Wasps up with parasite conservation in Britain?

I’m a few days late, but Happy Taxonomist Appreciation Day! Despite my tardiness, I am troubled by our global shortage of taxonomists, and I strongly support initiatives to (1) train more taxonomists, (2) provide them with livable and enjoyable career opportunities, and (3) find ways to integrate and value their important work amongst other science. I have mostly thought about this from a parasite conservation perspective, and I recently read an excellent paper that resonated with those thoughts. Below are some quotes (and my commentary) from Shaw and Hochberg (2001) that describe a parasite taxonomy crisis and some potential solutions:

Britain has a long history with natural history. For centuries, amateurs and professionals have been collecting and describing species from that relatively small land mass. In Britain, people probably don’t even seem like weirdos for gleefully wielding their custom-ordered extendable butterfly nets in public spaces. (Whatever, I’m not bitter, or anything.) Anyways, you might think that all of that enthusiasm for natural history has led to a complete inventory of Britain’s wildlife. But you’d be wrong.

“While the public may hold reasonably accurate perceptions that tropical ecosystems are teeming with unrecognized species, the average person in Britain is unaware that knowledge of the British biota – widely acknowledged as the best studied in the world – is also very limited.”

How could our taxonomic knowledge be so limited?! Parasites aren’t the only poorly known British taxa, but I’ll be talking about issues relevant to parasite taxonomy today. In particular, Shaw and Hochberg (2001) focused on a specific group of parasites: parasitoid wasps.

So how many parasitoid wasp species are there in Britain? When this paper was published in 2001, ~6000 species were known. To put that in perspective, that’s ~1/4 of the total known British insect biodiversity! To reiterate, without counting all of the other parasitic insects (e.g., fleas, lice), at least one quarter of the insect biodiversity is parasitic. I say “at least” because…

“…it seems likely that across parasitic Hymenoptera as a whole our knowledge of what is in the British fauna may be about 30-40% incorrect, or possibly even more…it strongly suggests that parasitic Hymenoptera will eventually turn out to be an even larger fraction of the total British insect fauna.”

Oh dear. We know that there must be many more parasitoid wasp species in the world (and Britain, specifically) than we currently know about for three reasons. First, whenever people conduct new field surveys or look at museum collections of parasitoids, they find that only a small fraction of the collected species have been described before. Second, many specimens are later found to be incorrectly identified, because morphological identification of parasitoid wasps is hard. And third, even when people think that they have nailed their morphological identifications, they might later find that the “species” that they are referring to is really a “morphospecies” representing 2 or 20 or even more cryptic species that are indistinguishable morphologically. For instance, here’s a quote from Smith et al. (2014) – an ambitious study matching morphological identifications to DNA barcodes for hundreds of parasitoid wasp species – regarding just one of the many cryptic species complexes that they uncovered:

“This minute black wasp with a distinctive white wing stigma was thought to parasitize 32 species of ACG hesperiid caterpillars, but barcoding revealed 36 provisional species, each attacking one or a very few closely related species of caterpillars.”

Yikes! When I read that, I thought, “Wow, if I ever need to do anything with parasitoid wasps, I’m going to need to find a collaborator who specializes in parasitoid wasp taxonomy.” So let’s say that I do need hypothetical help with an important biodiversity conservation project in Britain. Would I be able to find a parasitoid wasp expert to collaborate with? According to Shaw and Hochberg, in 2001, there were ~6 such experts – you know, approximately one expert for each thousand wasp species. (No, no, it’s fine, writing this isn’t giving me anxiety.) That seems like a tiny number of people who are responsible for ¼ of Britain’s insects! But at least there are some parasitoid wasp experts in Britain. The situation is likely worse in most other regions, where natural history is likely less popular and species diversity might be greater.

We must also remember that even if we can associate a DNA barcode or morphological description with a species name, we do not necessarily “know” that species. I brought this up a few weeks ago after reading John Lawton’s autecology and extinction crisis essay, and Shaw and Hochberg (2001) were clearly concerned about the biodiversity listicle phenomenon, too:

“Other insects are in a frame in which parasitic Hymenoptera are not, because parasitic wasps, with a low proportion of exceptions, are mostly just names.”

Are you having feelings about parasitoid wasps now?

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Of course, it is worth asking why we need to know more about parasitoid wasps. Research and conservation funding are limited, so why prioritize research regarding parasitoid wasps? I’ll give three possible reasons, but others could be suggested:

(1) If we are conserving biodiversity for its intrinsic value and/or because we believe that the most biodiverse or species-rich ecosystems are the best (for any given criteria), then we should perhaps prioritize research on the most species-rich and neglected taxa. Until we understand the most biodiverse taxa, we probably can’t maximize biodiversity conservation.

(2) Parasitoid wasps can be beneficial for humans. For instance, because parasitoid wasps tend to be highly host specific, they can be used as targeted biocontrols for agricultural pests. We’re talking serious economic worth. And given their documented effectiveness with controlling pests, it is likely that they play important roles in controlling populations of many other insect species that we don’t currently consider ‘pests’, but which might become problematic if they lost their parasitoid overlords. So maybe we should prioritize learning more about parasitoid wasps and conserving them to prevent potential economic losses.

(3) Parasitoid wasps might be especially vulnerable to extinction or co-extinction, so we might need to prioritize their conservation to prevent rapid biodiversity loss. As Shaw and Hochberg (2001) point out:

“The brief statement in the Insect Red Data Book (Shaw in Shirt (1987): 257-8) on parasitic wasps is to the effect that they must be considered among the most threatened of British insects, but that attempting a listing of endangered species would be quite hopeless in view of our poor knowledge. The message in this has, however, generally been as totally ignored as the parasitic wasps themselves.”


“..our parasitic Hymenoptera fauna…must – without any real doubt, given their high trophic level and characterizing levels of specialization and dependence – be happening at a rate that would surely be considered alarming, if only it could be noticed.”

Worryingly, that decline has been noticed outside of Britain:

“Therion (1976; 1981) reporting on the Ichneumonidae… fauna of Belgium, found that of the 122 species formally present 32 (26%) could not be found in a period of intensive collecting between 1950 and 1974/1979, with at least 30 further species (25%) showing major declines.”

After making cogent arguments for prioritizing parasitoid wasp conservation, Shaw and Hochberg (2001) provided several suggestions for improving those conservation efforts. One suggestion – including parasitoid wasps in Species Action Plans for better-known host species, like endangered butterflies – is something that I’ll come back to in my next parasite conservation post. For today, I’d just like to emphasize their number one suggestion:

“Nothing would do as much for the conservation of parasitic Hymenoptera as the provision of properly funded, career-length posts for alpha-taxonomists in major collection-building research institutions.”

Thank you, existing parasitoid wasp taxonomists! You rock. And I hope we can make and support many more scientists like you in the near future.


Shaw, M. R., and M. E. Hochberg. 2001. The Neglect of Parasitic Hymenoptera in Insect Conservation Strategies: The British Fauna as a Prime Example. Journal of Insect Conservation 5:253–263.

Smith, M. A., J. J. Rodriguez, J. B. Whitfield, A. R. Deans, D. H. Janzen, W. Hallwachs, and P. D. N. Hebert. 2008. Extreme diversity of tropical parasitoid wasps exposed by iterative integration of natural history, DNA barcoding, morphology, and collections. Proceedings of the National Academy of Sciences of the United States of America 105:12359–12364.

Why are caterpillars hairy?

Why is it advantageous to be a hairy caterpillar? One answer can be found in a beautiful paper by Sugiura and Yamazaki (2014). They put five species of caterpillars with various amounts of hairiness in containers with carabid beetles that naturally prey on caterpillars. The carabid beetles were always successful when attacking the smooth/hairless caterpillar species, and it usually only took beetles one try to successfully catch the caterpillar. When attacking a short-haired caterpillar species, the carabid beetles were still always successful, but it took them more tries. And when attacking a long-haired caterpillar species, the carabid beetles were only successful ~50% of the time. Even when they were successful, it took beetles more attempts to catch the caterpillars. Therefore, it looked like long hair protected caterpillars from beetle attacks. To test that idea, Sugiura and Yamazaki (2014) gave the long-haired caterpillars haircuts, so that the hairs were shorter than the beetles’ mandibles. The beetles were then way more successful at attacking the caterpillars with haircuts than the long-haired caterpillars! Now that is sexy science.

So, long-haired caterpillars are out there multiplying like crazy while their short-haired neighbors are getting mown down by beetles, right? Actually, having hairs may be a trade-off. Hairy caterpillars are more likely to be attacked by parasitoids, and a higher diversity of parasitoids attack hairy caterpillars than smooth caterpillars (Stireman and Singer 2003)! It might be beneficial for parasitoids to stick their eggs in hairy caterpillars because the eggs+caterpillars will be less likely to be eaten by a predator before the parasitoid emerges than if the caterpillar is smooth. Or it may be that hairy caterpillars – which are usually not cryptic – are easier for parasitoids to find. Either way, these papers have changed my life.



Stireman, J.O., and M.S. Singer. 2003. Determinants of parasitoid-host associations: insights from a natural tachinid-lepidopteran community. Ecology 84(2): 296-310.

Sugiura, S., and K. Yamazaki. 2014. Caterpillar hair as a physical barrier against invertebrate predators. Behavioral Ecology 25(4): 975–983.

Predator vs. Parasite vs. Parasitoid vs. Mutualist– A Simple Classification Scheme

One of the most frequently accessed posts on this blog defines the terms “predator,” “micropredator,” “parasite,” and “parasitoid” and then presents a classification scheme for differentiating among those natural enemies (Lafferty and Kuris 2002). If you haven’t read that post yet, I recommend taking a look before you read this one. I obviously fancy the dichotomous key presented in the previous post quite a bit. However, it is not be the best classification scheme for all situations. For example:

  1. Check out Britt Koskella’s comments on the previous post – it is difficult to classify the bacteriophages that she studies using that classification scheme.
  2. The previous key may be difficult to use for educational purposes. For instance, it requires explaining castration and trophic transmission, which are concepts that might be unnecessarily complicated for explaining the distinction between parasites and predators to non-specialists.
  3. The previous key only deals with natural enemies, so we can’t use it to explain how mutualists and commensalists fit into this group of trophic relationships.
  4. The previous key doesn’t show how relationships can vary with life stages, ecological conditions, and environmental conditions.
  5. The previous key doesn’t have any pictures of snails on it. (Priorities.)

Therefore, having an additional classification scheme that specializes in some of the aforementioned areas would be useful. And – you guessed it – one was recently(ish) published (Parmentier and Michel 2013)! This classification scheme uses two continuous variables to designate relationships: the relative duration of the association (RDA) and the fitness effects on the ‘host.’

The relative duration of association ranges from 0 to 1, where 0 means that the ‘symbiont’ (including predators) spends none of its lifetime associated with the ‘host’ (or prey), and 1 means that the symbiont spends all of its lifetime associated with the host. For instance, predators and micropredators have RDA’s close to zero – a lion spends only a small portion of its life with a single zebra prey. Conversely, an adult trematode parasite (the worm in the orange section of the figure) will spend that entire life stage in association with a single host. The RDA is the Y axis on the figure below.

In recent months, we’ve talked a lot about the fitness effects that symbionts have on their hosts. Briefly, symbionts may have both negative and positive effects on hosts, and it is the net effect that determines how we classify the relationship. However, the net effect can vary with ecological and environmental conditions (see here, here, and here). Therefore, whenever we place a point on this graph, we need to remember that it might slide left or right as conditions vary.


Parasitoid wasps spend the entirety of their larval life stage in the host, and they ultimately kill the host. In this figure, parasitic castrators – like the trematodes that castrate snails – end up in the same region as the parasitoids. And this is where bacteriophages and Cordyceps fungi would fall out, too. However, like we discuss in the previous post, the term “parasitoid” is probably not a good one for this group, because that term is usually used to refer specifically to the unique life cycles of parasitoid wasps. In this figure, it means any parasite that reduces host fitness to zero.

Predators like lions, frogs, and crayfish also reduce prey fitness to zero. However, micropredators and herbivores (e.g., mosquitoes and cows) are special classes of predators that do not kill their prey. Then there is a group of animals that consume plants, but are probably more appropriately classified as parasites because they spend the majority of their life span (or a single life stage) on a single host plant. Therefore, things that eat plants will typically fall out somewhere between the aphids and the micropredators. (I should note that herbivores can have more than minimal impacts on host plant fitness, but many grazers have small impacts.) Similarly, Mark Siddall – the man who went on a quest for the hippo ass leech – doesn’t like classifying leeches as micropredators, because some spend most of their time on a single host. Therefore, most leeches would also fall out somewhere along that line between aphids and mosquitoes. Except, yaknow, the ones that are actually predators, and fall out near the lions.

On the mutualism side of things, we have symbionts like pollinators, which are only very briefly associated with each host; they’re like micropredators, but with positive fitness effects on their hosts. And then there are symbionts that are associated with single hosts for most of their lifespans, like ants on Acacia trees or guard crabs on corals. But again, remember that those relationships might shift left on the X axis as conditions vary.

Speaking of which, finding an example of a commensalist is hard. I used the example of epibionts on hermit crab shells, which help protect the hermit crabs from some predators but make the hermit crabs more susceptible to other predators. The net effect is unclear, but the positive and negative effects might balance out to a zero net effect.

So, there you have it. I think this figure should be really useful, especially as a general framework.


Lafferty, K.D., and A.M. Kuris. 2002. Trophic strategies, animal diversity and body size.  TREE 17(11): 507-513. (Direct link to PDF download)

Parmentier, E., and L. Michel. 2013. Boundary lines in symbiosis forms. Symbiosis 60: 1-5.

How beneficial are defensive symbionts?

(This post is late. Sorry, Folks! In my defense… snail dissection. So. much. snail. dissection.)

In my aphid cartoons thus far, Sal the Aphid has been bragging about how she has H. defensa. But just how awesome is it to harbor defensive bacteria? Like we said last time, when aphids get attacked by parasitoid wasps, aphids with H. defensa are more likely to survive than aphids without H. defensa. That seems like a pretty big bonus provided by the symbionts. We might expect natural selection to then favor aphid lineages with symbionts, leading to domination by lineages with H. defensa. But that isn’t what we see. Instead, only some aphids have H. defensa – the symbionts are maintained at intermediate frequencies. So, if defensive symbionts are so great, then why don’t all aphids have them?

In some earlier work, researchers found that H. defensa may not always be beneficial for aphids. For instance, when no parasitoids are present, the frequency of H. defensa in aphid populations declines, suggesting that H. defensa may be costly to maintain (Oliver et al. 2008). So, Vorburger et al. (2013) set out to determine whether the cost of harboring H. defensa is constitutive, induced, or both. That is, is H. defensa always costly, regardless of parasitoid presence (constitutive cost), does the cost come after a parasitoids attack and H. defensa kill the parasitoid larvae (induced cost), or both?

To test this question, Vorburger et al. (2013) exposed aphids with and without H. defensa to attacks by parasitoid wasps. The first 2/3 of the aphids that the wasps attacked were put in an “attacked” treatment group, and the aphids that the wasps did not attack were put in an “unattacked” treatment group. And then Vorburger et al. (2013) kept track of aphid survival and reproductive output.

Like we said last time, when aphids had H. defensa, they were more likely to survive a wasp attack. But when aphids weren’t attacked by wasps, the aphids with H. defensa had reduced fitness in comparison to aphids without H. defensa. That’s the constitutive cost we mentioned before. For aphids without H. defensa, attacked aphids had lower lifetime reproduction than unattacked aphids. That makes sense, of course. But get this: for aphids with H. defensa, attacked aphids had higher lifetime reproduction than unattacked aphids. That’s the opposite of an induced cost! It’s an induced benefit!

So, what caused the “induced benefit”? Well, Vorburger et al. (2013) aren’t sure. But they have one amazing hypothesis. They suggest that maybe when a wasp injects all that venom into an aphid, it kills off a bunch of the H. defensa. That is, the induced benefit is to reduce the constitutive cost of harboring H. defensa by killing off some of those costly symbionts. In that case, H. defensa isn’t sounding so nice afterall, is it? 

I rest my case: parasites and parasitoids are crazy awesome.

Oh, and the paper is open access! Check it out!



Oliver, K.M., J. Campos, N.A. Moran, and M.S. Hunter. 2008. Population dynamics of defensive symbionts in aphids. Proc. R. Soc. B Biol. Sci. 275:293–299.

Vorburger, C., P. Ganesanandamoorthy, and M. Kwiatkowski1. 2013. Comparing constitutive and induced costs of symbiont conferred resistance to parasitoids in aphids. Ecology and Evolution 3(3):706-13.

Defended Hosts are Frassheads

Last week, I told you guys that parasitoid wasps respond to H. defensa, which is a bacterial endosymbiont that protects aphids from wasps.  Next week, I’m going to talk about how H. defensa affects aphid fitness.  But first, what is the magnitude of the protective effect?  Well, it varies with the strain of H. defensa and the aphid species and probably lots of other factors, too.  But in the example that I’m going to discuss next week, aphids without H. defensa have a 38% probability of becoming mummies if they get attacked by wasps, while aphids with H. defensa only have a 4% chance of becoming mummies if they get attacked by wasps (Vorburger et al. 2013).  So, H. defensa reduces the probability of mortality after attack by 89.5%!



Stay tuned to see what happens to Sal and Lisa.  Are they up Frass Creek without a paddle?


Vorburger, C., P. Ganesanandamoorthy, and M. Kwiatkowski. 2013. Comparing constitutive and induced costs of symbiont conferred resistance to parasitoids in aphids. Ecology and Evolution 3(3):706-13.


How do parasitoids respond to defended hosts?

Last week, I talked about the new Godzilla movie and how I thought that the MUTOs should have been parasitoids.  This week, let’s talk about some awesome, real life parasitoids: parasitoid wasps (Aphidius ervi).

Quickly, the life cycle works like this: the female wasp finds an aphid nymph, she stabs the aphid with her ovipositor, and then she typically lays one egg inside the aphid.  After one day, the egg hatches into a larval parasitoid, and the larva hangs out inside the aphid while eating the aphid’s innards.  After about one week of this, the aphid dies.  Actually, the aphid’s corpse becomes a “mummy,” and the larva pupates inside the mummy before eventually emerging as an adult parasitoid.  Mating happens, and then the female wasps go off to infect more aphids.

But here’s an interesting complication: some aphids are protected by bacterial symbionts (Hamiltonella defensa).  The degree to which aphids are protected varies with the strain of H. defensa, but the take-home message is that when a wasp lays an egg inside an aphid, the egg is much less likely to survive to adulthood if the aphid has H. defensa symbionts (Oliver et al. 2012).  However, if a wasp lays two eggs inside an aphid with H. defensa symbionts – which is not what wasps usually do – then one larva is more likely to survive than it would have been if it had been a single egg.  In other words, only one larva is going to make it out of there alive, even when two eggs are laid, but one of the larvae is better off than it would have been if it were a single egg.

You might be thinking, “Why, what an interesting tidbit.  Who cares?”  NATURAL SELECTION CARES.  Just kidding, natural selection isn’t sentient, but natural selection should favor any wasp strategies that increase wasp fitness.  And wasp fitness is higher when more wasp eggies turn into wasp larvae and then adult wasps.  And wasp larvae are less likely to die in aphids with H. defensa if two eggs are laid in the aphid, instead of the typical single egg.  See where I’m going with this?

Yes, wasps can differentiate between aphids with and without H. defensa.  And when aphids have H. defensa, wasps are much more likely to lay two eggs in those defended aphids than they are to lay two eggs in undefended aphids.  And that, my friends, is amazing.  While wasps still probably have reduced fitness when infecting defended aphids, the superinfecting tactic (=laying two eggs) likely compensates for some of the reduced fitness.



(Yes, sometimes aphids have conversations in my head, and I write them down. You’re welcome.)

Check out the open access paper to learn about the mechanism behind the success of superinfection:


Oliver, K.M., K. Noge, E.M. Huang, J.M. Campos, J.X. Becerra, and M.S. Hunter. 2012. Parasitic wasp responses to symbiont-based defense in aphids. BMC Biology 10:11.

Godzilla Parasites


A few weeks ago, I went to see Godzilla.  I hadn’t looked up the plot summary or anything beforehand, so imagine my surprise when out of the giant pulsing “spore” (ahem, egg) emerged something that looked a lot like a cross between a giant water bug and Alien…not Godzilla.  And then imagine my UTTER GLEE when they said that the thing that was not Godzilla was a parasiteSwoon.  I immediately conjured up all kinds of plot possibilities, and I couldn’t wait to see how the parasites attacked Godzilla!

But then I quickly realized that the “parasites” were not parasites at all.  The parasites acquired energy from radioactive material.  For instance, they ate nuclear warheads.  And that alone doesn’t make them parasites.*  It makes them autotrophs.  I thought I might have missed the parasite explanation, so after the movie, I did some googling.  But all I could find was some people saying that the parasites (or their young) might try to feed on Godzilla’s radioactive energy.  I would totally buy that, if the parasites had searched for Godzilla in the movie.  But instead, Godzilla searched for the parasites.  In fact, he was their “predator.”  WHAT?!  Yo, Hollywood.  You need a parasite ecology consultant?  HMU.

So, I wrote you guys a different plot, with actual Godzilla parasites in it.  Except that they aren’t parasites, per se.  They’re parasitoids.*  Enjoy!


The female parasitoid hatches from an egg in a mine in the Philippines.  The female parasitoid goes to the Janjira nuclear plant to feed and causes a giant explosion.  A lady dies, and it’s sad.  The female parasitoid forms a chrysalis in the wreckage.

Sometime in the next 15 years, the other egg from the mine in the Philippines is taken to the USA to be studied and whatnot.  Then the radioactive body of the male parasitoid – which is thought to be dead – is stored in Yucca Mountain.

After 15 years, the female parasitoid emerges from her chrysalis.  She has wings!  (Yes, it is the male who has wings in the movie, but I don’t like it that way.)  She destroys a bunch of stuff and kills a dude and it’s sad.

The male (he’s alive!) and female parasitoids start communicating via echolocation (ok, whatever, I’ll go with it).  They start trying to find each other, stopping only to ransack ships and whatnot so that they can eat radioactive material.  When they find each other, the male fertilizes the female.  The male also gives her a nuptial gift of a nuclear warhead, because that was really cute.  Then he dies because he’s a male and he no longer has a purpose in life.  ONE MONSTER DEAD.  Huzzah!

Now the female needs a host for her eggs.  So, while armed forces are trying to shoot her to bits, she uses her highly adapted sensory apparatus to seek out Godzilla.  When she finds Godzilla, she stabs her ovipositor (yes, she has one of those now) into Godzilla’s body cavity and deposits a single egg.

Godzilla2(And you guys thought my artwork was limited to snails!)

Then the female parasitoid tries to fly off to find another Godzilla so that she can lay another egg, because that’s what parasitoids do.  But Godzilla grabs her head and breathes plasma down her throat, and she dies. SECOND MONSTER DEAD.  Huzzah!

The world starts to rejoice because all the parasitoids are dead, but suddenly San Francisco is being trampled by Godzilla!  Someone left some giant war heads in San Francisco, and Godzilla is being manipulated by the parasitoid larvae into finding and eating more radioactive material!  Oh no!  But wait, one of the nuclear warheads has an analog detonator thingy, so the parasitoid’s EMP abilities can’t stop it from detonating now that it has been activated!  Godzilla eats it!  1 hour and 29 minutes later, Godzilla and the parasitoid within explode.  ALL THE MONSTERS ARE DEAD!

Some soldier and his lady kiss and stuff.  The end!

*If you don’t remember the difference between a parasite, a predator, and a parasitoid, check this out.

Some Definitions: Predator vs. Parasite vs. Parasitoid

Despite nearly one year’s worth of posts about parasite ecology, this blog has never defined the term ‘parasite.’  D’oh!  You might think, “Pft, the definition is obvious!”  But actually, it isn’t, and it isn’t without controversy, either.  I’m going to talk about a bunch of types of natural enemy, and then I’ll present a really good dichotomous key at the end.


Let’s start with predators.  Like parasites, predators are organisms that acquire energy by taking that energy from other organisms.  Therefore, we have a relationship that positively affects one organism (the predator) and negatively affects the other organism (the prey).  Predators have these important characteristics:

1)      One predator eats multiple prey during the predator’s lifetime.

2)      Predators tend to be bigger than their prey.

3)      Predators tend to kill their prey.


You’ll notice that I said that predators “tend to” be bigger than their prey and “tend to” kill their prey.  They don’t always!  A very good example of this is a vampire bat that takes blood meals from cows.  A single vampire bat will take blood meals from multiple cows during its lifetime.  It is eating multiple prey, therefore, it is a predator.  But it doesn’t kill the cows, and it isn’t bigger than them.  It’s a micropredator.


Parasites are different from predators because parasites only take resources from one host, whereas predators eat many prey.  A good example of this is the trematode parasite Schistosoma mansoni.  An adult schistosome parasite lives inside of just one human host.  It is never going to crawl out and go infect a different human.

You might be thinking, “Waaaaait…  Schistosoma mansoni has a complex life cycle!  It infects humans AND snails!  That’s two hosts!”  Yep.  But the rule is that parasites only infect one host during each stage of the life cycle.  One human.  One snail.

Here are some other common characteristics of parasites:

1)      They are smaller than their hosts.

2)      They don’t usually kill their hosts.*

Ok, so, the killing bit is confusing and wishywashy.  I’ll come back to it below.


Like a parasite, a parasitoid infects just one host per life stage.  But parasitoids always kill their hosts.

Parasite vs. Parasitoid:

So, what’s the difference between a parasite and a parasitoid?  If you’re about to take an exam or something and you want a quick answer, say that parasitoids always kill their hosts and parasites don’t usually kill their hosts.  You’ll find that in many introductory ecology textbooks.

In practice, we don’t really use that definition.  The term parasitoid is usually applied to certain insects that have free-living adult stages that lay eggs inside a host, and the eggs go on to parasitize and eventually kill the host.

There are many “parasites” that always kill their hosts, and we still call them parasites and not parasitoids.  Why, Scientists?  Why do you do this thing?  Well, it just doesn’t make sense to have a rule that says that parasites don’t kill their hosts.  For instance, if a parasite (say an acanthocephalan) in an intermediate host (a pillbug) makes the host more likely to get eaten by the next host (a bird) in the life cycle, then the parasite is often the cause of the host’s death.  Those kinds of parasites are called trophically-transmitted parasites.

A Very Nice Dichotomous Guide:

Lafferty and Kuris (2002) came up with a really nice dichotomous key for classifying natural enemies.  They used four dichotomies, but I’m only going to use the first three:

1. “Does the enemy attack more than one victim?”

2. “Does the enemy eliminate victim fitness?”  (‘Eliminating fitness’ could be killing the victim or sterilizing the victim so that it cannot reproduce.)

3. “Does the enemy require the death of the victim?”

Definition of Parasite Diagram

Figure adapted from Lafferty and Kuris (2002) – link below.

You might remember from one of my previous posts that we tend to divide parasites into microparasites and macroparasites.  As I described in that post, for microparasites, we care about presence/absence of infection, and for macroparasites, we care about intensity of infection.  The fourth dichotomy used by Lafferty and Kuris (2002) is “does the enemy cause intensity-dependent pathology?”  They include the fourth dichotomy in their figure.  It’s really useful, but I didn’t include it here to avoid confusion. Click the PDF link below to see their version.


Lafferty, K.D., and A.M. Kuris. 2002. Trophic strategies, animal diversity and body size.  TREE 17(11): 507-513. (Direct link to PDF download)