize any host life stage. In turn, koinobionts can be subdivided further into endoparasitoids, which develop inside body of the host, and ectoparasitoids, which develop outside the host body, though the parasitoids frequently are attached or embedded in the host's tissues.

It is fairly common for a parasitoid itself to serve as the host for another parasitoid's offspring. The latter is commonly termed a hyperparasite, but in most cases this term is slightly misleading, as both the host and the primary parasitoid are killed. A better term might be secondary parasitoid, or hyperparasitoid.

Most known specialist hyperparasite and hyperparasitoid species are in the insect order Hymenoptera, but a fair amount of incidental hyperparasitoidy results when a single host or a single food stash happens to house multiple guests and rations run short. Some members of the flesh fly family, Sarcophagidae, subfamily Miltogramminae, for example members of the genus Craticulina, are kleptoparasites of wasps in the subfamilies Bembicinae and Philanthinae (both currently placed in the family Crabronidae). Both those subfamilies tend to build nests by digging tunnels in sand, which they then stock with prey such as flies or bees, depending on the species. Kleptoparasitic flies such as Craticulina are much smaller than the host wasp and lay their eggs on the prey as the wasp returns to the nest on a victualing flight. The fly larvae are small, though faster-growing than the wasp larva, and if there is only one, the wasp is likely to complete its metamorphosis successfully, but when there are several it might suffer from malnutrition or even get eaten itself, which amounts to incidental kleptoparasitoidy.

In contrast though, as described in the following section, some insects, such as some members of the Trigonalidae, not only are specialist hyperparasitoids, but have advanced behavioural adaptations to support their speciality.

Note once again that there is no clear separation between the concepts of parasitism and parasitoidy. Many species of true parasites can cause the death of their host if for example they are present in overwhelming numbers or the host is in poor condition, or other compromising circumstances develop, such as secondary infections. For example, blood-sucking mites sometimes overwhelm nestlings of birds such as swallows to the point that the young birds cannot fledge successfully,.

Infestations of other mites cause various kinds of mange in mammals. Mange mites are generally in the families Demodicidae that cause Demodicosis or demodectic mange, Sarcoptidae that cause scabies or sarcoptic mange, and Psoroptidae that cause scab in sheep and rabbits. Severe mange can debilitate animals to the point that they cannot feed themselves adequately, so that in unfavourable circumstances they may die.

Again, various species of paralytic ticks sometimes kill dogs if the owners are insufficiently alert, and soft ticks can fatally poison a host such as a horse that might rest in an infested shady spot because it does not know the local hazards. Conversely, some parasitoids do somewhat shorten the lives of their hosts or constrain their reproduction, but without necessarily killing them as a part of their interaction. Almost any microbial disease could be defined as a parasitic condition, and some could be argued to amount to clear examples of parasitoidy. One converse argument is that when the death of the host is neither a logical nor necessarily even a desirable consequence from the point of view of the parasite, the relationship should be regarded as parasitic rather than parasitoidal. This certainly would apply to examples such as mange, and diseases in which the living victim acts as a natural reservoir or even a vector.

In their extreme forms the categories of parasitism and parasitoidy patently are distinct; one is in no doubt whether the larva of a Tarantula hawk wasp behaves more like a parasitoid, or even a predator, than a parasite; and similarly the biting midges that suck blood from large insects plainly are simply ectoparasites. However, there is a continuum of intermediate and contingent conditions that bridge those categories in practically every respect. This should not be taken too seriously as a material problem in terminology; the terms are useful in particular contexts and should not be abused by inappropriate application in contexts in which they create confusion rather than clarity. Many examples of species that are technically parasitoidal, at least facultatively, are not generally referred to as parasitoidal. Many microbial diseases and the aforementioned soft ticks constitute instructive examples.

Nor do those complete the list of examples that constitute the requirement for fuzzy distinctions in such matters; at the opposite extreme from parasitism, parasitoidy in turn grades into predation. Differences between various kinds of hunting wasps provide convenient illustrations. Predatory social wasps hunt flies, caterpillars and the like, grab them, butcher them, carry them home and feed them to their young. That is patent predation. Some solitary wasps, such as bee pirates, sting prey, sometimes fatally, before saving it, usually entire, in a nest or burrow for the young to feed on. That too is predation, fairly clearly.

In contrast, the best-known protelean solitary hunting wasps sting prey to paralyse it before storing it for the young in the nest. The larvae then proceed to eat the stored prey alive, sometimes according to very sophisticated schedules that delay killing the victim sooner than necessary, thereby avoiding having their rations rot before they could be consumed. Some authorities regard such larval behaviour as having a strong element of parasitoidy. That view is based largely on the view that the young larvae begin with small exactions like any parasite, then proceed to the point where they eat at such a rate that they might as well be predators.

Other wasps paralyse prey in the plant or other environment in which it feeds, before laying eggs nearby. The emerging young attack and feed on the paralysed prey organism in its own home. Some solitary parasitoids among insects lay their eggs on or in their live prey and any of a wide range of consumption schedules might follow. Some parasitoids even lay their eggs where the larvae must locate the prey for themselves when they hatch from the eggs. Examples include flies in the families Tachinidae and Bombyliidae. The physiological and strategic sophistication of such relationships, whether parasitoidal or parasitic, often are impressive.

Patently there is no point to trying to draw arbitrary lines of distinction between such vague, and often variable, life histories. In each ecological or ethological study the terms applied should reflect the facts in the contexts relevant to the matter in question. Such studies need not in all cases use the identical terminology, and there is no reason they should. All that is necessary is that the terminology in each study should be clear, useful and relevant.