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Ecologyecological relationship

Parasitism and predation

A predator eats many and kills fast. A parasite keeps its host alive. A parasitoid does one of each, which is why it needed its own word.

A predator eats many prey and kills each quickly. A parasite usually keeps its host alive. A parasitoid does exactly one of each: one host, consumed entirely, killed slowly from within — which is why it needed a word of its own.

These words are usually treated as points on a severity scale, which is why the boundaries feel arbitrary. They are better read as different structures, and the structure is about how many hosts and what happens to them. A predator consumes many prey over a lifetime and each dies quickly. A parasite typically has one host at a time and has a strong interest in that host continuing to live and function, because it is the parasite’s habitat. A grazer eats living tissue and the organism carries on. A scavenger consumes something already dead, having killed nothing. The parasitoid is the case that clarifies the rest, precisely because it belongs to neither. Its larva develops on or inside a single host, consumes it, and kills it. One host, like a parasite; consumed entirely and killed, like prey; and slowly from within, like neither. It is a coherent life history in its own right, it required its own theory, and it is enormously successful — parasitoid wasps are among the most species-rich groups of animals alive. Many of them manipulate their host’s behaviour, and this is where the site’s most famous case needs putting in context. The jewel wasp stings a cockroach in identified regions of its brain and suppresses its escape drive, then walks it to a burrow; the neural mechanism is genuinely well characterised. That is what direct manipulation of a nervous system looks like when somebody has worked it out. The far better known Ophiocordyceps, which makes ants climb and bite, is not in the ant’s brain at all. The two are routinely narrated in the same breath and they are doing different things — one of them understood, the other not.

Early coverage · 53% complete · reviewed 2026-09-03

What this page covers

The categories apply across all life. Parasitoids are concentrated overwhelmingly in insects — chiefly wasps and flies — which is itself something needing explanation.

Often confused with: A simple spectrum from gentle to lethal, when the categories differ in structure rather than severity; Parasites always being small, which is not what defines them; Herbivory being predation, which it usually is not, since the plant survives

Quick facts

The parasitoid
One host, consumed entirely, killed slowly from inside
Not a severity scale
The categories differ in structure, not in how unpleasant they are
Brain manipulation, done properly
The jewel wasp stings into identified brain regions — and that mechanism is understood
And the famous one
Ophiocordyceps is not in the ant’s brain at all

Four ways of living off something else

They differ in how many hosts, and in what happens to them.

A parasitoid sits exactly between the two categories: it consumes a single host and kills it, like a predator, but slowly and from inside, like a parasite. That is why it needed its own word.

Established

Specialists would state this without hedging. Multiple independent lines of evidence agree.

Parasitoidism is defined by development of a larva on or within a single host individual, which is consumed and killed. It differs from predation, in which multiple prey are consumed rapidly, and from parasitism, in which the host is typically kept alive. Parasitoids constitute one of the most species-rich animal life histories.

Who this applies to
Chiefly wasps and flies, where the life history is best documented.
Studied in
Hymenoptera, Diptera
Why we rate it this way, and what the caveats are
EstablishedHigh confidence

A definitional and life-history distinction, thoroughly documented and standard in the ecological literature.

How far it can be extended

The life history is characterised across many independently derived parasitoid lineages.

Caveats

  • The boundaries grade at the edges: some parasites kill their hosts, and some parasitoids attack more than one host individual.
  • The categories describe strategies, and a single species can shift between them across its life stages.

Still unanswered

  • Why parasitoidism has arisen so many times in insects and so rarely elsewhere.

Last reviewed 2026-09-03

The evidence (2 studies)
The categories, by structure rather than severity
RelationshipHow many hostsWhat happens to them
PredationMany over a lifetimeKilled, usually quickly
ParasitismOne at a timeKept alive — it is the habitat
ParasitoidismOne, per larvaConsumed entirely and killed, slowly
HerbivoryManyUsually survives, having lost tissue
ScavengingNot applicableAlready dead; the scavenger killed nothing

The edges grade, as biological categories do. Some parasites kill. Some predators take only part of an animal and leave it alive, which is closer to grazing. And a single species can move between categories across its life stages — many parasitoid wasps are nectar-feeders as adults, so the animal that lays the egg is not living off anything at all.

Two manipulators, one of them understood

And it is not the famous one.

The jewel wasp genuinely does work on a brain: it stings venom into specific regions of a cockroach’s, and the effect follows. That is the well-understood case — and it is not what the famous zombie fungus does.

Well supported

Good evidence backs this, though some details remain open.

Ampulex compressa delivers venom into identified regions of the cockroach central nervous system, producing a specific and reversible suppression of escape-related locomotor drive. Neural mechanisms of host manipulation are well characterised in this and a small number of other systems, and unresolved in most, including the fungal cases.

Who this applies to
Best characterised in the jewel wasp and cockroach; most manipulation systems are far less understood.Do not extend this beyond the taxa listed — the popular version over-reaches.
Studied in
Hymenoptera, Insecta
Why we rate it this way, and what the caveats are
Well supportedModerate confidence

The wasp system is characterised in real neural detail, which is why it can carry this claim. That most other manipulation systems lack such a mechanism is a statement about the state of the literature rather than about the parasites.

How far it can be extended

The point of the claim is that this system is exceptional in being understood; generalising its mechanism to other manipulators is the error it guards against.

Caveats

  • Direct neural intervention is the exception. Most documented manipulation is achieved by secreted compounds, altered host physiology, or routes nobody has identified.
  • Even in the jewel wasp, which venom components produce which effects is only partly resolved.

Still unanswered

  • Whether the chemical manipulators act on the nervous system indirectly, or on something else entirely that changes behaviour as a consequence.

Last reviewed 2026-09-03

The evidence (2 studies)

The jewel wasp is the case where the neurobiology is real. It stings a cockroach twice: once into the thorax, producing a brief paralysis of the front legs, and then — with the cockroach unable to defend itself — into the head, delivering venom to specific regions of the brain. The result is not paralysis but a selective loss of the drive to initiate escape. The cockroach can walk, and does, when led by an antenna. The wasp walks it to a burrow, lays an egg on it, and seals it in.

Two famous manipulators, compared honestly
AspectJewel waspOphiocordyceps
Where the parasite actsDirectly into identified brain regionsMuscles and body cavity; not the brain
Effect on the hostLoss of escape drive; movement retainedDirected climbing, orienting and biting
MechanismCharacterised in real neural detailUnidentified; a chemical route is likely
How famousBarely known outside entomologyEverywhere

Reading those two rows together is the point of putting them on one page. The system everybody has heard of is the one nobody can explain, and the system that has been worked out is largely unknown. That is a common shape in science communication, and it is worth naming when it appears.

Related

Why parasitoids are almost all insects

An open question with a plausible answer nobody has confirmed.

Parasitoidism has arisen many times in insects and hardly at all elsewhere, and the imbalance is striking enough to want explaining. The usual suggestion involves size and development: an insect larva can be small relative to a host, has a fixed developmental programme that can be completed inside one, and an adult stage that disperses and finds the next host. Whether that is sufficient, or whether something else about insects makes the life history reachable, is not settled.

  • Why is parasitoidism so overwhelmingly an insect strategy?

    Why it matters: A life history this successful — parasitoid wasps are among the most species-rich animal groups — arising repeatedly in one class and almost nowhere else suggests a constraint worth identifying. It would say something general about which life histories are reachable from which body plans.

The research behind this page

7 studies, newest first. Each one has a page explaining what it found and what it could not show.

2017Proceedings of the National Academy of Sciences

Three-dimensional visualization and a deep-learning model reveal complex fungal parasite networks in behaviorally manipulated ants

Fungal cells filled the ant’s body and formed an interconnected network surrounding and penetrating muscle fibres, including in the mandibles.

2014BMC Evolutionary Biology

Species-specific ant brain manipulation by a specialized fungal parasite

The fungus produced different sets of compounds depending on whose brain it was exposed to, secreting a distinct profile in the presence of its natural host’s brain compared with a non-host species.

2011BMC Ecology

Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection

Infected ants leave the canopy, descend to a consistent height and orientation, and bite into a leaf vein around solar noon, dying there.

2009The American Naturalist

The life of a dead ant: the expression of an adaptive extended phenotype

Ants died in a narrow band of height, orientation and microclimate.

2009Annual Review of Entomology

Manipulation of host behavior by parasitic insects and insect parasites

Behavioural manipulation is achieved by several distinct routes — direct injection into the nervous system, secretion of neuroactive compounds, and alteration of host physiology — and the neural mechanism is well characterised in very few systems.

1994Princeton University Press

Parasitoids: Behavioral and Evolutionary Ecology

Parasitoids occupy a distinct category between predators and parasites: a single host is consumed, as in predation, but slowly and from within, as in parasitism.

1879Karl J. Trübner

Die Erscheinung der Symbiose

Associations between unlike organisms form a continuum of outcomes, and the fact of living together does not determine which outcome applies.

This page is a stop on a longer route

A guided journey reads several subjects in a deliberate order, with an argument for why one follows another. You can join in the middle.

Where to go from here

Each of these follows from something on this page — a relationship in the evidence, a claim people ask about, or the next mechanism along.

How complete this page is, and what it is still missing

NatureHQ publishes its own gaps. This page is at 53% completeness against what we would call a finished subject, and was last reviewed on 2026-09-03. It carries 5 claims and answers 4 mapped search questions.

  • 5 high-priority search intent(s) not yet covered
  • no research from the last few years is attached — check for newer work
  • more experiments could be explained in plain English
  • no popular claim about this subject has been checked yet
  • Vertebrate parasites and their manipulation are barely covered; the evidence base here is entomological.
  • Brood parasitism — a bird raising another species’ chick — is a different phenomenon sharing the name, and is not covered.
  • Why parasitoidism is nearly confined to insects is unresolved.