How do parasites affect tadpole behavior?

Like I said last week, I saw a cool talk at ESA about tadpole behavior and trematode parasites.  And since I liked it, I thought that YOU might like it, too!

When tadpoles can sense that predators are nearby, they alter their behavior by becoming less active.  That’s a good way to avoid getting eaten.  But do tadpoles alter their activity levels when parasites are nearby?  Preston et al. (2014) say no!  This seems counter intuitive, because some parasites, like the trematode Ribeiroia ondatrae, can cause a lot of damage to the host tadpole, and even kill it.  But like we’ve discussed before, macroparasites (like Ribeiroia) tend to have intensity dependent effects on the host, so that pathology increases with the number of parasites.  And it might not be worth altering your behavior if you’re probably just going to get a few parasites, and they probably aren’t going to do much harm to you anyways.  Furthermore, just because parasites are nearby doesn’t mean that they’ll be able to successfully infect the tadpole, because tadpoles have other anti-parasite tactics, like immune responses.


But what about after the tadpoles get infected by the parasites?  Do the parasites affect tadpole behavior then?  You’ll just have to go check out the paper to find out!


Preston, D.L., C.E. Boland, J.T. Hoverman, and P.T.J. Johnson. 2014. Natural enemy ecology: comparing the effects of predation risk, infection risk and disease on host behaviour. Functional Ecology.

Parasite Ecology at ESA 2014

I didn’t go to all of the parasite ecology talks at ESA 2014, and I can’t even fit all the ones that I went to into one blog post.  But for those of you who weren’t there – and for those who were but just want to revel in your memories of awesome ESA 2014 parasite ecology – here are some of my favorites.  Also, just for fun (and because I may or may not have an addiction that I could totally give up at any time, I swear), I gave everyone a relative parasite cartoon score.

Monday was the ESA Early Career Fellows Symposium.  Meg Duffy and Pieter Johnson both gave brilliant parasite ecology talks.  Meg Duffy showed that parasite infection can reduce host feeding rates in Daphnia, which paradoxically ends up increasing host density via interactions with the algal resource (i.e., a hydra effect).  She had some sweet Daphnia + fungus cartoons, so I give her an 8 on my cartoon scoring scale (second place!).  Pieter Johnson showed that amphibian host diversity can increase parasite richness (“diversity begets diversity”) and decrease Ribeiroia infection risk (=dilution effect), and he emphasized that parasite richness and infection risk aren’t the same thing.  (Speaking of Ribeiroia, have you guys seen this tshirt?  You’re welcome.)  I may have missed some cartoons while I was scribbling in my notebook, but I think he had some silhouettes of snails and vertebrates and some fluorescent cercariae cartoons:  4.  

Christopher Johnson talked about competition between two mutualistic species (i.e., two butterfly species) for a shared resource (i.e., nectar from the host plant) – in other words, a symbiont competition model, where the resource is the host or services/resources from the host.  And instead of R*s, there were M*s.  And then there were phase diagrams and talk of symbiont species coexistence.  Yeah.  Amazing. 

On Tuesday, I saw a talk by Eric Schauber, who used agent based modeling to consider how the “need to be social” affects among group pathogen transmission.  Awwwesome.  He had some cartoon vertebrates (goats?), so that’s a 2. 

On Wednesday, Max Joseph gave a really cool talk about how a negative relationship between “disease risk” and host richness can emerge from a model that treats hosts as habitat patches, where symbionts have different niche requirements.  Oh, and so can a positive relationship between host and parasite richness!  I’d just been lamenting the loose way that people refer to “disease risk,” so I was really glad when he stressed the importance of quantifying what we mean by disease risk when talking about the dilution effect.  Get ready, World.  Big things are happening with dilution effect theory.

Angela Brennan asked:  what is the right scale to look at the impacts of host density on disease transmission?  That’s a tricky one…

Cat Searle gave a neat talk about invasive species, their competence as hosts, and their role in pathogen transmission, using a Daphnia model system.  She had some cartoons of space aliens as her invasive species, so that’s a 6.  Oh, and for all of the undergraduates reading this blog, she’s looking for grad students!

Continuing on the Daphnia vein, Alex Strauss looked at the outcomes of introducing diluting host species that both reduce parasite transmission to the focal host species and compete with the focal host species.  And DING DING DING DING!  For his cartoons of Daphnia, algae, fungi, and other tiny organisms (score = 10), Alex wins the Best Parasite Cartoons of ESA 2014 Award!! 

Finally, on Thursday, I really liked Dan Preston’s talk about tadpole behavioral responses to predators and parasites.  But I’m going to try to blog that one next week, so you’ll have to stay tuned!

The Last of Us

I thought that we’d do a quick, just-for-fun post today.  ABOUT ZOMBIES.  Now, I like me some zombie movies.  Sometimes their plots even have cool disease ecology components, like competition between the “zombie virus” and other host pathogens.  On the other hand, there are many, many biological inaccuracies involved in the popular zombie idea, as Neil deGrasse Tyson explains.  

But if you’ve been following this blog or popular science for any length of time, you know that in a way, Neil deGrasse Tyson is wrong in saying that if zombies exist, they only exist on other planets.  There are “zombies” on Earth: parasite zombies!  That is, some parasites can dramatically alter their host’s behavior, so that the host is effectively just a vehicle for the parasite.  The point of this manipulation is to get the host to behave in such a way as to increase the parasite’s probability of transmission to the next host.  Usually, this involves the host getting eaten by the next host, like when infected ants hang out at the top of blades of grass, where they are likely to be eaten by cows or sheep.  But that’s not always the case.  For instance, with rabies – the pathogen most similar to the classic idea of a zombie virus –  the virus makes (some) animals behave aggressively, and this increases the probability that the virus will be transmitted to new hosts via bites.  

Now, thankfully, there aren’t any parasites that re-animate dead corpses…yet.  But there are parasites that use corpses as points of transmission.  For instance, Cordyceps fungus makes ants leave their normal routines to go bite onto leaves above major areas of ant traffic.  Then the fungus sprouts a fruiting body out of the ant’s corpse and rains spores of death down on the ant’s extended family.  

Don’t you think that a Coryceps fungus apocalypse would be a cool video game plot?  Well, actually, it already is a cool video game plot!  The Last of Us has been out for a while on PS3, and a remastered version was just released on PS4.  From Wiki:

“In 2013, Joel (Troy Baker) is a single father living near Austin, Texas with his twelve-year-old daughter Sarah (Hana Hayes). One night, an outbreak of a mutant Cordyceps fungus ravages the United States, which transforms its human hosts into cannibalistic monsters…”

Now, real Cordyceps doesn’t turn insects into cannibals, but I’m willing to overlook this error in biology because – WAIT FOR IT – they’re going to make The Last of Us into a movie, too!  That’s right.  Cordyceps is coming to the big screen.  Awwyisss.     

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.